Hydrodynamics of flow through seagrass canopies : biological, physical, and geochemical interactions

Hydrodynamics of flow through seagrass canopies : biological, physical, and geochemical interactions

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Hydrodynamics of flow through seagrass canopies : biological, physical, and geochemical interactions
Koch, Evamaria Wysk
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Tampa, Florida
University of South Florida
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x, 123 leaves : ill. ; 29 cm.


Subjects / Keywords:
Seagrasses -- Ecology ( lcsh )
Thalassia testudinum -- Growth ( lcsh )
Boundary layer ( lcsh )
Dissertations, Academic -- Marine Science -- Doctoral -- USF ( lcsh )


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Thesis (Ph.D.)--University of South Florida, 1993. Includes bibliographical references (leaves 104-113).

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University of South Florida
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University of South Florida
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All applicable rights reserved by the source institution and holding location.
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029770638 ( ALEPH )
30755441 ( OCLC )
F51-00183 ( USFLDC DOI )
f51.183 ( USFLDC Handle )

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HYDRODYNAMICS OF FLOW THROUGH SEAGRASS CANOPIES: BIOLOGICAL, PHYSICAL, AND GEOCHEMICAL INTERACTIONS by / EVAMARIA WYSK KOCH A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Marine Science University of South Florida December 1993 Major Professor: Albert C Hine, Ph. D


Graduate School University of South Florida Tampa, Florida CERTIFICATE OF APPROVAL Ph. D Dissertation This is t o certify that the Ph.D. Dissertation of EVAMARIA WYSK KOCH with a major in Marine Science has been approved by the Examining Committee on September 3 1993 as satisfactory for the dissertation r equirements for the Ph. D degree Examining Committee: Major Hine, Ph. D Member : Ph.D. Member : Mimi A .R. Koehl, Ph.D. Memoer : Pamela Hallock-Muller, Ph.D.


DEDICATION To all those Americans and Israelis who accepted me as one of their own.


ACKNOWLEDGMENTS I would like to extend thanks: To Dr. Albert Hine for his constant encouragement and guidance and for the freedom I was allowed during my research; To Dr. Clinton Dawes for his friendship and endless source of inspiration during my studies in the United States; To Dr. Mimi Koehl for joining my commit tee during a difficult time and for her advice which contributed to the strength of my work and illuminated my plans for the future; To Dr. Michael Durako for lengthy and enlightening discussions and for his enthusiasm for my work which has been a continuous source of motivation; To Drs. Pamela Hallock-Muller and John Ogden for their suggestions on the manuscript; To Dr. Sven Beer for insightful discussions and true friendship which contributed to my professional and personal growth; To Dr. Markus Huettel for his comments which strengthened part of this manuscript; To the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (Brazil), the Department of Marine Science at the University of South Florida and the Center for Coastal Geology (United States Geological Survey-St. Petersburg) for making


this PhD possible through their financial support; To the Knight family for their generous investment in my career which enhanced my professional advancements in many ways and made the studies in Israel possible; To the Beers (Sven, Judith, Netta and Jonathan) for making me part of their family during my stay in Israel and for sharing their culture with so much enthusiasm; To Dr. Michael Friedlander and Yael Gonen for being my hosts and providing lab space in the 11 Israel Oceanographic and Limnological Research Institute11 in Haifa, Israel; To Dr. Giselher Gust for making the microcosm and the hot-film sensors (US Patent # 4986122) available; To Walter Bowles, Richard Eilers, Helen Talge, Maggie Toscano, Dr. Rick Stumpf, Steve Goodbred, Dr. Sven Beer, Netta Beer, Lily Pereg and Dr. Lipkin for their great companionship during the field deployments; To Chad Edmisten for preparing the graphs and figures. To Lisa Thurm for reviewing the manuscript; To Helen Talge for her true friendship; To my parents (Drs. Walter and Eva Koch) for introducing me to the fascination of Oceanography at an early age and for supporting me in all possible ways to make my dreams as an Oceanographer come true; To my fiancee, Richard M. Eilers, for being an endless source of support, strength and love during good and bad times.


TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS ABSTRACT CHAPTER 1. GENERAL INTRODUCTION What are Seagrasses? Effect of Water Flow on Seagrasses Morphology Bending of Blades Diffusion Boundary Layer Effect of Seagrasses on Water Flow Reduction of Current Velocity Wave Attenuation Turbulence Effect of Seagrasses on Porewater Fluxes Seagrasses in a Fluid Environment Purpose of the Study CHAPTER 2. HYDRODYNAMICS, DIFFUSION BOUNDARY LAYERS AND PHOTOSYNTHESIS OF THE SEAGRASSES Thalassia testudinum AND Cymodocea nodosa Introduction Methods Results Discussion CHAPTER 3. TURBULENCE DISTRIBUTION IN TIDE AND WAVE DOMINATED CANOPIES OF THE SEAGRASS Thalassia testudinum Introduction Methods Results Discussion i iii iv vi vii 1 2 3 4 5 6 7 8 9 10 13 17 25 35 43 49 53 65


CHAPTER 4. CURRENTS, POREWATER GEOCHEMISTRY AND THE MORPHOLOGY AND PRODUCTIVITY OF Thalassia testudinum SEEDLINGS Introduction Methods Results Discussion CHAPTER 5. SUMMARY AND CONCLUSIONS Does the boundary layer limit photosynthesis? Hydrodynamic implications of epiphytic covers Turbulence generation and attenuation Optimal hydrodynamic conditions for seagrasses Seagrasses and their fluid environment Adaptations to the fluid environment Recommendations for further research LITERATURE CITED APPENDIXES APPENDIX 1 Statistical analysis for data presented in Chapter 4 APPENDIX 2. Deployments in seagrass habitats using Tattle data loggers ii 72 77 80 86 92 93 93 96 97 98 100 104 114 115 116


LIST OF TABLES Table 1. Analysis of variance testing for significant differences between photosynthetic responses of Thalassia testudinurn exposed to different combinations of friction velocities (u*) and light levels 29 Table 2. Morphological characteristics of Thalassia testudinurn seedlings exposed to different flow conditions under laboratory controlled conditions 82 iii


LIST OF FIGURES Figure 1. Microcosm in a closed system used to test the effect of friction velocity on photosynthetic responses of the seagrasses Thalassia testudinum and Cymodocea nodosa 20 Figure 2. SEM photographs of Thalassia testudinum blades before and after removing epiphytes 26 Figure 3. Boundary roughness Reynolds numbers on seagrass blades colonized by different size organisms 27 Figure 4. Photosynthetic rates of Thalassia testudinum blades exposed to combinations of friction velocities and light levels 28 Figure 5. Photosynthetic rates of Thalassia testudinum blades exposed t o combinations of total DIC levels and friction velocities at saturating light in buffered seawater 30 Figure 6. Photosynthetic rates of Thalassia testudinum and Cymodocea nodosa blades exposed to various friction velocities at saturating light levels 31 Figure 7. Theoretical values of boundary layer resistance to the diffusion of C02 and HC03 on leaves of Thalassia testudinum and Cymodocea nodosa at increasing friction velocities 33 Figure 8. Friction velocities experienced over time by Thalassia testudinum and Cymodocea nodosa blades at 20 em above the bottom under low and high energy conditions 34 Figure 9. The effect of turbulent eddies on mixing of particles/molecules in a hypothetical seagrass bed 47 Figure 10. Generation of turbulence by epiphytes on a seagrass blade (cross view) 47 lV


Figure 11. Deployment of submersible data logger for evaluation of vertical speed profiles within seagrass canopies 50 Figure 12. Sites at which speed and depth were recorded within and above canopies of the seagrass Thalassia testudinum 52 Figure 13. Speed profiles, wave spectra and speed spectra for a Thalassia testudinum canopy in a tide dominated environment during calm and windy conditions Figure 14. Speed profiles, wave spectrum and speed spectra for a Thalassia testudinum canopy 55,56 in a wave dominated environment 60 Figure 15. Speed profiles during a high and an ebb tide, wave spectra and speed spectra for a Thalassia testudinum canopy in a mixed environment 63,64 Figure 16. Distribution of turbulent eddies in a dense Thalassia testudinum bed exposed to tide-dominated conditions 68 Figure 17. Distribution of turbulent eddies in a Thalassia testudinum bed exposed to wave-dominated conditions 69 Figure 18. Cross section of a Thalassia testudinum blade exposed to medium flow for 6 months 81 Figure 19. Morphology of Thalassia testudinum seedlings exposed to different flow regimes in microcosms for 6 months under laboratory controlled conditions 83 Figure 20. Cross section through the microcosms in which circular flow was generated for 6 months to test the effect of flow on the morphology of Thalassia testudinum seedlings and the geochemistry of the porewater 85 Figure 21. Ammonium, filterable reactive phosphorus and sulfide concentrations in the pore-water of 3 microcosms exposed to different flow regimes after 6 months under controlled conditions 86 v


D i 1\., l L N v p T LIST OF SYMBOLS concentration of "i" in the media concentration of "i" at the sink site molecular diffusion coefficient diffusion coefficient of the molecules "i" flux of the molecule "i" fundamental frequency Nyquist frequency epiphyte height solute (molecule dissolved in a liquid) transfer velocity average blade width diffusion boundary layer thickness number of data points in time-series blade roughness Reynolds number Schmidt number friction velocity sampling frequency boundary layer resistance dynamic viscosity kinematic viscosity density of the water shear stress vi


HYDRODYNAMICS OF FLOW THROUGH SEAGRASS CANOPIES: BIOLOGICAL, PHYSICAL AND GEOCHEMICAL INTERACTIONS by EVAMARIA WYSK KOCH An Abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Marine Science University of South Florida December 1993 Major Professor: Albert C. Hine, Ph.D. vii


Interactions between the hydrodynamics of Thalassia testudinum and Cymodocea nodosa seagrass beds and biological, physical and geochemical parameters were investigated. In bot h species, photosynthetic rates were limited due to carbon diffusion through the boundary layer only below relatively low current velocities (expressed as blade friction velocities, u.). Above a critical u. level (0 25 em s -1 for I. testudinum and 0. 64 em s-1 for Q nodosa), carbon fixation by the enzymatic system may have been the limiting photosynthetic processes. These results are factor to based on hydrodynamically smooth flow on the seagrass blade surface. The presence of epiphytes and attached debris on seagrass blades in situ induces intermediate to rough-turbulent flows causing the boundary layer thickness to fluctuate in time and space possibly enhancing carbon transport. High resolution speed and wave time series recorded in situ outside and within beds of the seagrass Thalassia testudinum indicate that canopies of t his species are efficient in attenuating turbulent energy at frequencies i n which most of the wave energy is concentrated (0.06 to 0.9 Hz), while generating turbulent energy at higher frequencies (>0.9 Hz). Additionally, the interaction between the water column above the canopy and a densely populated seagrass bed viii


in a tide-dominated environment was reduced when compared to a less dense canopy in a wave-dominated environment. This may be a result of the shoot biomass or the low blade flapping frequency in the tide-dominated environment when compared to the higher frequencies in the wave-dominated environment. Currents and the chemistry of the porewater acted synergistically on the growth and development of Thalassia testudinum seedlings exposed to stagnant, medium, and high current velocities under laboratory-controlled conditions. Blade widths tended to decrease, root length to increase and fiber bundles to develop thicker walls as the current velocity increased. The most favorable growth conditions (highest biomass, largest blade area) tended to occur at intermediate flow conditions. Stagnant and high velocity flows contributed to lower biomass through increased sulfide levels and reduced nutrient concentrations in the porewater, respectively. In summary, the productivity, growth, and development of seagrasses depend on feedbacks between the vegetative structure and the fluid environment. Abstract Approved: Major Professor: Albert C. Hine, Ph.D. Professor, Department of Marine Science Date Approved: X


What are seagrasses? CHAPTER 1 GENERAL INTRODUCTION 1 Seagrasses are the only angiosperms (monocots) found in the oceans and are believed to have evolved in the Cretaceous (den Hartog, 1970) when sea level was elevated and C02 concentration in the atmosphere was higher than present. These conditions could have facilitated the transition of angiosperms from the terrestrial to the marine environment. During their transition from land to sea, seagrasses had to ( 1) adapt to life in the saline environment, ( 2) be able to grow and reproduce completely submerged, and (3) develop an anchoring system strong enough to withstand drag in the more viscous medium (den Hartog, 1970) Seawater also posed a stronger resistance than air to diffusion of molecules between the water column and the seagrass surface. Seagrasses had to adapt to these conditions in order to successfully colonize coastal waters. The present study addresses some of the hydrodynamic conditions to which the seagrasses Thalassia testudinum Banks ex Koenig and Cymodocea nodosa ( Ucria) Aschers. had to adapt, water flow conditions which are created in seagrass canopies,


2 and their ecological implications. The two seagrass species addressed vary in the habitats they colonize. Thalassia testudinum is a climax species in the tropical and subtropical Atlantic (Zieman, 1982) which inhabits relatively calm environments. In contrast, Cymodocea nodosa is a pioneer species (Van der Velde and den Hartog, 1992) widely distributed in the Mediterranean (den Hartog, 1970) and inhabiting areas where current velocities are high and unfavorable to its competitors (den Hartog, 1971). Effect of water flow on seagrasses: I. Morphology Mechanical stress imposed by currents and waves in the marine environment may have influenced the morphology of the plants (Gerard, 1987), selecting for plants with long, thin and unbranched blades. As the general relationship between form and function in seagrasses is known to be related to the environment in which they occur (Cooper and McRoy, 1988), adaptations to higher current regimes also include larger hypodermal fibers (Cooper and McRoy, 1988; Kuo et al., 1988) and greater root mass (Short et al., 1985). The influence of flow conditions on the anatomy and morphology of the seagrass Thalassia testudinum is evaluated 1n Chapter 4. As currents and the geochemistry of the


3 porewater act synergistically, the porewater constituents were also taken into account in this study. II. Bending of the blades Another mechanism to reduce drag imposed on the canopy is the exposure of a lower area of vegetation to the flow (Vogel, 1989) This is accomplished through bending of the seagrass blades, causing a reduction in velocity within the canopy (Fonseca et al., 1982). As the canopy compresses with increasing flow, a closed structure is formed by bent blades and currents become more stratified with two distinctive flow zones, one within the canopy and one above it (Fonseca and Kenworthy, 1987) Currents are reduced inside the canopy and accelerated over it (Fonseca et al., 1982; Gambi et al., 1990). This had only been observed in flumes but is reported here, for the first time, for field conditions (Chapter 3). The reduction in water velocities within canopies promotes accretion and stabilization of the sediment (Clarke and Kirkman, 1989) within seagrass beds exposed to waves and currents (Fonseca and Cahalan, 1992). This sheltered environment is a valuable nursery and refuge habitat for associated fauna (Peterson, 1982), but its degree of protection may vary due to different shoot densities (Eckman, 1987) and to hydrodynamic microclimates within the canopy (Fonseca and Fisher, 1986). The flow environment within tide


4 and wave-dominated conditions in Thalassia testudinum beds is evaluated in Chapter 3. III. Diffusion boundary layer Immediately next to the surface of any aquatic plant, there is an unstirred layer of water, the diffusion boundary layer, that carbon molecules must diffuse across before they can be incorporated in the photosynthetic processes (Parker, 1981) The thickness of this layer diminishes (1) at the edge of the blade (Larkum et al., 1989), (2) under rapidly flowing water (Hillman et al., 1989) and (3) under high level s of turbulence (Fonseca and Kenwo rthy, 1987). As s ynthetic reactions of the photosynthetic process are limited, in part, by C02 availability (Parker, 1981), diffusion across the boundary layer can be an important limiting step in photosynthesis of aquatic macrophytes (Kirk, 1983). Due to the lack of information on the specific responses of individual species to water motion (Parker, 1981), the effect of currents on seagrass physiology may be severely underestimated (Fonseca and Kenworthy, 1987). The present study contributes with the photosynthetic response of the seagrasses Thalassia testudinum and Cymodocea nodosa under different flow conditions (Chapter 2). Most studies concerning the effect of boundary layers on photosynthetic processes have been based on smooth surfaces


5 which allow the boundary layer to fully develop. Seagrass blades are covered by epiphytic organisms which, at relatively low densities, act as isolated objects protruding through the viscous sublayer. Epiphytes increase turbulence levels in the fluid among the objects (up to a threshold density, Nowell and Jumars, 1984) removing the "diffusion shell" that prevents rapid exchange of molecules (Boston et al., 1989). At densities above threshold, the fluid tends to flow over rather than through the objects and turbulence levels decrease (Nowell and Jumars, 1984). As the rates of transfer and mixing are several orders of magnitude greater in turbulent flow when compared to molecular diffusion (Tennekes and Lumley, 1972) epiphytes may affect seagrasses by reducing the boundary layer thickness. This hypothesis is tested in this study (Chapter 2) Effect of seagrasses on water flow: I. Reduction of current velocity Seagrasses reduce currents inside the canopy (Fonseca et al. 1982). The degree of current reduction is a function of current speed, wave frequency, distance into the canopy, seagrass species (Heller, 1987) and shoot density (Eckman, 1987). The decrease in velocity is associated with a decrease in the sediment carrying capacity (Fonseca et al., 1982);


6 therefore, sedimentation of particles is enhanced within seagrass canopies (Clarke and Kirkman, 1989) Current measurements have not been taken routinely or in a repeatable manner in studies of seagrass beds (Fonseca, 1990). Most previous work on the effects of water flow on seagrass ecosystems has been concerned with unidirectional, flume-generated flow interactions of the seagrass canopy (Fonseca and Cahalan, 1992). The present study contributes with high-resolution speed profiles within Thalassia testudinum beds collected in situ over several hours and meteorological conditions in tide and wave-dominated habitats (Chapter 3). II. Wave attenuation Seagrasses wave back and forth under even very mild surface waves (Vogel, 1989) Energy is lost due to the repeated bending of the blades, therefore, seagrasses have the potential to absorb large amounts of wave energy (Wayne, 1975) Seagrasses loose their effectiveness in reducing wave energy as the water depth increases over the canopy (Fonseca and Cahalan, 1992). As a consequence, particulate matter is resuspended in canopies exposed to wave dominated conditions (Ward et al., 1984) in relatively deep waters (Fonseca and Cahalan, 1992). Unfortunately, the measurement of waves in seagrass


7 systems is a relatively unstudied science (Fonseca 1990) The attenuation of waves propagating over a Thalassia testudinum bed is described in this study in Chapter 3 III. Turbulence A highly turbulent flow pattern results when currents move through seagrass blades (Heller, 1987) due to the formation of a dense mesh when blades bend over with currents and waves (Fonseca et al. 1982) Mesh-like structures produce turbulent eddies at scales comparable to the mesh size (Nowell and Jumars, 1984) The scale of turbulence in the canopy may be important to the ability of seagrasses to effectively utilize available water-column nutrients for photosynthesis (Fonseca and Kenworthy, 1987; Ackerman and Okubo, 1993). A characteristic of turbulent motion is its ability to transport or mix particles and molecules (Tennekes and Lumley, 1972) The rates of transfer and mixing in turbulent flows are several orders of magnitude greater than the rates dominated by molecular diffusion (Tennekes and Lumley, 1972). Based on the findings of Anderson and Charters (1982) that marine plants are able to rescale turbulence to levels that enhance metabolic exchange, Fonseca and Kenworthy (1987) suggested that turbulence scales in seagrass canopies are tuned to the dimensions of the blade mesh. Ackerman and Okubo (1993) observed that turbulence frequency within the canopy of


8 Zostera marina is a function of plant movement rather than of ambient conditions. In the present study, the vertical distribution of turbulent energy is described for canopies in tide and wavedominated environments (Chapter 3). Effect of seagrasses on porewater fluxes Water flow can greatly enhance solute transfer between the sediment and the overlying water (Webster and Taylor, 1992) ; therefore, the mobility of nutrients in the upper layer of the sediment exceeds that of molecular diffusion alone (Rutgers van der Loeff, 1991). The capacity of seagrasses to reduce current velocities and to attenuate waves promotes the formation and maintenance of anoxic conditions (Fenchel, 1977), although a minimum of water movement through the sediment-water interface may be required to flush phytotoxins from the sediment (Lodge et al., 1989) Sediment oxidation can have positive effects on macrophyte growth by enhancing mineralization of organic matter and by oxidizing reduced phytotoxins such as sulfides which can be potentially hazardous to seagrass roots and rhizomes (Smith et al., 1988; Pezeshki et al., 1991). On the other hand, sediment oxidation can reduce porewater nutrient availability, potentially decreasing macrophyte growth (Barko et al., 1991). Morse et al. (1987) did not find any detectable


9 sulfate reduction in the sediments of a seagrass bed and attributed it to the well-irrigated sediments of this wave-dominated area. In contrast, Penhale and Wetzel (1983) observed that the accumulation of sulfides in less irrigated sediments resulted in toxic effects to the roots of Zostera marina. Unfortunately, turnover rates of nutrients in the sediment have been estimated based on assumptions that the porewater is stationary and no replacement by mass flow through the sediment occurs (Moriarty and Boon, 1989) The present study is the first to directly address the feedback between water flow, porewater geochemistry and seagrass productivity (Chapter 4). Seagrasses in a fluid environment: feedback between currents, turbulence, productivity, and porewater chemistry Physical conditions in seagrass beds mediate biological, geological and chemical processes (Fonseca, 1990) and appear to be driven to a point where an equilibrium is reached between the structure of the meadow and localized current flow dynamics ( Fonseca et al., 1983). Plants may only persist in an area based o n a delicate balance between their growth and the dominant physical factors (Kenworthy et al., 1982) Seagrasses may benefit from fast currents due to a decrease in boundary layer thickness, enhancing C02 and


10 nutrient uptake at the leaf surface (Hillman et al., 1989). Another benefit from fast currents may come from enhanced porewater fluxes which decrease the concentration of phytotoxins (Lodge et al., 1989). In contrast, enhanced porewater fluxes may also decrease nutrient availability in the sediments (Barko et al., 1991). Therefore, the capacity of seagrasses to reduce currents and attenuate wave energy seems advantageous when nutrients are concerned but detrimental when boundary layers and phytotoxins in the porewater are concerned. Purpose of the study The lack of information on the effect of water flow on seagrass physiology and ecology is widely recognized (Fonseca and Kenworthy, 1987). Therefore, the following questions regarding the effect of water flow on biological, physical and geochemical parameters in seagrass habitats and their ecological implications are addressed in this study. As seagrasses can benefit from fast moving water due to a reduction in the blade boundary layer thickness (Hillman et al., 1989), what is the minimum flow required for maximum photosynthesis in different seagrass species? Could there be differences in the minimum flow required by species inhabiting environments with fast and slow moving waters? Are seagrasses ever exposed to boundary-layer limiting conditions in situ?


11 what is the effect of an epiphytic cover on the blade boundary layer? These questions are addressed in Chapter 2. It seems contradictory that seagrasses can benefit from fast currents but are also known to reduce current velocity within the canopy (Fonseca et al., 1982), to attenuate wave energy (Wayne, 1975) and to reduce turbulent mixing (Ackerman and Okubo, 1993) What is the extent to which currents are reduced in the canopy? How does that affect the distribution of turbulent eddies? Do these responses change for canopies inhabiting tide or wave-dominated environments? How does the density of the canopy affect the currents? To what extent are waves attenuated? Could seagrass canopies attenuate turbulence at one frequency while generating turbulence at another, biologically more beneficial, frequency? These questions are addressed in Chapter 3. Seagrasses have colonized shallow coastal areas since the Cretaceous (den Hartog, 1970) and their canopies are likely to have always reduced currents and attenuated waves. Therefore, what is the ecological advantage of reduced flow within seagrass canopies? How does flow interact with the porewater geochemistry? Does flow affect the nutrient availability in the sediments? Can currents induce porewater fluxes which remove phytotoxins from the sediments? How does this feed-back to the productivity of seagrasses? What are the optimal hydrodynamic conditions to support seagrass beds? These questions are addressed in Chapter 4.


12 Ultimately, the hydrodynamics of seagrass beds have to be evaluated interactively with biological, physical and geochemical parameters 1n order to address the equilibrium created and maintained by seagrass canopies leading to the optimization of growth and development. This is discussed in Chapter 5


13 CHAPTER 2 HYDRODYNAMICS, D I FFUSION BOUNDARY LAYERS AND PHOTOSYNTHESIS OF THE SEAGRASSES Thalassia testudinum AND Cymodocea nodosa. Introduction The tendency of water molecules surrounding aquatic vegetation to adhere to blade surfaces is responsible for the development of a steep velocity gradient, or boundary layer (BL), adjacent to the leaves of aquatic plants exposed to flowing waters (Whitford and Kim, 1966). The diffusion BL is the zone of the BL which is immediately adjacent to the leaves. Here, turbulent transport of molecules becomes unimportant and fluxes occur only through molecular diffusion (Larkum et al., 1989). Thus, the diffusion BL (also referred to as the unstirred layer and diffusive sublayer) is a barrier for carbon incorporation into photosynthetic processes (Kirk, 1983) The thickness of the diffusion BL and, consequently, the f lux of carbon to the plant surface, can be affected by current velocity (Wheeler, 1980A; Koehl and Alberte, 1988), as well as by surface roughness (Nowell and Jumars, 1984) Increasing current velocity decreases the BL thickness; however, rigid objects (epifauna, epiflora, hairs, debris) protruding through the diffusion BL increase the level of


14 turbulence. Since a turbulent BL provides less of a barrier to the transport of ions (Vogel, 1989), a rough blade surface may enhance hydrodynamic conditions required for the transport of carbon to the host plant. Calculations of productivity under different flow regimes have assumed a variety of values for BL thickness which would limit carbon availability to aquatic plant surfaces (Smith and Walker, 1980; Larkum et al. 1989; Prins and Elzenga, 1989; Raven, 1991). Only Sand-Jensen et al. (1985) directly measured the BL thickness of a seagrass (Zostera marina L.) using 02 profiles near the leaf surface. Boundary layer thicknesses which are constant in time and space (i.e. equilibrium BL thicknesses) have also been assumed. Seagrasses in situ are exposed to the oscillatory movement of waves which may cause the BL to constantly fluctuate in thickness over time and space (Nowell and Jumars, 1984) Fluctuations in the BL thickness a seagrass blade experiences over varying flow regimes in situ have not been adequately measured to determine if flow is a limiting factor to photosynthesis. Only Koehl and Worcester (1991) have quantified in situ flow experienced by blades of a seagrass (Zostera marina), albeit with low temporal resolution. Several hypothesis have been proposed to explain the impact of hydrodynamic conditions on marine plants. Conover (1966) and Fonseca and Kenworthy (1987) suggested an increase in Zostera marina productivity with increasing current


15 velocities due to the decrease in BL thickness and therefore, higher carbon availability to the plant surface. In contrast, Lucas (1983) proposed that increasing current velocities could decrease the BL thickness to the point of washing away extracellular carbonic anhydrase (CA) For seagrasses which utilize this enzyme to catalyze the reaction between HC03 and C02 this would result in a decrease in C02 availability and therefore a decrease in photosynthetic rates and plant growth with increasing flow velocities. The fact that diffusion no longer limits photosynthesis above a certain water-velocity saturation point (Wheeler, 1980A; Koehl and Alberte, 1988) was neglected in earlier studies relating the effect o f currents on photosynthetic processes of seagrasses. Recently, this pattern has been reported for photosynthetic responses of the seagrasses Zostera marina (Koehl and Worcester, 1991) and Thalassia testudinum Banks ex Koenig (Koch and Gust, 1991). The cause of this saturation in photosynthetic rates at a certain current velocity is attributed to a limitation of the enzymatic uptake system of the plants (Wheeler, 1980A). The flux of biologically important molecules through the BL depends not only on the diffusion BL thickness ( L) but also on the concentration gradient of the diffusing molecules as described by Fick's law, (Raven and Richardson, 1986):


16 where Fi is the flux of molecule I I I Di the diffusion coefficient of i, em the concentration of i in the water column, and C8 the concentration at the sink site. The diffusion coefficient in Fick's Law is constant for a given molecule while its concentration may vary in time and space. Total dissolved inorganic carbon (DIC) in seawater occurs at limiting concentrations for seagrass photosynthesis under saturating light levels (Adams et al., 1978 i Beer, 1989 i Durako, 1993i Durako and Hall, 1992). Thick diffusion BLs, under slow current regimes and in environments naturally low in DIC, may limit carbon availability to photosynthetic processes of seagrasses even further. An intracellular C02 concentration of 2 mM m -3 (compensation concentration for ribulose-1,5-biphosphate carboxylase oxygenase -Rubisco) is required in order to maintain maximum photosynthesis (Raven, 1991). Thus, carbon has to diffuse not only through the diffusion BL but also through the cuticle, cell wall, plasmalemma and chloroplast envelope before it can be incorporated into photosynthetic processes. The resistance imposed by the diffusion BL has been estimated to be larger than that of the biological components (Larkum et al., 1989). Shear stress (T) is responsible for momentum flux as well as carbon, nutrient, and particle fluxes to the plant surface, thus making friction velocity (u.) the best "velocity-like" parameter to use in describing near-leaf flow effects (Jumars and Nowell, 1984). What is the effect of u. on photosynthetic


17 rates of the seagrasses Thalassia testudinum and Cymodocea nodosa (Ucria) Aschers. ? Can the photosynthetic rates ofT testudinum be affected by synergistic interactions between light levels and u .. and between DIC concentrations and u .. ? What are the in situ u.. lev e l s experienced by b lades of T testudinum and nodosa o ver time at sites characterized by various hydrodynamic regimes? What is the effect of the epiphytic cover on T testudinum blades on the hydrodynamics of the d iffusion BL? Could it actually contribute to higher carbon f luxes to the seagrass surface? Methods Collection sites Youngest, mature leaves of Thalassia testudinum shortshoots were collected off Lassing Park, in St. Petersburg, FL (2745'N, 8238'W) during the spring and summer of 1990 and 1991. This site is a shallow embayment protected from the direct impact of waves by an offshore sand bar. Behind the bar, patches of T. testudinum grow intermixed with other seagrasses, Syringodium filiforme Kuetz. and Halodule wrightii Aschers.; T testudinum is the climax species at this site. The oldest parts of the leaves of T testudinum were heavily epiphytized by micro-and macroalgae, as well as epifauna. Leaves o f Cymodocea nodosa were collected off Tel


18 Shikmona (32'N, 34'E) in Israel during the summer of 1992. This site is characterized by a rocky substrate with little sediment deposited between crevices. Cymodocea nodosa is a pioneer species (den Hartog, 1970; van der Velde and den Hartog, 1992) which inhabits, among others, high energy areas unfavorable to its competitors (den Hartog, 1970). Blades of this species are directly exposed to waves breaking on the rocks during high tide and are colonized at their oldest part, the tips, by the encrusting red alga, Fosliella farinosa (Lamour.) Howe. (Lipkin, 1977). SEM photographs Seagrass blades were transported to the lab within 30 minutes of collection where they were cleaned by gently wiping the surface. T o determine the efficiency of the cleaning procedure, SEM photographs were taken. Samples of cleaned and non-cleaned Thalassia testudinum leaf tissue were prepared by fixation in 3% gluderaldehyde (0.1M phosphate buffer pH 8.0) for one hour at room temperature followed by 1 % osmium tetroxide fixation for 2 h (Dawes 1989) The samples were then dehydrated in ethanol, critical point dried, and sputtered with gold palladium prior to photographing.


19 'Microcosm The microcosm used to measure photosynthetic responses at different u* values was a 2. 8 1 closed system (Fig. 1 ) Homogeneous bottom T was created by manipulating angular velocities generated by a spinning plate superimposed with pumping rates controlled by a rotameter (Gust, 1989; Huettel and Gust, 1992A). Bottom T relates to u* through u*=(T/p)1 1 2 The temperature of the closed system was maintained constant by circulating the seawater through a glass coil in a cooled water bath (Fig. 1). A 300 ml BOD bottle in the circulating system allowed measurements (02 pH) and the addition o f chemicals (bicarbonate) without opening the system. A Clarktype oxygen electrode attached to a YSI 02 meter (Model 57) was used to measure 02 concentrations. Light levels within the microcosm were measured with a 2w LiCor Quantum Sensor (Model LI 185A) and controlled by rheostats attached to two 1000 W halogen lamps above the microcosm containing Thalassia and 12 fluorescent lamps around the microcosm containing Cymodocea. Specimen preparation The upper 3 to 4 em of 40 to 50 cleaned Thalassia testudinum leaves or the basal parts of 90 to 100 Cymodocea nodosa leaves were placed in a microcosm by attaching one end to the bottom with wax. The leaves were allowed to acclimate


LIGHT / MICROCOSM pH PROBE 02PROBE Ci COOLING SYSTEM 2 0 ROTAMETER Figure 1 Closed system microcosm used to test the effect of friction velocity (u.) on photosynthetic responses of the seagrasses Thalassia testudinum and Cymodocea nodosa. The microcosm is connected to a BOD bottle which allows measurements without opening the system. A glass coil in a cooling unit maintains the temperature constant. A pump and a rotameter control the flow rate. A motor on top of the microcosm control s the spinning of a flat plate in the microcosm. Arrows indicate the direction of water flow.


21 for a minimum of one h prior to the experiments; they were, at most 1 day old at the time the experiments were performed. Experimental setup Synthetic seawater (Instant Ocean) at 28/oo salinity and 26 C was used in all experiments involving Thalassia testudinum. This was the average salinity at the collection site during the summer months and temperature of the water during the early morning collection hours. Natural filtered Mediterranean seawater was used for experiments involving Cymodocea nodosa. Salinity was 39/oo and the temperature was maintained at 25C. In order to manipulate total DIC concentrations, the synthetic seawater used for experiments with I testudinum was buffered with 10 mM bicine, acidified with HCl to a pH below 4 and purged with N2 for 20 to 24 hours to remove all C02 (Durako, 1993). The pH of the C02-free media was adjusted to 8 .21 with carbonate-free NaOH (Vogel, 1961) and used with the addition of NaHC03 (Durako, 1993) Light and u* experiment The effect of u* levels on photosynthetic responses of Thalassia testudinum at different light levels was tested in a factorial experiment using synthetic seawater without any


22 modifications of total DIC (2 2 mM DIC H 7 9) Stag nt p . na water (u.=O.OO em s-1 ) was always the first condition tested, followed by increasing u. levels (0.22, 0.37, 0.57, 0.83, 1.28, 1.62 and 2.20 em s -1 ) within different light levels (41, 73, 181, 380 and 780 J.Lmol photons m-2 s -1 PAR). After a combination of u. and light level was set, the system was equilibrated for at least 10 minutes and the photosynthetic response was determined by measuring initial and final dissolved 02 concentrations every 20 to 30 minutes. Three replicates of each combination o f u. and light levels were tested. A similar protocol was used to test the effect of u. levels on photosynthetic responses of Cymodocea nodosa. Only saturating light conditions were used, and u. levels were run in a rando m order. Four replicates of each u. level were tested. One and two-way ANOVAs (Sokal and Rohlf, 1981) 1n a SAS program (SAS Institute Inc. 1985) were used to compare the photosynthetic rates of Thalassia testudinum and Cymodocea nodosa at different u. and light levels (when available) The data were tested for normality and homogeneity of variances. Student-Newman-Keuls (SNK) multiple range tests (Hicks,1982) were carried out to test for significant differences (P < 0.05)


23 DIC and u. experiment In the experiment testing for the effect of DIC concentration on the photosynthetic rates of Thalassia testudinum at various u. levels, light intensity was kept above saturation (380 p.mol photons m -2 s -1 PAR} Levels of u were run randomly and NaHC03 was added to the carbon-free synthetic seawater such that the total C02 level increased from 0.75 to 2.35 to 6.64 mM within each u. level. After a combination of u. and DIC was set, the system was equilibrated for 30 minutes and photosynthetic responses determined. Boundary layer resistance The boundary layer resistance (o} of HC03 and C02 was calculated based on the transfer velocity (Km, Opdyke et al. 1987} : 2 1 2 Km where D is the molecular diffusion coefficient (for seawater 1989}, Sc the Schmidt Number (Sc=v/D}, v the kinematic viscosity, 1 the average blade width (0. 8 em for Thalassia and 0.2 em for Cymodocea}, and u. the friction velocity


24 experienced by the blade. The BL thickness at which carbon limitation occurs for each carbon species was determined as the point where uccit intercepts the BL resistance curve. measurements on blades In situ u. levels experienced by blades of Thalassia testudinum and Cymodocea nodosa were recorded at a 5 Hz frequency using a microprocessor-controlled, autonomous recorder and hot-film sensors (modified from Gust, 1989; US Patent #4986122). The sensors were attached to the blades at a height of 20 em above the bottom and moved freely with the flapping seagrass blades. In situ data collection for u. on T testudinum leaves was repeated at two sites in Florida, USA: a wave-dominated environment (Summerland Key, 2439'N, 81'W) and a tide-dominated environment (St. Joseph Bay, 2942'N, 85'W). In situ u. levels nodosa leaves were recorded at two sites in Israel: a surf environment during a flood tide (Mikhmoret, 32 'N, 34' E) and an exposed environment during slack water at low tide (Tel Shikmona, 32'N, 3457'E). The voltage output of the sensors placed on the seagrass blades was calibrated in a microcosm for u. level s ranging from 0 to 2. 2 em s -1


25 Boundary roughness Boundary roughness Reynolds numbers (ReR) were calculated for seagrass leaves for the range of u. values and epiphyte sizes observed for Thalassia testudinum according to the equation suggested by Nikuradse ( 1933) for flat granular surfaces: where p is the density of the medium, H the epiphyte height the dynamic viscosity. The flow on the blade surface was characterized as hydrodynamically smooth if Re R < 3 5, as rough-turbulent if ReR > 100 and as transitional if 3.5 < ReR < 100 (Jumars and Nowell, 1984). Results Effect of epiphytes on hydrodynamics of the BL The manual cleaning of Thalassia testudinum leaf segment s utilized in this study proved to be efficient in removing attached debris and epiphytes (Figs. 2A, 2B). The resulting surfaces were smooth with epidermal cells directly exposed to the flow (Fig. 2B) without obstacles altering the BL development (Fig. 2A ) Flow over the cleaned surfaces was defined as hydrodynamically smooth, based on their ReR numbers


26 (letter B in Fig. 3). A continuous diffusive BL is formed on such surfaces even at the highest u* values (Fig. 3). The flow experienced by the 11 clean to the naked eye 11 J:. testudinum surfaces (Fig. 2A) found in young blades was also characterized as hydrodynamically smooth (letter A in Fig. 3) Older blades with heavier epiphytic cover are exposed to hydrodynamically smooth flow at the lowest u* levels but may experience a transitional type of flow under higher u* values (letter C in Fig. 3). Thus, blades with an epiphytic cover thicker than 100 may be exposed to a diffusive BL discontinuous in time and space. When the epiphytic cover is thicker than 1.5 mm, the blade is constantly exposed to this type of fluctuating diffusion BL (Fig. 3). Figure 2. SEM photographs of Thalassia testudinum blades before (A) and after (B) cleaning. Note epidermal cells visible after cleaning and epiphytic load on seagrass blade which looked clean to the naked eye.


a: 103 w CD 102 :::::> z (f) >-0 101 a:_J <{0 oz 100 :::>a: (f) 10 1 w z I <.9 1 0 :::::> 0 a: 1 0 3 ROUGH TURBULENT (no diffu sive s ub laye r ) 27 u =3 5cm s 1 =2 2cm s -1 u = 1.6cm s 1 u :l.Ocm s 1 u =0 6cm s 1 TRANSITIONAL (discon tinuou s diffusive sublayer) HYDRODYNAMICALLY SMOOTH (continuous diffusive sublayer) 1 I Jl 2 3 2 3 20 30 200 300 1 Oj..tm 1 OOJlm 1mm ROUGHNESS HEIGHT (H) u =0.2cm s 1 I I Figure 3. Boundary roughness Reynolds numbers on seagrass blades colonized by different size organisms characterizing the flow on the blade as hydrodynamically smooth, rough-turbulent or transitional at different blade u levels. (A) indicates a 11 clean to the naked eye 11 Thalassia testudinum blade shown in Fig. 2A, (B) indicates the cleaned blades used in this study and shown in Fig. 2B (C) represents an older blade with heavier epiphytic cover. Effect of u and light on photosynthetic rates Blades o f Thalassia testudinum exposed to combinations of u and light levels under controlled laboratory and hydrodynamically smooth conditions showed significantly lower photosynthetic rates under stagnant state when compared to rates at any intensity of water motion (Fig. 4, Table 1). No significant difference was observed in photosynthetic rates of


28 4.0 ........ 3.6 ..c ..._ + + 780 )lmo l photons "0 3.2 0> ..._ (\J 0 2.8 0> E 2.4 380 Jlmol photons (/) (/) 2.0 w I 1 .6 73 )lmol photons fz 1.2 > 181 )lmol photons (/) 0 0.8 f-0 I 0.4 4 1 JlmOI photons 0... 0.0 0.00 0.2 5 0.50 0.75 1 .00 1 .2 5 1 .50 1.75 2.00 2.25 2.50 u*(cm / s) Figure 4 Lowess plot of photosynthetic rates of Thalassia testudinum blades exposed to combinations of friction velocities (u.) and light levels (circles: 41; x: 73; triangles: 181; diamonds: 380 and+: 780 11mol photons m 2 s1 ) at 26C, 28/oo salinity and pH 7.9 in synthetic seawater. Each point represents the mean of three replicas. The box represents the area where limitation due to diffusion BL t hickness occurs. blades exposed to u levels above 0 .25 em s1 Thus, photosynthetic rates were not enhanced by increasing flow regimes above a critical u value ( ucric.) A peak in photosynthetic rates (not significant) which was most pronounced at higher light levels wa s observed in the transitional zone between stagnant conditions and lowest u. values (Fig. 4). Photosynthetic responses at the lowest light


29 level (41 11mol photons m 2 s 1 ) were significantly lower than at 73 and 181 11mol photons m2 s 11 which in turn were significantly lower than at 380 and 780 11mol photons m2 s 1 (Fig. 41 Table 1 ) Table 1. ANOVA testing for significant differences between photosynthetic responses of Thalassia testudinum exposed to different combinations of friction velocities (u.) and light levels. Source Light u. Light u. DF 4 6 24 F value 33.60 6 .04 O.S7 Probability 0.0001 0 .0001 0.9389 The same pattern of response to different u. levels was observed at all light levels tested for Thalassia testudinum (Figs. 4 and SA) and for the saturating light level tested for Cymodocea nodosa (Fig. SB) Photosynthesis was inhibited under stagnant conditions (u.=O O em s1 ) r a peak in photosynthetic rates was observed in the transitional zone between limiting and saturating flow conditions and photosynthetic rates were not enhanced by increasing flow velocities after ucrit was reached (Figs. SA1 SB). For nodosar both the lower photosynthesis under stagnant conditions and the peak in the


.c C\J 0 O'J E .3 Cf) Cf) w I 2 1-z >Cf) 0 t o I a.. Thalassia testudinum l 0 0 0 0.2 0.4 0.6 0.8 1 0 1 .2 1.4 1 .6 1.8 2.0 2.2 5 ..c 4 0'> C\J 0 0'> .3 E Cf) Cf) w 2 I 1-z >-Cf) 0 to I ra.. 0 0 .0 Cymodocea nodosa v I I I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 .6 1 8 2.0 2.2 30 A 8 F igure 5. Lowess plot of photosynthetic rates of Thalassia testudinum and Cymodocea nodosa blades exposed to various friction velocities (u. ) at saturating light levels under controlled conditions. Bars indicate standard error. Boxes indicate conditions under which the diffusion boundary layer limits photosynthesis.


2 .50 2 2 2 5 ....... 2 .00 Ol ....... 0 I .75 Ol E I .50 -(f) (f) I .25 w I f-1.00 z >-(f) 0.75 0 f-0 0.50 -I / 0... 0.25 -0 .0 0 \l -- / v 31 7 6.64mM 2 35m M .... .... -_. 0 .75m M -0.00 0.25 0.50 0 .75 1 .00 1 .25 1 .50 1 .75 2.00 2.25 2.50 uAcm /s) Figure 6. Lowess plot of photosynthetic rates of Thalassia testudinum blades exposed to combinations of total DIC levels (ci rcles: 0 .75i squares: 2.35 and triangles: 6.64 mM) and friction velocities (u.) at saturating light (380 11mol photons m -2 s 1 ) I 26 C1 28/oosalinity and pH 8.21 in buffered seawater. transition zone were non-significant (P=0.220). The main difference in r esponses between both species was the magnitude of u.crit 1 which occurred at u. values 2 5 time s higher f o r n o dosa (u.crit=0.64 em s 1 ) than for I testudinum (u.crit=0.25 em


32 Effect of u. and DIC concentrations on photosynthetic rates. The manipulation of DIC in the medium did not alter the pattern of photosynthetic response of Thalassia to different u. levels; increasing DIC concentrations within u. levels did not resulted in a proportional increase in photosynthetic rates (Fig. 6). A 3-fold increase in low DIC levels resulted in a 2-fold increase in photosynthetic rates, while a further 3 -fold increase in DIC resulted in only a 1.1-fold increase in photosynthetic rates. Boundary layer resistance Equations 2 and 3 predicting the resistance imposed by the BL on the diffusion of C02 and HC03 -to the blade surface show an exponential decline in BL resistance, with greater changes in resistance occurring at the lower u. levels (Fig. 7) At equivalent u. values, BL resistances are relatively higher for Thalassia testudinum (Fig. 7A) than for Cymodocea nodosa (Fig. 7B), due to wider blades in the first species. The u*crit for T testudinum is equivalent to a BL thickness of 280 and 330 p.m for HC03 -and C02 respectively; the equivalent values for Cymodocea nodosa are 98 and 115 p.m, respectively. This suggests a physiological requirement for a shorter diffusional distance for the species inhabiting higher energy environments.


...-... 700 E -_._ ..__. w 630 0 z 560 <{ I(/) 490 (/) w 420 (( (( 350 w >:3 280 >2 1 0 (( <{ 0 z 1 40 :J 70 0 (J) 0 .-. L QO r;----------, E :::t 350 0 300 I (/) (/) 2 50 w (( (( 200 w ><{ _J >(( <{ 0 z :J 0 (J) 1 5 0 33 Tha lassi a testudinum A C ymodocea nodosa B 0 oc 0.25 0.50 0.75 1.00 :.::25 1 5 0 1.75 2.00 2.25 2 5 0 Figure 7. Theoretical values of boundary layer resistance to the diffusion of C02 and HC03 (equations 2 and 3) on leaves of Thalassia testudinum and Cymodocea nodosa at increasing frictio n velocities (u.) Boxes represent conditions under which the diffusion b oundary layer limits photosynthesis as indicated in Fig. 6.


Thala ss i a t es tudinum LOW ENERG Y HIGH ENERGY 4 3 u (cm/s) 2 1 r15 10 u (cm /s) 5 ---Cymodocea nodosa LOW ENERGY HIGH ENERGY 15 10 5 20 40 60 20 40 TIME (s) TIME (s) 34 6 0 Figure 8 Friction velocities (u. ) experienced over time by Thalassia testudinum and C ymodocea nodosa blades at 20 em above the bottom unde r low and high energy conditions. Shaded areas indicate carbon l imitation due to the thickness of the diffusion boundary layer. In situ u. levels Values of u experienced on Tha lassia testudinum and Cymodocea nodosa blades in s itu, 20 em above the bottom, change between sites and oscillate with time (Fig. 8 ) Data


35 compiled for low and high energy environments inhabited by each seagrass showed u. levels were significantly higher for nodosa (5.04+-0.55 cm-1 ) than for I. testudinum (1.10+-0.12 em s -1 P=0.00001). The u. values recorded for nodosa are higher than those covered by the calibration curve. Thus, although the fit of the calibration curve is good (R2=99.7%), these data may not be as accurate as those reported for I testudinum. Although the average in situ u. values experienced by the two seagrasses in this study showed a 5 fold difference, they fluctuated 10% about the mean in both species. As mean in situ u. values at 20 em from the bottom are above ucrit' fluctuations in blade u. rarely expose Thalassia testudinum and Cymodocea nodosa to conditions where the diffusion BL limits photosynthesis (Fig. 8). Such limitation only occurs under extremely calm conditions and only for fractions of a second. As the BL thickness oscillates, conditions fluctuate between non-limiting and limiting at a high frequency. Discussion The thickness of the diffusion BL adjacent to seagrass blades fluctuates in time and space and only affects photosynthetic of Thalassia testudinum and Cymodocea nodosa at relatively low u. values. When these are below ucrit photosynthetic rates of hydrodynamically smooth blades may become carbon-limited due to physical factors (i.e. thickness


36 of the diffusion sublayer) Above u*crit ( 0. 25 and 0. 64 em s -1 for .T. testudinum and nodosa, respectively), the ratelimiting process becomes biochemical (i.e., the enzymatic system) Here, an increase in carbon concentrations at the surface of the plant caused by a reduction in the thickness of the diffusion BL does not translate into a further increase in carbon uptake rate by the plant. This is attributed to saturation of the photosynthetic enzymes (Koehl and Alberte, 1988). Beer et al. (1980) suggested that the inefficient cellular HC03 -uptake system is the limiting factor for seagrass photosynthesis under natural inorganic carbon concentrations. This may also be the cause of the plateau in photosynthetic rates above u*crit. Inefficiency in the cellular carbon uptake system becomes evident by the non-proportional increase in photosynthetic rates with increasing inorganic carbon concentration in the medium. According to Fick' s law, higher inorganic carbon concentrations are available at the plant surface when this parameter increases in the water column. H owever, photosynthetic rates of Thalassia testudinum blades did not increase proportionally with DIC concentrations in the medium. This suggests that conditions were close to saturation at the higher DIC concentrations tested (approximately 3 times the concentration of seawater) and confirms the carbon-limiting conditions found by Adams et al. (1978), Beer (1989), Durako and Hall (1992), and Durako (1993) for seagrasses in the


37 marine system. Similar experiments on the effect of flow on photosynthesis of the alga Ul va lactuca L. showed that hydrodynamic effects were overridden by low light levels (Parker, 1981). This was expressed as a decrease in photosyntheti c rates of lactuca at high current velocities under the lowest light level tested (40 J.Lmol photons m-2 s -1 ) This was not observed for Thalassia testudinum, for which hydrodynamic effects were always more significant than light levels and carbon concentrations. The time of collection was responsible for the non-significant difference between the two highest and two intermediate light levels tested. Although the responses to light were seasonal, the pattern of hydrodynamic effects on photosynthesis remained constant over the year. The origin and significance of the peak in photosynthetic rates observed immediately above u*cri t for Thalassia testudinum and Cymodocea nodosa are not understood. It could be either a physical or physiological mechanism. Perhaps, under boundary limiting conditions (below u*crit), an alternative pathway for the utilization of internal carbon reserves may be active. Onc e no further boundary limitation occurs (above u*crit) another pathway which utilizes carbon from the external source may become active. At the peak, both pathways may be active simultaneously. More research is needed to investigate this hypothesis. The significantl y higher u*c rit for Cymodocea nodosa from


38 a surf zone when compared to the u*crit for Thalassia testudinum from a relatively calmer habitat may reflect an acclimation to environmental conditions rather than a true physiological need for higher carbon availability at the surface of Q. nodosa. Seagrasses exposed to strong currents in their environments may develop thicker cell walls and/or cuticles in order to protect their tissues from mechanical damage (Cooper and McRoy, 1988). Thus, the additional thickness from reinforced cell walls and/or cuticles may account for an extra diffusional distance that has to be covered by carbon molecules. To test this, plants of the same species would have to be grown under controlled laboratory conditions in which the flow environment is the only variable. Photosynthetic and morphological measurements could then determine if u*crit levels are species specific or a mechanical adaptation to environmental conditions. Values for BL thickness determined experimentally in this study are in general agreement with the range estimated for various seagrass species (Larkum et al. 1989) and refer to the total diffusional distances, including the cellular components, as well as the diffusion BL. This overestimates the true diffusion BL thickness. Theoretical BL resistances calculated according to equations 2 and 3 are larger for T testudinum than for Q. nodosa, due to the wider blade of the first species. This allows for more area for the development of a BL and less influence of the edge, where the BL is


39 and turbulence higher. Also, the reported BL resistances are for smooth, epiphyte-free blade surfaces which rarely occur in situ. Epiphyte-free plants used in the laboratory experiments were exposed to smooth flow at all u. levels tested. a hydrodynamically This type of flow develops a continuous diffusive sublayer where the flux of ions is limited by the diffusion BL thickness. The presence of roughness elements found on leaves in situ may generate a transitional type of flow when epiphytic covers are moderate and u. values are high. Under these conditions, the diffusion BL becomes discontinuous in space and time. Small-scale hydrodynamic conditions to which seagrass blades are exposed in situ are more complex than the steady flow assumed thus far. Levels of u. on blades of Thalassia testudinum and Cymodocea nodosa fluctuate constantly, exposing the plant to various BL thicknesses in short periods of time. If the BL thickness is limiting to the diffusion of carbon to the plant surface at one moment, it will not be so a few milliseconds later. This might bring light to the controversy that inexplicably thin theoretical diffusive BLs on blade surfaces of marine plants are required for saturating photosynthesis. Based on calculations, Larkum (1989) and Raven (1991) concluded that inorganic carbon can not be supplied to the plant surface at the rate required for saturating photosynthesis by diffusion through the BL alone. Raven (1991) suggested that only the "catalysis of the extracellular


40 conversion of HC03 -to C02 could maintain the C02 concentration at the plasmalemma surface very close to the value found in the bulk, air-equilibrated medium." Although the presence of carbonic anhydrase (CA) in the C3 seagrass T. testudinum was suggested to contribute to the minimization of carbon limitation imposed by the diffusion BL (Durako, 1993) by converting HC03 -to C02 extracellularly, Beer et al. ( 1980) found this enzyme to have a minimal effect in this process. In contrast, the C4 seagrass nodosa has no CA activity and probably fixes HC03 -directly through phosphoenolpyruvate carboxylase (PEPcase, Beer et al. 1980). Thus, both of these seagrasses have no or little extracellular conversion of HC03 -to C02 but are able to photosynthesize at saturating levels at relatively thick diffusion BLs. Furthermore, if CA was present extracellularly on Thalassia blades, a decrease in BL thickness with increasing u. values should wash away this enzyme and reduce photosynthetic rates (Lucas, 1983). This was not observed. Theoretical diffusion BL requirements are based on smooth, epiphyte-free surfaces under steady, unidirectional velocities, conditions which rarely occur in situ. Seagrass blades in nature are covered with epiphytes and attached debris. Sand-Jensen et al. (1985) observed that the removal of an epiphyte cover 0.2 to 2 mm thick from Zostera marina blades caused a 49% reduction in the diffusion BL. Thus, the diffusion BL values suggested for submerged macrophytes are


41 underestimated when epiphytic cover is thick. In contrast, protuberances, like hairs, and epiphyte/debris covers less dense than those described by Sand-Jensen et al. (1985) may enhance carbon transport to the plant surface by increasing the blade roughness. The epiphytic cover of Thalassia testudinum blades 11clean to the naked eye11 actually have some roughness elements. These small roughnesses, however, will not alter the development of the diffusion BL which will be continuous in time and space. But, moderate epiphytic covers at high u. levels may create a transitional flow on the blade surface. In this type of flow, the diffusion BL thickness varies in time and space due to vortices and areas of accelerated flow through the epiphyte/ debris mesh. During short periods of time, the epiphytized blade surface could be bathed in carbon of the average BL concentration (Jumars and Nowell, 1984). The diffusion BL of seagrass blades should thus be seen as a dynamic barrier to the transport of dissolved substances. This barrier constantly changes its thickness in time and space not only with the intensity and direction of water flow but also with other hydrodynamic conditions such as wave action, the distance from the edge, and the roughness of the blade surface (indirectly, blade age) Therefore, the blade surface may be receiving pulses of carbon and nutrients at the concentration of the bulk medium. The detrimental effects of an epiphytic cover on light attenuation, increased transfer resistance and competition for


42 carbon and nutrients may overcome the benefits from a rough turbulent or transitional flow at a certain point. Furthermore, a threshold thickness may be reached where water will tend to flow over the epiphytic cover rather than through it. This will decrease the velocity and turbulence adjacent to the leaf surface and generate a thicker diffusion BL as described by Sand-Jensen et al. ( 1985) for Zostera marina. Experimental studies under controlled conditions are needed to test this theoretical concept.


43 CHAPTER 3 TURBULENCE DISTRIBUTION IN TIDE AND Wave -dominated CANOPIES OF THE SEAGRASS Thalassia testudinum. Introduction The microclimate within canopies of aquatic plants is important to biological processes and is regulated by the exchange of momentum, heat and mass between the surrounding water and the plants (Burke, 1982). The rate of mass exchange determines the concentration and residence time of carbon and nutrients i n the canopy and strongly depends on hydrodynamic characteristics, particularly the level o f turbulence (Anderson and Charters, 1982) Seagrass canopies could benefit from increased turbulence level s through the faster removal of undesired substances as well as the intensified transport of carbon and nutrients through the water column and the blade boundary layer (Gerard and Mann, 1979; Wheeler, 1980B; Fonseca and Kenworthy, 1987; Koehl and Alberte, 1987; Hillman et al., 1989) F o r example, small scale eddies formed near rough surfaces of epiphytized plants (Fig. 10) are responsible for t h e intermittent f lushing of stagnant water that accumulates between epiphytic elements (Dade, 1993; see Chapter 2). These epiphytes may contribute to the generation of small scale turbulent eddies which are injected into the flow, adding to


44 the level of high frequency turbulence (Fig. 10, Denny and Shibata, 1989) and minimizing boundary layer limitations (see Chapter 2). The ability of seagrasses to attenuate waves (Ward et al., 1984; Fonseca and Fisher, 1986; Fonseca and Cahalan, 1992) and slow currents (Fonseca et al., 1982; 1983; Fonseca and Fisher, 1986; Clarke, 1987; Heller, 1987; Gambi et al., 1990) seems to counter benefits seagrasses would experience under increased velocity and turbulence levels. What could be the advantages of reduced turbulent energy within seagrass beds? The dissipation of turbulent energy through wave attenuation by seagrass canopies result s in a lower concentration of suspended particulate matter (Ward et al., 1984), which translates into higher light availability for the benthic vegetation (Orth and Moore, 1983; Cambridge and McComb, 1984; Giesen et al. 1990; Zimmerman and Alberte, 1991) Dissipation of turbulent energy by canopies may also diminish porewater fluxes (Dade, 1993), thereby decreasing nutrient availability in the water column (less epiphytic growth) and favoring reducing conditions in the sediment underlying seagrass canopies (see Chapter 3) Since ammonium is the form of nitrogen most readily assimilated by seagrasses (Short, 1987), anoxic conditions induced by decreased turbulence levels in the canopy could be advantageous to growth (see Chapter 4). Although there are advantages to slowing current velocity


45 and attenuating waves in seagrass canopies, the question regarding the apparent reduced flux of molecules of carbon and nutrients to and from the seagrass blades induced by slower flow conditions still remains. Could attenuation of turbulent energy in seagrass canopies be selective at certain microhabitats and turbulence frequencies? Ackerman and Okubo (1993) who pioneered the measurements of turbulence in seagrass beds in situ concluded that Zostera marina can generate mechanical turbulence at blade waving frequencies. Other work addressing turbulence in seagrass canopies was done in flumes (Heller, 1987; Gambi et al., 1990) and suggests that a highly turbulent flow results from the presence of seagrass blades. In this study, the distribution of turbulent energy at different frequencies in canopies of the seagrass Thalassia testudinum was investigated in situ in an attempt to address several ecologically important questions: (1) What are the levels of turbulent energy within seagrass canopies under different hydrodynamic and climatological conditions? (2) Do seagrasses attenuate turbulent eddies or do they generate turbulence as suggested for some terrestrial crops (Seginer et al. 1976; Finnigan, 1979; Raupach et al. 1991) ? ( 3 ) Is turbulent energy equally attenuated with depth in canopies exposed to wave versus tide-dominated environments? (4) Is the attenuation of turbulent energy dependent on the density of the seagrasses or the water depth? ( 5 ) What is the


46 contribution of epiphytes to the attenuation or generation of turbulent energy? Turbulence Turbulence consists of random fluctuations at different frequencies and amplitudes level) over time at a in a parameter (velocity, given point (Motzfeld, water 1938; Schlichting, 1968) and is generated under conditions which also develop eddies (Denny and Shibata, 1989) Large eddies are unstable and transfer their kinetic energy to a cascade of successively smaller eddies, leading eventually to dissipation of the kinetic energy as heat (Raupach, 1989). The pattern in which turbulent energy is distributed over different frequencies (frequency spectrum) and the rate at which it dissipates are important for ecological processes (Burke, 1982; McKenzie and Leggett, 1993) Low frequency (large) turbulent eddies can move substantial water parcels over relatively large distances; (Raupauch, 1989; Kundu, 1990), high frequency (small) eddies are responsible for the homogenization of larger eddies (Fig. 9). Because eddies of all sizes coexist (Raupach, 1989; Dade, 1993), low frequency turbulent eddies could move nutrients vertically from the sediment surface, through the canopy, into the water column and could move inorganic carbon in the opposite direction (Fig. 9A) Currents, in turn, account for the horizontal transport of nutrients and particles (Ward et al., 1984) High


47 LARGE TURBU LENT EDDIES .. A 8 c Figure 9 The effect of turbulent eddies on mixing of particles/molecules in a hypothetical seagrass bed. Figure 10. (A) large (low frequency) eddies transport larger particles over wider distances than (B) small (high frequency) eddies while turbulent eddies of mixed sizes (C) transport a variety of particles and are capable of mixing bodies of water. ",;)t;;) TURBULENT q EDDIES EPIPHYTES A B Generation of turbulence by epiphytes on a seagrass blade (cross view) (A) Water flowing over a "clean" blade produces less turbulent eddies than when flowing over an epiphytized (B) blade due to t h e increased blade roughness cause d by epiphytic g rowth.


48 and low frequency eddies superimposed in the flow could minimize the concentration gradient through eddy diffusion (Fig. 9C) Measuring current speeds at varying heights above the bottom (speed profile) is the basis for determining the effects of seagrasses on currents (Fonseca et al., 1982). The most appropriate method for measuring current speed in seagrass systems is based on the principle of hot-film anemometry (Fonseca, 1990). This method provides rapid and accurate data collection necessary for the evaluation of turbulence in the flow. The distribution of turbulent eddies is determined by the frequency spectrum (Townsend, 1980) through a Fourier cosine transformation of the autocovariance function of fluctuations in speed and water depth over time. Frequency spectra give the variance of a process (speed, water level) as a function of its frequency (Denman, 1975) The resolvable range of spectra depends on the sampling frequency and the number of data points in the time-series ( N ) The lowest resolvable frequency (the Fundamental frequency) is defined by and the highest resolvable frequency (the Nyquist frequency) by Speed frequency spectra analyze the distribution of turbulent kinetic energy (TKE) as a function of frequencies at which speed varies. Wave frequency spectra analyze the distribution of turbulent potential energy (TPE) based on measurements of pressure (water height) variation as a


49 function of wave frequency. Levels of TKE and TPE tend to decrease vertically as the bottom is approached and horizontally as waves move onshore, respectively. Both forms of turbulent energy contribute to mixing processes in seagrass canopies. Methods Time-series of current speeds at different heights within and above seagrass canopies were recorded simultaneously in situ at a 5 Hz frequency using a microprocessor-controlled autonomous data-logger and hot-film sensors (Gust, 1988; US Patent #4986122). The sensors were positioned horizontally in arrays and placed within the seagrass beds (Fig. 11) The position of the sensors in the vertical array ranged from heights immediately above the sediment surface ( 5 em) to heights at the top (20 em) and above (40 em) the canopy (Fig. 11). The voltage output of the sensors was calibrated in front of a nozzle at known velocities and temperatures. Velocities ranging from 0. 8 to 36.9 em s -1 were generated in a temperature controlled water bath by pumping water at a known flow rate (cm3 min-1 ) through a nozzle of known area (2 1 6 cm2 ) Flow velocities were calculated by dividing the flow rate by the nozzle area and calibration curves (speed versus voltage) were obtained from the voltage output at the different combinations of temperature and velocity.


50 u PROBE S Figure 11. Deployment of submersible data logger for evaluation of vertical speed profiles within seagrass canopies. Speed (u) probes in vertical arrays were positioned at various heights within and above the canopy. Water depths (waves and tides) were recorded concurrently to speeds at a 5Hz frequency by a 15 PSI pressure transducer (Transmetrics) located in the data-logger positioned on the sediment surface. Depths were calculated from a factory provided equation relating voltage output of the pressure transducer and depths using software packages (Statgraphics and Tattletools) Tidal fluctuations were confirmed through tide tables. In situ data collection was repeated within Thalassia testudinum beds at three sites in Florida, USA (Fig. 12): a wave-dominated (Summerland Key, 24 39'N, 8126'W), a tide-d d ( h B 2942'N, 8522'W) om1nate St. Josep ay, and a mixed environment (Mullet Key, 2737'N, 82 44'W). The architecture


51 (3-dimensional structure) of each seagrass bed was characterized through measurements of shoot density (n=3 25 versus 25 em random quadrats) and leaf length of all blades (n=30) of 6 to 7 shoots along the side of the quadrat which was nearest to the diver. Vertical arrays of probes were positioned within I testudinum beds at a minimum distance of 5 m from the canopy edge. At Mullet Key, arrays of probes were positioned at three sites: bare sand area 5 m from the canopy edge, 5 m into the canopy where the seagrasses were only slightly epiphytized ("clean"), and SO m shoreward where seagrasses were heavily epiphytized. Speed and depth were recorded simultaneously over time at all three sites. Vertical water speed profiles were calculated based on speeds recorded for several hours (3 to 15 hrs, see Appendix 2) at the various depths and averaged over 28 min intervals. Fluctuations in speed and in water depth (waves) were analyzed by means of smoothed frequency spectral estimates obtained by Fourier transforming 8400 points, equivalent to 28 minutes of data, using a modified Matlab program. Frequency spectra were plotted only for resolvable frequencies ranging from the fundamental to the Nyquist frequencies. Data sets here presented include 28 min of data but are representative of the general responses observed over the entire recording period (up to 15 hrs) wave period (T=l/f) was calculated based on the wave frequencies (f) in the wave spectra. Wave height (H) and wave


S T JCS EPH BAY GULF OF MEXICO 52 I o .,P F igure 12. Sites at which speed and depth were recorded within and above canopies of the seagrass Thalassia testudinum. St. Joseph Bay is c haracterized as a tide-dominated environment, Mullet Key a mixed environment and Summerland Key a wave-dominated envir onme nt. attenuation by the vegetation we r e estimated from the wave spectra based on the principles that wave energy is proportional to the square of wave height, and that wave attenuation involves dissipation of energy. Wave energy (TPE)


53 attenuation by a seagrass canopy was calculated by comparing simultaneously recorded frequencies in 3 habitats: (1) bare sand area before waves reach the seagrasses; (2) 5 m into the canopy where seagrass blades are "clean" and (3) 55 m into the canopy where seagrass blades are heavily epiphytized. Results St. Joseph Bay Canopy architecture and behavior Leaves of the relatively dense (1147 33 shoots m-2 ) Thalassia testudinum bed in a tide-dominated habitat, St. Joseph Bay, showed no flapping movement during the recording time in this tide-dominated environment. The 28 8 em long blades remained in an upright position with slight oscillations only during a wind event. The water level extended 30 to 40 em above the canopy during the recording period. Speed profiles Speed profiles recorded during an ebb tide indicate that the weak currents.at this site were slowed within the canopy and that current speeds did not change throughout the canopy (Fig 13A) Windy conditions in the beginning of the recording


54 period increased speeds in the water column above the canopy but did not affect the average speed within the canopy (Fig 13A) Wave spectra The turbulent potential energy (TPE) at all frequencies in the St. Joseph Bay site (Fig. 13B) was in the lowest range (10-4 to lo-s cm2 Hz -1 ) observed for sites in this study (compare Figs. 13B, 14B and lSB). While TPE at other sites was concentrated in bands which reached levels up to 3 x 10-3 cm2 Hz-1 (Fig. 14B), St. Joseph Bay only showed an energy concentration in a relatively low frequency band between 10-2 and 5 x 10-2Hz (T=lOO to 20 s) during a wind event (Fig. 13B). No defined peaks in a specific wave band frequency were observed during calm conditions (Fig. 13B). Speed spectra The low TPE levels observed for St. Joseph Bay's wave spectra were reflected in the relatively lower turbulent kinetic energy (TKE) levels in the canopy at this site (Figs. 13C, 13D) when compared to other sites (Figs. 14C, lSC, lSD). Speed spectra within and above the seagrass bed in St. Joseph Bay during calm (Fig. 13C) and windy (Fig. 13D) conditions indicate that TKE is lower within than above the canopy and


0 0 Th a la ssia testudinum (St. Jos e ph Bay) co 60 0 E 40 u.. I-20 I C) w I 0 0 10A ---N I -N E () ..__.. >I(f) z w 0 a: w s 0 0.... 1Q-5 1 o-3 2 4 0 2 4 0 2 SPEED (em s -1) WAVE SPECTRUM +-WINDY ----i CALM 10-2 1 o-1 1 o o FREQUENCY (Hz} 4 0 ----CA NOPY HEIGHT 2 4 10 1 5 5 Figure 1 3 Speed profiles (A} wave spectra (B) and speed spectra (C and D note different vertical scale) 5 m into a Thalassia testudinum c anopy in a tide dominated environment (St. J oseph 3ay) during calm and windy c onditions. Each line r epresents 28 minutes of data recorded at a 5 P.z frequen cy.


56 VELOCITY SPECTRA ---10 1 N I CALM --C\.1 --C/) -E (.) 20 em >l 10 (/) z w ..... __ -... ....... ------....... 0 ...... ........ Scm ... ... a: ... ... ... w ... ... 0 Q_ 10 1 10 3 10-2 10 1 10 10 1 FREQUENCY {Hz) VELOCITY SPECTRA ---10 2 N I WINDY --C\.1 --C/) -E (.) 10 1 --20 em >-_ _ J _ _ ....._ (/) z ....... w 100 0 /.,/. a: ____ f ______ . \_r -----w -@ Scm .................. 0 Q_ 1Q 1 1 o 3 1 o 2 10 1 10 101 FREQUENCY (Hz) F i gure 13. (Conti n u e d )


57 further decreases towards the substratum. Turbulent energy in the water column penetrates deeper into the canopy when it is exposed to windy rather than calm conditions (Figs. 13C, 13D). Independent of the speed in the water column which was a result of different meteorological conditions, TKE levels at all turbulence frequencies did not change at 5 em from the bottom. At 20 em from the bottom, TKE levels were always higher than at 5 em. During calm and windy conditions, most of the TKE in the water column was found at the lowest frequencies (Figs. 13C, 13D) The transfer of TKE from low to high frequencies within the canopy (represented by the slope of the spectra) was not as strong as in the water column above it (Figs. 13C, 13D). Summerland Key Canopy architecture and behavior Leaves of Thalassia testudinum in this wave-dominated bed were 32 3 em long and flapped back and forth covering a 120 angle in the wave-dominated environment. This population was less dense (539 40 shoots m -2 ) having half the number of short-shoots found in St. Joseph Bay. The water level extended 30 to 40 em above the canopy during the recording period.


58 Speed profiles Speed profiles recorded during a flooding tide indicate that velocities tended to decrease towards the bottom (Fig. 14A) but the slowing of currents by the seagrass canopy was not as evident as for St. Joseph Bay (Fig. 14A) Speeds progressively decreased from 40 em (water column) through the canopy, to 5 em from the bottom. Little change in the shape of the speed profiles was observed during the 2 h this timeseries was recorded (Fig. 14A). Wave spectrum Most of the TPE at this site was concentrated in the frequency band between 0.06 and 0.8 Hz (T =16.7 and 1.3 s; Fig. 14B) characteristic for wind generated gravity waves Brown et al., 1989). A peak in TPE was observed at the 0.25 Hz (T= 4 s) frequency. Speed spectra Speed spectra for the seagrass canopy adjacent to Summerland Key indicate that levels of TKE in the water column and at the top of the vegetation are similar (Fig. 14C) while TKE is lower in the bottom of the canopy. Levels of TKE within the canopy were always higher at 20 em than at 5 em from the


59 bottom (Fig. l4C). The slopes of the TKE of the 5 and 20 em spectra in Fig. l4C suggest that lower frequency turbulent eddies were broken into smaller (high frequency) eddies at similar rates in the water column and within the canopy. A -peak in TKE was observed in the frequency band between 0 2 and 0.3 Hz (T=S and 3.3 s, Fig. l4C), which was also the band in which most of the TPE was concentrated (Fig. l4B). Mullet Key Canopy architecture and behavior Density within this population exposed to mixed hydrodynamic conditions did not change between the area with "clean" (454 72 shoots m-2 ) and heavily-epiphytized (464 96 shoots m-2 ) plants, but epiphytized blades were shorter (22. l 8 6 em) than 11clean" ones (26.6 9.8 em) Flapping movement during the recording time was gentle. Back and forth oscillations of "clean" blades covered a wider angle (approximately 90) than heavily-epiphytized blades which were collapsed onto each other and only moved up and down at high tide covering a narrower angle (approximately 30) During the recording period, the water level extended 20 to 30 em above the "clean" canopy and lO to 20 em above the heavilyepiphytized area.


0 0 co 60 w >0 8 4 0 co t: (f) z w 0 a: UJ 3: 0 n.. 10 2 10 3 1 0 1 0 5 10 10 7 10 3 1 01 WAVE SPECTRUM 10 10 1 10 101 FREQUENCY (Hz) VELOCITY SPECTRA Scm--1 0 10 1 100 101 FREQUENCY (Hz) 60 Figure 14. Speed profiles ( A ) wave spectrum (B) and speed spectra (C) 5 m into a Thalassia testudinum canopy in a wave-dominated environment (Summerland Key) Each line represents 28 minutes of data recorded at a 5 Hz frequency.


61 Speed profiles Speed profiles recorded during a high (Fig 15A top) and an ebb (Fig. 15A bottom) tide indicate that speeds may be influenced by water height (Fig 7A). During high water (Fig. 15A top) speeds in the water column above the epiphytized seagrasses were relatively high. Speed was slowly attenuated vertically through the epiphytized canopy (profile in an angle). At the same tidal stage, speeds above the 11clean11 seagrasses were not as high as for the epiphytized plants and flow within the canopy was faster through the lower than the upper part of the canopy. During ebb (Fig. 1 5A bottom) velocities at different heights were similar within the epiphytized habitat while the 11Clean11 habitat showed faster flow through the bottom of the canopy and reduced speeds at the upper portion of the vegetation. The shape of the speed profile in the bare sand area changed during the tidal cycle. While highest speeds in the water column were found during high tide, highest speeds near the bottom were observed during ebb. Wave spectra During high tide, most of the TPE was concentrated in the frequency band between 0 06 and 0.9 Hz (T=16.6 and 1.1 s), equivalent to wind generated gravity waves (Fig. 15B). In the


62 bare sand area adjacent to the seagrass canopy, two peaks in TPE were observed: at 0. 2 and 0 3 Hz (T=5 s and T=3. 3 s respectively) The lower frequency peak ( 0. 2 Hz) was not observed in the "clean" seagrass canopy while the higher frequency peak (0.3 Hz) shifted to a 0.35 Hz frequency (T=2.9 s) as waves moved from the bare sand over the "clean" seagrasses. Between the bare sand and the seagrass canopy TPE at the 0 .35 Hz frequency was attenuated approximately 7 .7% m-1 (Fig. 15B). This same wave frequency was further attenuated 1. 6% m -1 as waves propagated from the "clean" to the heavilyepiphytized area. TPE in the epiphytized canopy tended to be more equally distributed over all frequencies (Fig. 15B). At relatively high frequencies (1 to 2 5 Hz, T=1 to 0.4 s, smaller eddies), TPE was highest in the epiphytized canopy, intermediate in the "clean" canopy and lowest in the bare sand area (Fig. 15B) Speed spectra The speed spectra from a bare sand area and the adjacent Thalassia testudinum bed measured at 20 em from the bottom (upper canopy) during a high tide indicate that TKE was higher in the bare sand area than in the seagrass canopy (Fig. 15C). TKE in "clean" seagrasses was always lower than in the epiphytized ones (Fig. 7C) This suggests that levels of TKE are reduced at all frequencies as currents move from the bare


2 0 60 II-40 0 0) 20 W....._ 6 5 0) .._.. <( 20 I-I <.9 0 EPIPHYTIZED CLEAN BARE S AND c 0 1 0 20 0 1 0 20 0 10 20 w I SPEED (em s-1) >1-(f) 104 z w 0 a: w s 10-5 WAVE SPECTRA "CLEAN" SEAGRASSES I \ BARE----">'' SAND / ______ ,,-" I J / -,l EPIPHYTIZED SEAGRASSES 0 0... 1(}3 l02 1(}-1 100 101 FREQUENCY (Hz) 63 Figure 15. Speed profiles (A) during a high (A, top) and an ebb (A, bottom) tide, wave spectra (B) and speed spectra (C and D, note different vertical scale) for a Thalassia testudi num canopy in a mixed environment (Mullet Key) Data were simultaneously recorded i n the bare sand area 5 m from the edge of the seagrass canopy; 5 m into the lightly epiphytized canopy ("clean") adjacent to the bare sand area, and 50 m shoreward, in a heavilyepiphytized part of the same seagrass bed (epiphytized) Each line represents 28 minutes of data recorded at a 5 Hz frequency.


---N I --(\J ---(/) --E (.) >r(f) z w 0 a: w s 0 0... VELOCITY SPECTRA (HIGH TIDE 20c m) EPIPHYTIZED 101 1 o-3 1 o 2 10 1 1 oo 1 o 1 FREQUENCY (Hz) VELOCITY SPECTRA 'N (HIGH TIDE 5 em) I ---(\J ---(/) --SAND 5 102 ------------EPIPHYTIZED ..... \ '' .. I \.\',1 L ., I ';. Figure 15. (Continued) 64


65 sand area into the 11clean11 canopy. Strongest reduction in TKE in the 11clean11 seagrasses occurred between 0. 27 to 0. 7 Hz (T=3.7 to 1.4 s) which was also the frequency band at which most of the TPE (wave) energy was concentrated (Fig. 15B) In contrast, higher levels of TKE seem to be generated as currents continue to move another so m over a more epiphytized part of the seagrass bed (Fig. 15C). This is most noticeable at high frequencies. At 5 em from the bottom, also at high tide, levels of TKE at all frequencies decreased when moving from the bare sand area to the adjacent 11clean11 canopy (Fig. 15D). In contrast levels of TKE increased when moving from the 11Clean11 to the heavily-epiphytized part of the seagrass bed. TKE energy at frequencies above 0 15 Hz was higher in the epiphytized canopy than in the adjacent bare sand area (15D) Discussion Because turbulence is an extremely variable process (Lazier and Mann, 1989), profiles obtained within minutes from each other can differ significantly. Therefore, individual turbulence profiles may not be truly indicative of the dominant mechanism responsible for turbulence in an environment (Gibson, 1990). Data obtained in this study represent general trends observed, but should be interpreted with limitations in time and space. The majority of previous observations regarding flow


66 through seagrass canopies come from theoretical models (Wayne, 1975), from unidirectional flow experiments in flumes (e.g. Fonseca et al., 1982; Fonseca and Cahalan, 1992) or from waves superimposed on unidirectional flow also in a flume (e.g Heller, 1987). The results here reported are the first based exclusively on high resolution data collected in situ and confirm previous findings that seagrass canopies attenuate wave energy and slow currents. Additional observed trends include: (1) stronger vertical reduction of turbulent energy within a dense canopy in a tide-dominated environment when compared to a less dense canopy in a wave-dominated environment; ( 2) the contribution of epiphytes (increased complexity in seagrass architecture) to increased high frequency turbulent energy levels within the canopy; and (3) the generation of high frequency turbulent energy (small eddies) by waves propagating over a Thalassia testudinum bed. The shape of speed profiles depend on the hydrodynamic forces prevailing in the environment (waves versus tides), the seagrass architecture, and the tidal stage (water depth and speed). In the tide-dominated environment (St. Joseph Bay), the speed profile maintained constant speeds over depth within the canopy even during windy conditions. As a result, a sharp speed gradient developed above the canopy, indicating little interaction between the water column and the seagrass bed. This was also observed in the speed spectra, and may have been a result of the dense canopy and the lack of blade flapping.


67 Visual observations indicate that blades in the tide-dominated environment tend to slightly bend over forming a sealed structure (Fig. 16) over which the currents flow. Little interaction occurs between the water column and the seagrasses enclosed in this area. This parallels the findings of Fonseca et al. (1982), Ward et al. (1984)and Gambi et al. (1990), who suggested that low water levels and unidirectional currents form such structures. The lack of oscillatory motion of waves maintains the seagrass blades bent over for extended periods of time (hrs, Fig. 16). As a consequence, suspended particles can be deposited (Ward et al., 1984) and anoxic conditions in the upper sediment can be maintained (see Chapter 4). In the present study, it is not clear if this is a consequence of seagrass density or the tide-dominated nature of the environment. In contrast, profiles in the wave-dominated environment (Summerland Key) showed a less steep gradient between speeds above and within the canopy (i.e. more interaction between the water column and the seagrass bed) This was also observed ia the speed spectra (Fig. 14C) and may be due to the less dense canopy or to the flapping of the blades. When blades flap back and forth, the canopy opens and closes (Fig. 17), increasing the interaction between the seagrasses and the water column during periods in which the canopy is open. A consequence would be that more particles could be maintained in suspension and the upper sediments could be more oxidized than under


ST. JOSEPH BAY TIDE-DOMINATED HABITAT TIDE 68 Figure 16. Diagram ilustrating the probable distribution of turbulent eddies in a dense Thalassia testudinum bed exposed to tide-dominated conditions. Based on speed measurements as well as visual observation of the behavior of seagrass blades it can be suggested that blades lay on a side for extended periods of time (hrs) forming a sealed structure wh ich possibly allows for reduced interaction between the water above and within the canopy. tide-dominated conditions (see Chapter 4) Tidal stages influenced the speed profiles in the mixed-energy environment (Mullet Key). At high tide, the heavily-epiphytized canopy showed a profile similar to St. Joseph Bay, although the density of the canopy at thi s site was only half that of St. Joseph Bay. As flow above the canopy reached velocities comparable to St. Joseph Bay (ebb), flow within the canopy also did not change with depth. This indicates that heavy epiphytic cover on the upper portion of Thalassia testudinum blades may contribute to the formation of a sealed structure. Therefore, the architecture of the population may be more important than seagrass density in determining the shape of the speed profile. As suggested by Fonseca et al.


SUMMERLAND KEY WAVEDOMINATED HABITAT TIME 2 TIME3 6 9 F igure 17.Diagram ilustrating possibl e distribution of turbulent eddies in a Thalassia testudinum bed exposed to wave-dominated conditions. Based on speed and wave attenuation measurements as we l l as visual observation of the behavior of seagrass blades it can be suggested that blades flap back and forth constantly closing and opening the canopy allowing for strong interactions between the water above and within the canopy.


70 (1982) the capacity to reduce currents in "clean" seagrasses was inversely proportional to water depth. At high tide, speeds through the lower part of the 11 clean 11 canopy were faster than in the upper part, perhaps due to less obstruction to the flow (sheaths holding the blades together) This was also observed for a Zostera marina canopy (Ackerman and Okubo, 1993). Speed acceleration over the bottom was enhanced during an ebb tide and may have important biological (larvae, spore and seedling settlement) geological (erosion and deposition) and geochemical (porewater flux) implications (see Nowell and Jumars, 1984). The TPE in canopies exposed to wave-dominated and mixed environments was concentrated in the 0.06 to 0.9 Hz frequency (16 to 1.1 s period) band representing wind generated gravity waves. These frequencies are lower than those used by Fonseca and Cahalan (1992) in a flume experiment (2 5 to 1.4 Hz). In this study, the upper portion of Thalassia testudinum canopies exposed to wave action showed selectively stronger reduction of turbulent energy in the dominant wave frequency band when compared to other non-dominant frequencies. This may be due to the loss of momentum due to the flapping of blades at the wave frequencies of wave orbital motion. To maintain particles in suspension, energy (large eddies) is required. Less relatively higher energy (smaller eddies) is required to maintain molecules mixed in a turbulent flow. seagrass productivity benefits from well mixed nutrients


71 and carbon in the medium but is negatively affected under high concentrations of suspended matter due to l ower light levels available for the benthic vegetation. Therefore, the capacity of seagrass beds to attenuate high energy (large) eddies could contribute to the settling of particles in suspension thereby decreasing turbidity while generating low energy (small) eddies which could enhance mixing of nutrients and carbon in the water. A similar mechanism of rescaling of turbulent energy with possible benefits for carbon and nutrient acquisition was observed by Anderson and Charters (1982) for the alga Gelidium nudifrons Gardn ..


CHAPTER 4 CURRENTS, POREWATER GEOCHEMISTRY AND THE MORPHOLOGY AND PRODUCTIVIT Y OF Thalassia testudinum SEEDLINGS. Introduction 72 Effects of seagrass beds on water flow and porewater chemistry. Seagrass beds reduce current velocity (Fonseca et al., 1982, 1983; Fonseca and Fisher, 1986; Garnbi et al., 1990) and attenuate waves (Fonseca and Cahalan, 1992) causing a decrease in physical stress on the sediment-water interface (expressed as bottom friction velocity, u*) As the rate of porewater exchange between permeable sediments and the overlying water decreases proportionally with u* (Rutgers van der Loeff, 1981) and with the attenuation of surface waves (Webb and Theodor, 1972), reduction in current velocity and wave attenuation in seagrass beds inhabiting permeable sediments may minimize porewater flux through the sediments. Seagrasses also enhance the deposition of organic matter (Fenchel, 1977; Fonseca et al., 1983) and decrease the thickness of the oxidized sediment surface layer within the canop y (Rutgers van der Loeff 1981). These processes may


73 promote the formation and preservation of anoxic conditions in the sediments (Fenchel, 1977) and maintain increased porewater nitrogen (Kenworthy et al., 1982; Morse et al., 1987) and phosphorus (Fourqurean et al., 1992) concentrations. Reduction of porewater flux may also have significant negative effects on productivity and/or nutrient uptake of submersed plants due to the accumulation of sulfide (Barko et al., 1991; Pezeshki et al., 1991; Rey et al., 1992). The rootrhizome system of seagrasses generally occurs in highly anaerobic sediments where no free oxygen is available and the accumulation of end products of microbial anaerobic respiration (hydrogen sulfide) is high (Penhale and Wetzel, 1983; Smith et al., 1988). Mechanisms to tolerate such conditions include oxygen release from the roots (Smith et al., 1988; Moriarty and Boon, 1989; Pezeshki et al. 1991) and alternate metabolic pathways (Penhale and Wetzel, 1983). Although seagrasses are well adapted to anaerobic sediment conditions (Penhale and Wetzel, 1983), increased porewater sulfide levels were linked to the decreased root respiration rates in Zostera marina (Pezeshki et al., 1991) and to the die-back of Thalassia testudinum in Florida Bay (Carlson, 1991; Yarbro et a l 1991). Therefore, the negative effects of high sulfide concentrations could be mitigated by increased current velocities and wave action (Lodge et al., 1989; Barko et al., 1991) or by the flux of oxygen from the roots and rhizomes into the anoxic sediments (Moriarty and Boon, 1989)


74 The main biological importances of porewater movement include the above described mechanism of phytotoxin (sulfide) release from the porewater and the transport of nutrients generated in the sediments into the water column (Riedl et al., 1972). The consequences of increased water column nutrient concentrations to Thalassia testudinum include an enhancement in epiphytic cover, and a decrease in biomass and shoot density (Tomasko and Lapointe, 1991). Morphology of seagrasses exposed to different hydrodynamic environments. Morphological characteristics of aquatic plants are related to environmental conditions in which they occur (Cooper and McRoy, 1988) The withstand hydrodynamic forces in function of their morphology ability of seagrasses to fast moving waters is a and size (Koehl, 1986; Carrington, 1990). Drag, the primary force exerted on blades exposed to currents, which tends to push the plant in the direction of the flow, depends on the area of the blade, and the density and velocity of the medium (Koehl, 1986; Vogel, 1989) The best evidence for acclimation to hydrodynamic conditions may come from internal structures which provide mechanical support to the plant. This is observed in blades of Phyllospadix, which inhabits wave-exposed areas. Blades of this genus have reduced lacunae, increased cell wall


75 thickness, greater hypodermal fibers (Cooper and McRoy, 1988), and enhanced fiber wall thickness (Kuo et al., 1988) when compared to seagrasses inhabiting more slowly moving waters. Zostera marina L. exposed to relatively high current velocities has a tendency to develop lower blade area (Fenchel, 1977) and greater root mass (Short et al. 1985). Such morphological features reduce drag and increase anchoring capacity, respectively. Synergism between currents and porewater chemistry. The morphology of Zostera marina has been strongly linked to availability of nutrient resources (Short, 1987). In Thalassia testudinum Banks ex Koenig, reduced blade widths have been associated with low salinity, reduced plant vigor (Tomlinson, 1969), low light conditions (McMillan and Phillips, 1978) geographical distribution (biogeographical extremes, McMillan, 1978), sex o f the short-shoot (female, Durako and Moffler, 1985), younger age (Tomlinson, 1969), ecotypic and/or clonal variation and environmental stress (see Durako and Moffler, 1985). Johnstone (1979) suggested that leaf width in Enhalus acoroides (L. f.) Royale is not a function of environmental factors, but of plant development. Thus, the morphology of seagrasses may not only be an acclimation that minimizes drag, but also a response to other environmental parameters.


76 If seagrass morphology and growth are strongly linked to nutrient availability (Short, 1987), and waves and currents enhance solute transfer between the sediment and the overlying water column (Webster and Taylor, 1992), then hydrodynamic conditions and porewater geochemistry within seagrass beds may act synergistically on seagrass morphology. Fonseca and Kenworthy (1987) found that blades of Zostera marina exposed to different current velocities in situ tended to be longer when exposed to higher current regimes. Short ( 1987) observed that blade length of this species increased with porewater phosphorus concentrations. As phosphorus levels in the porewater are expected to decrease with increasing current velocities on the sediment surface, the findings of Fonseca and Kenworth (1987) and Short (1987) seem contradictory. The interaction between the capacity of seagrasses to reduce curent velocity and porewater geochemistry may explain, at least in part, this contradiction. Kenworthy et al. (1982) found the highest nutrient concentrations in the center of seagrass beds, which is also the area in which currents have their maximum reduction in velocity (Fonseca et al., 1982). This suggests that the growth and development of seagrasses may be affected synergistically by currents and porewater geochemistry. The hypothesis that (1) currents affect the morphological development of seedlings of Thalassia testudinum and that (2) hydrodynamic conditions and porewater geochemistry act


77 synergistically on seagrass morphology were investigated under controlled conditions in benthic chambers (microcosms) Methods Seedlings of Thalassia testudinum were collected from drift material at Long Key, FL (24'N, 8047'W) on August 31, 1991 and transported to the laboratory in plastic bags containing paper towels saturated with seawater. The seedlings were placed in shellfish grow-out racks floating in outdoor tanks fitted with a closed seawater c irculation system (adapted from Short, 1985) The natural photon flux level reaching the tanks was reduced by 30% through the use of neutral density screens. Temperatures fluctuated between 28 and 32C and salinity between 30 and 32/oo. Seedlings were cultured in the floating racks for 2 months and were allowed to acclimate to experimental conditions for a week prior to the exposure to hydrodynamic treatments in microcosms. Fine quartz sand was sieved with a 1 mm mesh and washed with synthetic seawater (Instant Ocean, 32/oo salinity) to remove debris and organic matter. The washed sediment was placed in three benthic chambers (microcosms as shown in Fig. 20; Gust, 1989) to a depth of 15 em The sediment surface was smoothed using a rotating blade to minimize advective porewater flow associated with surface roughness (Huettel and Gust, 1992A, 1992B) Eight Thalassia testudinum seedlings were planted into each microcosm at equal distances (4 em ) from the


78 side walls. The microcosms were filled with 6 1 of synthetic seawater (Instant Ocean, 32/oo salinity) each, using a thin plastic film on the top of the sediment to avoid disturbances. The same methodology was used in monthly seawater exchanges (50% o f the total volume) The experimental setup consisted of circular steady state flow induced at different levels in each microcosm through the rotation of a horizontal disk 20 em in diameter (Gust, 1989; Huettel and Gust, 1992A). Currents were quantified as bottom friction velocity (u.= (TIp) 112, where T = bottom shear stress and p = fluid density) according to tables relating u. and sediment level in the microcosm. These tables were obtained by subjecting pre-calibrated u. probes placed on the sediment surface in a microcosm to different spinning velocities. The experimental design consisted of exposing one microcosm to stagnant conditions (u.=O.O em s -1 ) a second to flow regimes frequently encountered in local sheltered, tide dominated Thalassia testudinum beds (u.=0.3 em s -1 ) and a third to flow regimes prevailing in more exposed local seagrass beds (u*=1.0 e m s -1 ; Koch, in preparation) Seedlings were grown for 6 months in the 3 microcosms at room temperature (26 2C) Epiphytic cover was removed biweekly from the blades through gentle wiping and from the sediment surface through suction. Eight Gro and Sho fluorescent lamps (40W) on a 12:12 hr light/dark photoperiod provided a PAR photon flux density of approximately 180


79 photons m -2s"1 After two months in the microcosms, blade width and length measurements were taken bimonthly for an additional 4 months. Senescent and detached blades were collected during the period in which growth occurred in all three microcosms and dried at 60C to quantify biomass produced per seedling. At the end of the experiment, seedlings were carefully removed from the microcosms and the length of each root, as well as the number of roots per seedling, recorded. SEM photographs were used to analyze fiber bundles in Thalassia testudinum blades after 6 months of exposure to the different flow conditions. The tissue was prepared by fixation in 3% glutaraldehyde (0.1 M phosphate buffer pH 8 0) for one hour at room temperature followed by 1% osmium tetroxide fixation for 2 h (Dawes, 1989) The samples were then dehydrated in ethanol, critical point dried, and sputter coated with gold palladium prior to SEM photographing. Measurements of wall thickness of haphazardly selected fibers (Fig. 18B) were measured with a caliper for 5 leaves of different plants within each flow treatment until a total of 110 fibers per treatment was reached. After 5 months, porewater sippers (5 em long "bubblewand" sections) were permanently installed horizontally at 4 depths in the sediment (1, 3, 6 and 10 em), 2 replicates at each depth, at equal distances from the side walls (4 em) The system was allowed to equilibrate for 1 month. Porewater and water column samples were taken and sulfide, ammonium and


80 filterable reactive phosphorus (FRP) were quantified according to Carlson et al. (1983). Cutting the drained sediment in a radial direction after completion of the experiment revealed the depth and profile of the anoxic laye r defined as the layer of dark reduced sediment. Statistical analysis of the data was not possible due to the lack of true replicates. limited number of microcosms, Due to the availability of a only one microcosm with 8 seedlings each was used per hydrodynamic condition tested. Therefore, measurements regarding seedling morphology and porewater concentrations were subreplicates. For the information of the reader, an ANOVA of the data assuming seedlings and porewater measurements to be true replicates is presented in the appendix. Results External morphology. After an incubation time of six months in microcosms with controlled currents, 100% of the Thalassia testudinurn seedlings died under stagnant conditions (bottom u.=O.O ern s -1), 38% under medium flow (bottom u.=0.3 ern s -1 ) and 50% under high flow (bottom u.=1.0 ern s -1 Fig. 19). During the duration of the experiment, seedlings exposed to stagnant conditions tended to have shorter blades and roots (Table 2) as well as


81 A Figure 18. Cross sections of a Thalassia testudinum blade exposed to medium flow (characterized as u.=0. 3 em s-1 on the sediment surface) for 6 months. (A) Arrow indicates a fiber bundle. (B) Lines indicate examples of measurements of fiber wall thickness.


82 reduced total blade area, biomass and root number (Table 2), compared to seedlings grown under moving water. Seedlings exposed to u .. =O. 3 ern s -1 tended to have the longest blades, largest blade area, highest number of roots and greatest biomass (Table 2) Seedlings exposed to u .. =1. 0 ern s-1 tended to have intermediate responses in terms of blade length, blade area, number of roots and biomass (Table 2) but roots tended to be longer than in any other treatment. Blade widths of seedlings decreased with increasing bottom u.. (Table 2) Visual observation of epiphytic growth showed highest cover during the initial months of the experiment and a tendency for increased growth with increasing u .. levels. After two months, a constant epiphytic level seemed to be reached in all microcosms. Table 2. Morphological characteristics of Thalassia Blade (ern) Blade (ern) Blade ( crn2 ) Blade testudinurn seedlings exposed to different flow regimes (characterized as friction velocity on the sediment surface, bottom u .. ) after 6 months under laboratory controlled conditions. Stagnant u .. =O. 3 u .. =1 0 ern s-l ern s l length 5.66.37 7.15.32 6 .85.39 width 0.482.015 0.476.013 0 .460.017 area 2 .82.26 3.61.25 3.310.30 biomass 0.30.22 0.58.43 0.50.41 (rng dw seedling-1 day-1 ) Root number 2.88.35 5.13.40 4 .25.70 (roots seedling-1 ) Root length 3.90.65 5.65 0 .48 5 .98.70 (ern)


SEDIMENT no flow shootsI \ u* =0.3 cm/s u* 1 0 em/ \i--shoots 1 1"1"111"" 1 1 5c m SEDIMENT 83 Figure 19. Morphology of Thalassia testudinum seedlings after 6 months of exposure to different flow regimes (characterized as friction velocity on the sediment surface, u ) in microcosms under laboratory controlled conditions.


84 Internal anatomy. The internal anatomy of Thalassia testudinum blades (Fig. 18) proved to be more responsive to flow than the external morphology. Blades exposed to u.=1. 0 em s-1 for 6 months had thicker fiber bundle walls (1.266 0.033 than blades exposed to u.=0.3 em s-1 (1.091 0 .037 The death of all seedlings exposed to stagnant conditions made it impossible to collect data on fiber wall thickness for this experimental condition. Anoxic layer. microcosm under The depth of the dark anoxic layer in the stagnant conditions was parallel to the sediment surface at a depth of 0 6 em and extended downwards along the transparent plexiglass walls (Fig. 20). At u .=0. 3 em s-1 the profile of the anoxic layer presented some similarity to the porewater washout pattern described for this benthic chamber by Huettel and Gust (1992A); it was deepest at the side walls and reached the surface in the center (Fig. 20). While the washout profile found by Huettel and Gust (1992A, 9 days at u.=O.S em s -1 ) was concavely bent towards the center area, in the present study (6 months at u.=0. 3 em s -1), the anoxic layer followed a convex pattern (Fig. 20, 1.6 em deep at the location of the seedlings). At u.=1. 0 em s -1 there was no clear anoxic layer, although an elliptical area immediately beneath the central surface was s lightly darker (Fig. 20), indicating reducing conditions.


u = 0.0 cm/s WATER LEVEL u.= 0 .3 cm/ s OXIDIZE LAYER [r; D 85 c I I I 1 ............. S.Ocm OXIDIZED SEDIMENT u.= 1 0 cm/ s Figure 20. Cross section through the microcosms in which circular flow was generated for 6 months to test the effect of flow on the morphology of Thalassia testudinum seedlings and the geochemistry of the porewater. Dashed lines indicate the depth and shape of the anoxic profile after 6 months at steady bottom friction velocities (u.) Shaded areas represent reducing conditions. Porewater chemistry. Concentrations of ammonium, filterable reactive phosphorus (FRP) and sulfide in the porewaters of the three microcosms decreased with increasing u. (Fig. 21). Nitrogen, FRP and sulfide concentrations in the porewater tended to be higher under stagnant conditions (u.=O. 0 em s 1 ) than under either flow regime.

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86 9 8 7 NITR OG E N 6 5 ::1. ..__.. '
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87 D i s c ussion The use of microcosms in the evaluation of interactions between currents, porewater geochemistry and seagrass morphology has the advantage of allowing the control of certain environmental conditions while changing others, but caution has to be used in extrapolating data from microcosms to field conditions. In enclosed benthic chambers such as microcosms, the porewater flux tends to be controlled by pressure differences between the outer rim and the central area (Huettel and Gust, 1992A). Thus, microcosms are valuable tools to make relative observations, but the magnitude of porewater flux and the profile of the anoxic layer are not compatible with field observations. For example: light transmitted through the transparent acrylic material of the microcosm walls support photosynthetic microorganisms, thus causing the strong vertical dip in the profile of the anoxic layer on the outer edge of the microcosms (Huettel, pers. comm.). This is not found under natural conditions. Diffusive processes and the biological activity of microorganisms may have been responsible for the depth and shape of the anoxic layer profile under stagnant conditions, while advection may have been the dominant factor under moving water. The observation of Rutgers van der Loeff (1981) that porewater flux increases with u. is supported by the measurements in this study of: ( 1) increased depth of the anoxic layer, and (2 ) decreased ammonium, FRP and, sulfide

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88 concentrations in the porewater with increasing flow. Seedlings of Thalassia testudinurn may have contributed to advective processes through the generation of differential pressures in front of and behind the p lant, causing localized porewater flow (Huettel and Gust, 1992B). Epiphytic growth in the enclosed microcosms can be viewed as a biological indicator of flux of nutrients from the sediments into the water column. The bloom observed under high flow (u*=l. 0 ern s -1 ) during the initial period of the experiment indicates that 2 months was the approximate period needed to flush the nutrients from the sediment column 15 ern deep. Under medium flow (u*=O. 3 ern s-1 ) the epiphytic bloom subsided at an earlier time, depth of the anoxic layer indicating that an equilibrium had been reached and that a relatively lower porewater volume had been exchanged with the water column. Under natural conditions, currents may dilute and/or transport advected nutrients, hence the blooms encountered during the initial period of this experiment may not occur in situ. By removing the epiphytic cover from the seedlings in the microcosms, its effect on growth and development was minimized. Morphological changes due to currents. As reserves seedlings of Thalassia testudinurn have nutrient to support three months of growth (Durako and

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89 Mofflerr 1981) r and seedlings were ten months old at the end of the experiment r surrounding conditions were likely to affect growth and development, However/ morphological changes in seedlings of this species were not clearly related to one physical (u*) or geochemical (porewater chemistry) parameter. Perhaps the synergistic effect between currents and geochemistry obscured seedling responses to isolated factors. Replicate microcosms would be needed to clarify synergistic effects using multivariate analysis. In order to evaluate the isolated effect of currents on seagrasses, a more conservative structure directly linked to the mechanical strength of the plant may be more adequate than morphological features (blade width and length) r which are also affected by other environmental parameters. Fiber bundles (Kuo et al.1 1988) and epidermal cell-walls (Cooper and McRoy, 1988) are suggested to provide mechanical support to seagrass blades/ making them the most likely structures to be directly affected by hydrodynamic conditions. Here, only blade width and fiber wall thickness of Thalassia testudinum seedlings were correlated to different hydrodynamic conditions. Blade width was also influenced by porewater geochemistry thus, the best evidence for acclimation to hydrodynamic conditions may come from internal structures directly linked to mechanical support to the plant.

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90 Synergistic effect of current and porewater chemistry on growth and development. Biomass and morphological characteristics of Thalassia testudinum seedlings tended to have maximum responses at intermediate u. levels (except for blade width and root length) The lowest responses during the growth period and the death of all seedlings under stagnant conditions may be attributed to a combination of higher sulfide levels in the porewater1 creating potentially lethal conditions for I. testudinum (Carlson et al.1 1991) 1 and to a thick diffusion boundary layer on the blade surface/ limiting carbon availability to photosynthetic processes (Koch1 in preparation) Root l ength increased with u levels 1 indicating that anoxic sediments may be detrimental to root development in I. testudinum seedlings. At the other extreme1 under high flow conditions1 porewater sulfide concentrat ions may not be detrimental to the seagrasses1 but ammonium and FRP in the sediment are at reduced levels. In this case/ productivity may be limited by nutrient availability and by the form in which these nutrients are found. The nitrogen form most readily assimilated by seagrasses/ ammonium (Short/ 1987) I is the dominant form of nitrogen under anoxic condition s thus I oxyge n transported into the sediment through increased u. l evels will enhance nitrification and thereby increase nitrate concentrations 1 a form of nitrogen less readily assimilated by

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91 seagrasses. Maximal responses in Ttestudinum biomass, blade length, blade area and root number were found under intermediate flow conditions where sulfide is reduced and nutrients are at higher concentrations than in relatively stronger currents. This intermediate response may be of importance in the establishment of seagrass beds and healing of boat scars. When seedlings colonize or seagrasses are transplanted into a baresand area, porewater nutrient (Fourqurean et al., 1992) and sulfide concentrations are low. The geochemistry of the sediment may be altered as current velocity is reduced and organic matter accumulates, creating an ideal environment in terms of nutrient availability (concentration and form) However, the sulfide concentration in the sediment will also increase. Therefore, an intermediate level of porewater flux has to be maintained in order to support maximum seagrass growth. The opposite may occur in boat-scars: porewater is flushed from a deeper level, oxidizing nutrients and decreasing nutrient concentrations. Field data is needed to verify this hypothesis.

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CHAPTER 5 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS Seagrasses successfully habitats by adapting to the invaded coastal marine environment 92 submerged where the diffusion of molecules is slower and the resistance of the medium is stronger than in air. Therefore, water flow became an important parameter in seagrass ecology influencing biological, physical and geochemical interactions with seagrass populations. Does the boundary layer (BL) limit photosynthesis in seagrasses? The availability of carbon to seagrass blades depends on the current velocity they are exposed to. Fast moving currents reduce the BL thickness making more carbon available t o the blade surface. Photosynthetic rates of the seagrasses Thalassia testudinum and Cymodocea nodosa exposed to different current velocities (expressed as b lade friction velocities, u.) under laboratory-controlled conditions were limited by the blade boundary layer thickness only at relatively low friction velocities. Photosynthetic rates reached a plateau after

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93 critical u. values were reached, indicating a biochemical limitation of the enzymatic system. The saturation of the photosynthetic enzymes occurred at lower friction velocities (u.:::: 0. 25 em s-1 ) for J:. testudinum taken from relatively calmer waters, compared to Q. nodosa (u.::::0.64 em s-1 ) from a surf zone. This may be an acclimation to the physical conditions prevailing in the different habitats or a speciesspecific response. The measurement of u. levels experienced by Thalassia testudinum and Cymodocea nodosa blades in situ indicate that the boundary layer on seagrass blades is constantly changing in thickness in accordance with local hydrodynamic conditions. The boundary layer may impose short periods of carbon limitation due to increased diffusional distances but, due to constant fluctuations in the thickness of this layer, the carbon limiting conditions are transient. Hydrodynamic implications of epiphytic covers: Epiphytes on seagrass blades can be viewed as roughness elements which contribute to reduced blade BL thickness. Therefore, epiphytes directly affect the hydrodynamic conditions and the carbon availability on seagrass blades. All previous studies on the effects of boundary layers (BL's) on aquatic plants were based on smooth blade surfaces. When analyzing a seagrass blade in situ, epiphytic organisms

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94 in a variety of shapes and sizes can be found. The roughness of seagrass blades with different levels of epiphytic covers was calculated based on boundary roughness Reynolds number. The results indicate that the roughness introduced by organisms lightly colonizing seagrass blades contributes to the breakdown of the diffusion BL by promoting the formation of microscale eddies which fluctuate in time and space. Thus, the blade surface may be bathed in carbon and nutrient concentrations of the bulk medium over short periods of time, simulating a pulse feeding process (short periods of high carbon and nutrient availability intermittent with periods of low or no carbon and nutrient availability) Epiphytic growth may reach a leve l at which the water tends to flow over, rather than through, the epiphytes. At that point, the presence of epiphytes will increase the boundary layer thickness. One has to keep in mind that epiphytes, even when at a low density, also compete for light and nutrients on the seagrass surface. Turbulence generation and attenuation in seagrass beds: Seagrasses and capacity to change their epiphytic communities the hydrodynamic conditions have the in their habitats. The extent to which this occurs was determined for a tide-and a wave-dominated environment. Speed and wave time-series were recorded at a 5 Hz

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95 frequency at different depths within canopies of the seagrass Thalassia testudinum. Data recorded in a wave-dominated, a tide-dominated and a mixed environment confirmed that J:. testudinum attenuates wave energy and reduces current velocities. Turbulent potential energy (TPE), originated from passing waves, showed strongest attenuating at frequencies at which most of the wave energy was concentrated (0.06 to 0.9 Hz) Simultaneously TPE was generated at higher frequencies (>0.9 Hz) The above rescaling of TPE may allow particles in suspension to settle, decreasing turbidity while maintaining an adequate level of mixing of carbon and nutrients in the water surrounding the seagrass blades. High-density canopies exposed to tide-dominated conditions slowed currents more than did canopies with lower densities exposed to wave-dominated conditions. The blades in unidirectional flow (tides) bent over forming a closed structure which reduced the interaction between the water above and within the canopy. Under oscillatory flows (waves), this interaction was increased due to constant closing and opening of the structure formed by seagrass blades. This process may have a strong influence on the sedimentary processes occurring within seagrass canopies in tide and wavedominated environments. Environments with heavily epiphytized Thalassia testudinum blades had elevated levels of high frequency turbulence when compared to environments with 11Clean11

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96 seagrasses. Therefore, the architecture of the canopy may be important in determining hydrodynamic microclimates. Because seagrass beds are important nursery and refuge habitats, the effect of hydrodynamic microclimates in the canopy may be important for the settlement of larvae. Optimal hydrodynamic conditions for seagrass growth: By modifying the hydrodynamic conditions in their habitats, seagrasses also affect the geochemistry of the sediments they colonize by changing the rate of porewater flux. This in turn, affects seagrass growth and development. This feedback loop was investigated under laboratory controlled conditions. Seedlings of the seagrass Thalassia testudinum grown under different flow regimes in microcosms tended to develop thicker fiber walls and narrower blades with increasing currents. These are mechanisms which compensate for higher drag. Currents generated in microcosms suggested to have a strong effect on the geochemistry of the porewater. Under stagnant conditions, the exchange between the water column and the sediment could only occur through molecular diffusion. Under these conditions, sulfides was enhanced. enhance growth, high the concentration of nutrients and While high nutrient concentrations sulfide levels can be toxic to

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97 seagrasses. Under intermediate flow conditions (as found in seagrass beds during a calm day), the most favorable growth conditions were generated: nutrient concentrations were still enhanced but sulfides were reduced. Under high flow conditions, lower biomass was a result of reduced nutrient concentrations in the porewater. Feedback between seagrasses and the fluid environment: The hydrodynamics of seagrass beds affect biological, physical and geochemical parameters in seagrass habitats. These parameters are directly or indirectly linked forming several feedback loops within a seagrass system. Seagrasses reduce the flow through canopies, therefore establishing sheltered conditions in which elevated nutrient concentrations in the porewater can be generated and maintained. However, these sheltered conditions could impose restrictions on the diffusion through the blade boundary layer and the mixing of carbon and nutrients in the water column. A light epiphytic cover on seagrass blades should contribute to a reduced boundary layer by creating small vortices on the plant surface. Although the bulk currents in the seagrass canopy are reduced, the microclimate o n the seagrass blade may counteract the generally reduced flow. Epiphytes also have a detrimental effect on seagrass blades by competing for light, carbon, and nutrients. A relativel y low

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98 epiphytic cover (with hydrodynamic benefits but no significant competition) can be maintained through decreased porewater fluxes (decreased current velocity) This is a closed feedback system where carbon availability depends on BL thickness which is directly linked to currents and epiphytes but epiphytes depend on the porewater flux of nutrients and on currents. Another part of this feedback system is observed when turbulent energy is attenuated at low frequencies which contain more energy and are able to maintain particles in suspension. This creates the conditions necessary for increased sedimentation rates in seagrass beds, resulting in decreased turbidity in the water column and more light availability for seagrass growth. However, sufficient nutrient and carbon mixing in the water column is necessary to maintain seagrass growth. This is accomplished by the rescaling of turbulent energy from low to high frequencies. In summary, hydrodynamic conditions in a seagrass bed interact with all levels of the system, creating and maintaining an equilibrium which can be altered by changing any of the levels involved. Adaptations of seagrasses to their fluid environment: Seagrasses successfully invaded the oceans 66 million year s ago and have colonized it ever since (den Hartog, 1970). To do this, seagrasses must. have adapted morphologically to

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99 life in moving waters. Several hypothesis have been suggested regarding the fact that only monocots and not dicots were able to make the transition from land to sea. For example, seagrasses have a basal meristem in the leaves, a major advantage in an environment where leaves can be consumed by herbivores. Additionally, the morphology of monocots which needed to adapt to a medium {water) which imposed higher drag than air may be an important factor to be considered. In general, monocots have a more streamline anatomy than dicots therefore making them more likely {lower drag) to successfully colonize the marine environment. Within those seagrasses which effectively invaded coastal areas, those with long and narrow blades are dominant. The advantage of such a morphology is not only in reduced drag but also in reduced boundary layer thickness due to a large blade edge effect. In a thin and long blade, a boundary layer may not fully develop. This is of importance when considering the slower carbon diffusion rates in the marine environment when compared to the those in air. The flexibility of seagrass blades allows them to flap back and forth in moving waters. This may have also contributed to the transition of seagrass ancestors into the oceans. Flexible blades experience less drag than rigid ones and also allow light to reach both sides of the blade as they move back and forth in this environment where light is attenuated by the water column.

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100 By reducing current velocities through blade interaction with the water, organic matter would accumulate in seagrass communities. With their subterranean rhizomaceous growth, seagrasses could take advantage of the nutrients available in the sediment unlike their algal counterparts. In summary, seagrasses developed a close equilibrium with their fluid environment during their evolution and are well adapted to the hydrodynamic conditions to which they are exposed. Recommendations for Further Research: Areas which still need to be further investigated became clear during this study. For example, the higher u*cr i t observed for photosynthetic responses of Cymodocea nodosa exposed to different u* levels when compared to responses of Thalassia testudinum suggests that (1) different species may have different responses to water flow or (2) u*crit may be dependent on the hydrodynamic conditions to which the seagrass has been exposed during its development. This could be tested by generating water flow versus photosynthesis curves for (1) a broad range of seagrass species grown under similar hydrodynamic conditions and (2) for one species exposed to a variety of flow conditions. Additionally, because a healthy looking seagrass population was shown to be exposed to boundary limiting conditions in situ for up to 3 hours during one tidal cycle,

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101 the physiological mechanism which allows Thalassia testudinum plants to tolerate such conditions needs to be investigated. Seagrasses may have alternate photosynthetic pathways in which internal carbon sources may be utilized when not enough external carbon is available. Questions which still need to be addressed regarding this topic include: ( 1) How does this mechanism function? (2) What is (are) the alternate carbon source(s)? (3) For how long can a seagrass support photosynthesis when the boundary layer thickness is limiting to carbon acquisition? If epiphytic growth on seagrass blades generates turbulent eddies resulting in a carbon pulse feeding mechanism on seagrass blades, how fast can a carbon molecule be incorporated in the photosynthetic pathway? Is this an instantaneous process? Does the molecule have to remain for a certain amount of time on the plant surface before its uptake? I have found no answer to this question in my literature search. Therefore, in order to determine if seagrass blades can benefit from the short pulses of carbon and nutrient enriched water suggested here, the speed of carbon incorporation in the photosynthetic process needs to be investigated. The bending of seagrass blades at a low frequency (hours) in a tide-dominated environment compared to the high frequency (seconds) blades bend in wave-dominated environments may have strong geological implications. In tide-dominated

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102 environments, the blades have the tendency to lay on one side for several hours, as the tidal flow changes direction, the blades switch sides and lay there for several more hours. This creates a closed structure of blades over which the currents move. In contrast, in a wave-dominated environment, the blades are constantly moving back and forth. Therefore a truly closed blade structure is not generated and the interaction between the water column above the canopy and the canopy itself is intensified. The effect of this total separation of the s ediments within seagrass beds in tide-dominated environments and the constant exposure/protection cycle of sediments within seagrass beds in wave-dominated environments needs to be addressed regarding sedimentation and erosion. Seagrass beds serve as nursery habitats, supporting a large number of diverse organisms. As vertical velocity profiles and turbulent spectra revealed several hydrodynamic microclimates within seagrass canopies, the effects of these conditions on settling and growth of the associated organisms need to be addressed. For example, the current velocity is reduced in the top of the canopy but accelerated towards the bottom where the sheath holds the blades of Thalassia testudinum together. How does that affect larvae settlement? Could it be beneficial for the supply of food to filter feeding organisms within the canopy? In order to confirm the trends observed in the experiment testing for the effect of currents on the porewater

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103 geochemistry and the growth and development of Thalassia testudinum seedlings, the experiment should be replicated using more microcosms per flow conditions. Field data on currents, porewater geochemistry and seagrass biomass from different hydrodynamic environments and meteorological conditions would also prove helpful.

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104 LITERATURE CITED Ackerman,J.D. and A.Okubo. 1993. Reduced mixing in a marine macrophyte canopy. Functional Ecology 7:305-309. Adams,M.S., P.Guilizzoni, and S.Adams, 1978. Relationship of dissolved inorganic carbon to macrophyte photosynthesis in some Italian lakes. Limnology and Oceanography 23:912-919. Anderson,S.M. and A.C.Charters. 1982. A fluid dynamics study of seawater flow through Gelidium nudifrons. Limnology and Oceanography 27:399-412. Barko,J.W., D.Gunnison, S.R.Carpenter. 1991. Sediment interaction with submersed macrophyte growth and community dynamics. Aquatic Botany 41:41-65. Beer,S. 1989. Photosynthesis and photorespiration of marine angiosperms. Aquatic Botany 34:153-166. Beer,S. A.Shomer-Ilan, and Y.Waisel, 1980. Carbon metabolism in seagrasses. II. Patterns of photosynthetic C02 incorporation. Journal of Experimental Botany 31:1019-1026. Boston,H.L., M.S. Adams and J.D.Madsen. 1989. Photosynthetic strategies and productivity in aquatic systems. Aquatic Botany 34:27-57. Brown,J., A.Collins, D.Park, J.Phillips, D.Rothery and J. Wright. 198 9. Waves, Tides and Shallow-water Processes. Pergamon Press, N.Y., 187 pp. Burke,R.W. 1982. Free surface flow through salt marsh grass. PhD Thesis. Mass. Inst. Tech. /Woods Hole Oceanographic Institute WHOI-82 -50, 252 pp. Cambridge,M.L. and A .J.McComb. 1984. The loss of seagrasses in Cockburn Sound,Western Australia. I. The time course and magnitude of seagrass decline in relation to industrial development. Aquatic Botany 20:229-243. Carlson,P.R., L.A.Yarbro, C.F.Zimmerman, J.R.Montgomery. 1983. Pore water chemistry of an overwashed mangrove island. Florida Science 46:239-249.

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105 Carlson,P.R., T .R.Barber, L.A.Yarbro. 1991. Effects of sediment sulfide on the die-back of Thalassia testudinum in Florida Bay. Abs. 11th International Estuarine Research Conference, 21-22. Carrington,E. 1990. Drag and dislodgement of an intertidal macroalga: consequences of morphological variation in Mastocarpus papillatus Kuetzing. Journal of Experimental Marine Biology and Ecology 139:185-200. Clarke,S. M 1987. Seagrass-sediment dynamics in Holdfast Bay: summary. Safish 11:4-10. Clarke,S. M and H.Kirkman. 1989. Seagrass Dynamics. In: Larkum,A.W. D A.J. McComb and S A .Shepherd (eds.), Biology of Seagrasses, Elsevier, N Y pp 304-345. Conover,J.T. 1966. The importance of natural diffusion gradients and transport of substances related to benthic marine plant metabolism. Botanica Marina 11:1-9. Cooper,L.W. and C P McRoy, 1988. Anatomical adaptations to rocky substrates and surf exposure by the seagrass genus Phyllospadix. Aquatic Botany 32:365-381. Dade,W. B 1993. Near-bed turbulence and hydrodynamic control of diffusional mass transfer at the sea floor.Limnology and Oceanography 38:52-69 Dawes,C.J. 1989. Introduction to Electron Microscopy: Theory and Techniques. Ladd Res. Industries, Inc., Burlington, Vermont. 315 pp. den Hartog, C. 1970. The Seagrasses of the World. North-Holland Publishing Co Amsterdam. 295 pp. Denman,K.L. 1975. Spectral analysis: a summary of the theory and techniques. Department of the Environment, Fisheries and Marine Services Technical Report No. 539. Denny,M.W. and M.F.Shibata. 1989. Consequences of surf-zone turbulence for settlement and external fertilization. American Naturalist 134:859-889. Durako,M. J 1991. Carbon dynamics of the seagrass Thalassia testudinum. PhD Thesis, University of South Florida, 161 pp. Durako, M. J. 1993. Photosynthetic utilization of C02 (aql and HC03 -in Thalassia testudinum (Hydrocharitaceae) Marine Biology 115:373-380.

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106 Durako,M.J., M.D .Moffler. 1981. Variations in Thalassia testudinum seedling growth related to geographic origin. Proceedings of the Eighth Conference on Wetlands Restoration and Creation, pp 100-108. Durako,M.J., M.D.Moffler. 1985. Observations on the reproductive ecology of Thalassia testudinum (Hydrocharitaceae). II. Leaf width as a secondary sex character. Aquatic Botany 21:265-275. Durako,M.J. and M.O.Hall. 1992. Effects of light on the stable carbon isotope composition of the seagrass Thalassia testudinum. Marine Ecology Progress Series 86:99-101. Eckman,J.E. 1987. The role of hydrodynamics in recruitment, growth, and survival of Agropecten irradians (L.) and Anomia simplex (D'Orbigny) within eelgrass meadows. Journal of Experimental Marine Biology and Ecology 106:165-191. Fenchel,T. 1977. Aspects of the decomposition of seagrasses. In: C.P.McRoy, C.Helfferich (eds.) Seagrass Ecosystems. Marcel Dekker, Inc., N.Y. pp 123-145. Finnigan, J. J. 1979. Turbulence in waving wheat. Boundary Layer Meteorology 16:181-211. Fonseca,M.S. 1990. Physical measurements. In: Phillips,R.C. and C.P.McRoy (eds.), Seagrass research methods, UNESCO, pp 139-145. Fonseca,M.S. and J.A.Cahalan. 1992. A preliminary evaluation of wave attenuation by four species of seagrasses. Estuarine, Coastal and Shelf Science 35:565-576. Fonseca,M.S. and J.S.Fisher. 1986. A comparison of canopy friction and sediment movement between four species of seagrasses with reference to their ecology and restoration. Marine Ecology Progress Series 29:15-22. Fonseca,M.S., J.S.Fisher, J .C.Zieman and G.W .Thayer. 1982. Influence of the seagrass Zostera marina (L.) on current flow. Estuarine, Coastal and Shelf Science 15:351-364. Fonseca,M.S. and J.Kenworthy. 1987. Effects of current on photosynthesis and distribution of seagrasses. Aquatic Botany 27:59-78. Fonseca,M. S J .C.Zieman, G W.Thayer and J.S.Fisher. 1983. The role of current velocity in structuring seagrass meadows. Estuarine, Coastal and Shelf Science 17:367-380.

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107 Fourqurean,J.W., J.C.Zieman, G.V.N.Powell. 1992. Relationships between porewater nutrients and seagrasses in a tropical carbonate environment. Marine Biology 114:57-65. Gambi, M. C. A R. M Nowell, P. A Jumars. 1990. Flume observations on flow dynamics in Zostera marina (eelgrass) beds. Marine Ecology Progress Series 61:159-169. Gerard, V .A. 1987. Hydrodynamic streamlining of Laminaria saccharina Lamour. in response to mechanical stress. Journal of Experimental Marine Biology and Ecology 107:237-244. Gerard,V. and K.H.Mann. 1979. Growth and production of Laminaria lonoicuris populations exposed to different intensities of water movement. Journal o f Phycology 15:33-41. Gibson,C.H. 1990. Turbulence, mixing, and microstructure. In: B.LeMehaute, D.M.Hanes (eds), The Sea, vol. 9, part A, Ocean engineering science, John Wiley and Sons, Inc., N.Y. pp 631-660. Giesen,W.B.J.T., M.M.van Katwijk and C .den Hartog. 1990. Eelgrass condition and turbidity in the Dutch Wadden Sea. Aquatic Botany 37:71-85. Gust,G. 1988. Skin friction probes for field applications. Journal of Geophysical Research 93:14,121-14,132. Gust, G 1989. Method and apparatus to generate preciselydefined wall shearing stresses. U.S.Patent No 4884892. Heller,D. Y 1987. Sediment transport through seagrass beds. MSc Thesis, University of Virginia, 72 pp. Hicks,C.R. 1982. Fundamental Concepts in the Design of Experiments. Holt, Rinehart and Winston, NY. pp 36-58. Hillman,K.,D. I.Walker,A.W.D.Larkum and A J McComb 1989. Productivity and nutrient limitation. In: Larkum,A. W .D., A .J.McComb and S.A.Shepherd (eds.), Biology of Seagrasses, Elsevier,N.Y., pp 635-685. Huettel,M. and G.Gust. 1992A. Solute release mechanism from confined sediment cores in stirred benthic chambers and flume flows. Marine Ecology Progress Series 82:187-197. Huettel,M., G.Gust. 1992B. Impact of bioroughness on interfacial solute exchange in permeable sediments. Marine Ecology Progress Series 89:253-267.

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108 Johnstone,I.M. 1979. Papua New Guinea seagrasses and aspects of the biology and growth of Enhalus acoroides (L.f. ) Royale. Aquatic Botany 7:197-208. Jumars,P.A. andA.R.M.Nowell, 1984. Fluid and sediment dynamic effects on marine benthic community structure. American Zoologist 24:45-55. Kenworthy, W. J., J. C. Zieman, G W. Thayer. 1982. Evidence for the influence of seagrasses on the benthic nitrogen cycle in a coastal plain estuary near Beaufort, North Carolina (USA) Oecologia (Berl) 54:152-158. Kirk,J.T.O. 1983. Photosynthesis in the aquatic environment. In: Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press, N.Y. pp 262-269. Koch,E.W. and G.Gust. 1991. Near stagnant waters as a disturbance to the seagrass Thalassia testudinum. Proc. 11th Int.Estuarine Research Conference. Koehl,M.A.R. 1986. Seaweeds in moving water: form and mechanical function. In: Givnish,T.J. (ed.) On the economy of plant form and function. Cambridge Uni v Press, N.Y., pp 603-634. Koehl,M.A.R. and R.S.Alberte, 1988. Flow, flapping and photosynthesis of Nereocystis luetkeanna: a functional comparison of undulate and flat blade morphologies. Marine Biology 99:435-444. Koehl,M.A. and S.A.Wainwright. 1977. Mechanical adaptations of a giant kelp. Limnology and Oceanography 22:1067-1071. Koehl,M. and S.E.Worcester, 1991. Effects of seagrass on water flow at several biologically-important spatial scales. Proc. 11th Int.Estuarine Research Conference. Kundu,P.K. 1990. Fluid Mechanics. pp 416-473. Kuo,J., K.Aioi, H.Iizumi. 1988. Comparative leaf structure and its functional significance in Phyllospadix iwatensis Makino and Phyllospadix japonicus Makino (Zosteraceae). Aquatic Botany 30:169-187. Larkum,A.W.D., G.Roberts, J.Kuo, and S.Strother, 1989. Gaseous s .A. Shepherd (Eds) Biology of Seagrasses A treatise on the biology of seagrasses with special reference to the Australian Region. Elsevier, NY. 686-722 pp.

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109 Lazier,J.R.N. and K.H.Mann. 1989. Turbulence and diffusive layers around small organisms. Deep Sea Research 36:1721-1733. Lipkin, Y. 1977. Seagrass vegetation of Sinai and Israel. In: McRoy,C.P. and Helferich,C. (Eds. ) Seagrass Ecosystemsa scientific perspective. Marcel Dekker, Inc., New York, 263-293 pp. Lodge,D.M., D.P.Krabbenhoft, R.G.Striegl. 1989. A positive relationship between groundwater velocity and submersed macrophyte biomass in Sparkling Lake, Wisconsin. Limnology and Oceanography 34:235-239. Lucas,W.L. 1983. Photosynthetic assimilation of exogenous HC03 by aquatic plants. Annual Review of Plant Physiology 34:71-104. McKenzie,B.R. and W.C .Leggett. 1993. Wind-based models for estimating the dissipation rates of turbulent energy in aquatic environments: empirical comparisons. Marine Ecology Progress Series 94:207-216. McMillan,C. 1978. Morphogeographic variation under controlled conditions in five seagrasses, Thalassia testudinum, Halodule wrightii, Svrinoodium filiforme, Haloohila engelmanii, and Zostera marina. Aquatic Botany 4:169-189. McMillan, C and R.C.Phillips. 1978. Differentiation in habitat response among populations of New World seagrasses. Aquatic Botany 7:183-196. Moriarty,D.J.W. P.I.Boon. 1989. Interactions of seagrasses with sediment and water. In: A.W.D.Larkum, A .J.McComb, S. A. Shepherd ( eds) Biology of Seagrasses. Elsevier, N.Y. pp 500-535. Morse,J.W. J.J.Zullig, R.L.Iverson, G.R.Choppin, A .Mucci, F. J. Millero. 198 7. The influence of seagrass beds on carbonate sediments in the Bahamas. Marine Chemistry 22:71-83. Motzfeld,H. 1938. Zeitschrift 18:326-365 Frequenzanalyse turbulenter Schwankungen. fuer angewandte Mathematik und Mechanik Nikuradse, J. 1933. Laws of flow in rough pipes. National Advisory Committee on Aeronautics Technology Mem. 1292:1-62 (translation from German, 1950).

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112 Tennekes,H. and J.Lumley. 1972. A 1st course in turbulence. The MIT Press. Cambridge, Mass., 300 pp. Tomasko,D.A. and B.E.Lapointe. 1991. Productivity and biomass of Thalassia testudinum as related to water column nutrient availability and epiphyte levels: field observations and experimental studies. Marine Ecology progress Series 75:9-17. Tomlinson,P.B. 1969. On the morphology and anatomy of turtl e grass, Thalassia testudinum (Hydrocharitaceae). III. Floral morphology and anatomy. Bulletin of Marine Science 19:286-305. Townsend,A.A. 1980. The structure of turbulent shear f low. Cambridge Univ. Press, N.Y. 429 pp. van der Velde,G. and C. den Hartog. 1992. Continuing range extension of Halophila stipulacea (Forssk.) Aschers. (Hydrocharitaceae) in the Mediterranean -now found at Kefallina and Ithaki (Ionian Sea) Acta Botanica Nederland 41:345-348. Vogel,A. I. 1961. A Textbook of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis. 3rd Edition, L o ngmans, Green and Co., London. pp 239-242. Vogel,S. 1989. Ve locity gradients and boundary layers. In: Life in moving fluids-the physical biology of flow. Princeton Univ. Press. pp 127-140. Ward,L.G., W.M.K emp and W.R.Boynton. 1984. The influence of waves and seagrass communities on suspended particles in an estuarine embayment. Marine Geology 59:85-103. Wayne,C.J. 1975. Sea and marsh grasses: their effect on wave energy and nearshore sand transport. MSc thesis, Florida State University, Tallahassee, 135 pp. Webb,J.E., J.L.Theodor. 1972. Wave-induced circulation in submerged sands. Journal of Marine Biology (Ass.U.K.) 52:903-914. Webster,I.T., J.H.Taylor. 1992. Rotational dispersion in porous media due to fluctuating flows. Water Resources Research 28:109-119. Wheeler,W.N. 1980A. Effect of boundary layer transport on the fixation of carbon by the giant kelp Macrocystis pyrifera. Marine Biology 56:103-110

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113 Wheeler,W. N. 1980B. Pigment content and photosynthetic rate of the fronds of Macrocyst is pyrifera. Marine Biology 56: 97102. Whitford,L.A. and C.S. Kim, 1966. The effect of light and water movement on some species of marine algae. Revue Algologique 3:251-254. Yarbro,L.A., P.R.Carlson Jr., T.R.Barber. 1991. Temporal, spatial, and tissue effects on hypoxic metabolism of Thalassia testudinum rhizomes in Florida Bay. Abs. 11th International Estuarine Research Conference, 166 pp. Zieman, J C. 1982. The ecology of the seagrasses of south Florida: a community profile. U S Fish and Wildlife Service, Office Biological Services, Washington, D .C. 123 pp. Zimmerman,R.C. and R .S.Alberte. 1991. Light requirements of temperate seagrasses. In: Kenworthy,J.W. and D .E.Haunert (eds.). The light requirements of seagrasses. Proc. of a workshop to examine the capacity of water quality criteria, standards and monitoring programs to protect seagrasses. NOAA Technical Memorandum NMFS-SEFC-287, pp 26-37.

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115 APPENDI X 1. STATISTICAL ANALYSIS FOR DATA PRESENTED IN CHAPTER 4 A sufficient number of microcosms was not available to allow for appropriate replication in the experiment regarding the effect of water flow on porewater geochemistry and the growth of Thalassia testudinum seedlings. Three microcosms (one with stagnant conditions, one with u.=0.3 em s -1 and one with u.=1. 0 em s-1 ) contained 8 seedlings each. Thus, seedlings were not true replicates and a statistical analysis using ANOVAs would not be inappropriate. However, if seedlings were assumed to be true replicates, the ANOVA for biomass production and morphological characteristics of T. testudinum seedlings as well as for physical and geochemical conditions to which the plants were exposed, would reveal the following significant differences: n d. f. F p Blade length 130 2 4.883 0.0091 Blade width 130 2 0.549 0.5786 Blade area 130 2 2.712 0.0703 Root length 98 2 3.380 0 0382 Root number 24 2 4.997 0.0168 Biomass 6 2 0.909 0.4240 Fiber bundles 110 1 12.241 0.0007 Sulfide 24 2 8.712 0 0018 Ammonium 24 2 19.837 0 0000 Phosphorus 24 2 5.108 0.0156

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All speeds and blade friction velocities were measured at a 5 Hz frequenc y at the Lndicated height above the sediment. B indicates a probe which broke during deployment. No data is available for suc h probes. indicate simultaneous deployments DATE/HRS SPECIES 0 4 /30/91 Thalassia testudinum 7:46AM to 6:46 PM 10/03/91 Thalassia testudinum 9:10AM to 12:10 PM SITE TIDE Mullet Key flood & ebb in seagrasses St. Joseph Bay high & ebb in seagrasses MEASUREMENTS temperature pressure (waves ) speed at 40 em 20 e m 5 em 0 em blade u. 4 em 14 em temperature pressure (waves ) speed at 40 em 20 e m 5 em 0 em blade u 5 e m 3 5 e m ;po 'U ."0 trl a H X tv L'O 0(11 Gl 0 or (110 ;;o" (f) t!J z r-3 {f) H z (f) [lj )';:' Gl (f) {f) ::r: )';:' OJ 1-t r-3 ;p .-] (f) c (f) 1-1 z 0 r-3 : t J .-J "'":] l'i 0 )';:' "'":] : t ... f-' 1-' 0"1

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DATE/HRS SPECIES 06/01/92 Thalassia testudinum 11:18 AM to 2:18AM 06/01/92 Thalassia testudinum 11:18 AM to 2:18AM 06/01/92 Thalassia testudinum 11:18 AM to 2:18AM SITE Mullet Key in bare sand 5 m from the Mullet Key edge of seagrasses Mullet Key in seagrasses 5 m from the edge TIDE ebb & flood (spring) ebb & flood (spring) ebb & flood {spring) M EASUREMENTS temperature pressure (waves ) speed at 40 emu* canopy 2 0 em&* 10 em 5 em o em temperature speed at 4 0 em 20 em 10 em 5 em 0 em temperature pressure {waves) speed at 40 e m 20 em 10 em 5 em* 0 cm8* :t'0 '0 trl 8 H :>< N n 0 :J rr 1-' :J c !; 0.. 1-' 1-' ..._J

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DATE/HRS SPECIES 08/31/91 Thalassia testudinum 2:37 PM to 5:37 PM 01/08/91 Ulva laetuea {alga) 1:46 PM to 3:46 PM SITE TIDE Summerland Key flood & high in seagrasses Skyway jetties high & ebb MEASUREMENTS temperature pressure (waves) speed at 4 0 em 20 em 5 em 0 e m blade u 5 em 20 em temperature pressure ( waves) speed at 40 e m 20 e m8 5 e m 0 em frond u. exposed prct:.ec.tErl )>' 'tl 'tl 1:':1 s H >< N n 0 :J rr I-' :J (() Q. 1-' 1-' co

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DATE/HRS SPECIES SIT E 09/05/92 C ymodoeea nodosa N ikhmoret 10:27 AM to (Mediterranean, 12:27 PM Israel) 09/11/92 Cymodoeea nodosa Tel Shikmona 2 :06 PM to (Medit e rranean, 3:0 6 PM Israel ) TIDE surf zone surf zone MEASUREMENTS temperature pressure (waves) speed at 50 em 30 e m 10 em 0 em blade u 20 e m 7 em temperature pressure (waves) blade u 20 em 5 em '0 '0 ttl 8 H :>< tv n 0 ;:J n 1-' ;:J (!) 0. 1-' 1-' I.D

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DATE/HRS SPECIES 06/11/91 Thalassia testudinum 9 :30 AM to 7 :30 PM 06/11/91 Thalassia testudinum 9:30AM to 7 :30 PM 06/11/91 Thalassia testudinum 9:30AM to 7:30 PM SITE Mullet Key in bare sand 5 m from the canopy TIDE high, ebb & low Mullet Key high, ebb & low in seagrasses (low epiphytic cover) 5 m into the canopy Mullet Key high, ebb & low in seagrasses (heavy epiphytic cover) MEASUREMENTS temperature 40cm temperature Ocm pressure (waves) speed at 40 em 20 em 5 an* 0 an temperature pressure (waves)* speed at 40 em 20 em 5 em 0 e m blade u 24 em 5 em temperature pressure (waves) speed at 30 em 20 e m 5 e m 0 on blade u 1 7 em* 5 an "0 "0 txl a H :>< N n 0 ;:l rr ..... ;:l ro 0.. f-' N 0

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DATE/HRS SPECIES 06/23/92 Thalassia testudinum 9:30AM to 8:30 PM 06/23/92 Thalassia testudinum 9:30 AM to 8:30 PM 06/23/92 Thalassia testudinum 9:30 AM to 8:30 P M SITE Looe Key in bare sand 5 m from the canopy Looe Key edge o f seagrasses TIDE windy/wavy windy/wavy Mullet Key windy/wavy in seagrasses 5 m from the edge MEASUREMENTS temperature pressure (waves) speed at 40 cm8* 20 em 1 0 em 5 em 0 on* tem perature pressure (waves) speed at 40 em 20 cm0* 1 0 cm9* 5 em 0 on temperature pressure (waves) speed at 40 e m 20 em 1 0 em 5 on 0 c m8* :t>' tU !"() M s H X tv n 0 ;:J IT !-' ;:J c tli 0... f--1 tv f--1

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DATE/HRS SPECIES 04/11/92 Thalassia testudinum 9:30 AM to 8:30 PM 04/11/92 Thalassia testudinum 9:30AM to 8:30 PM 04/11/92 Thalassia testudinum 9:30AM to 8:30 PM SITE Mullet Key in bare sand 5 m from the canopy Mullet Key edge of seagrasses Mullet Key in seagrasses 5 m from the edge TIDE ebb & flood (neap) ebb & flood (neap) ebb & flood (neap) MEASUREMENTS temperature pressure (waves) speed at 40 em 20 em 10 em 5 em o em temperature speed at 40 em 20 em 10 em 5 cm8 0 em temperature pressure (waves) speed at 40 cm8 20 em 10 cm8 5 em 0 e m )"' 1"0 1"0 tij a H X N () 0 ::J rr !-" ::J Ill 0. 1-' N N

PAGE 137

DATE/HRS SPECIES 09/05/92 Haloohila stioulaeea 11:50 AM to 2:50 PM 09/05/92 Halophila stipulaeea 4:05PM to 6:05PM SITE Eilat (Red Sea, Israel) Eilat (Red Sea, Israel) TIDE stormy calm MEASUREMENTS temperature pressure (waves) speed at 20 em8 10 em 5 em 0 em blade u. 4 em 3 em8 temperature pressure (waves) speed at 20 e m8 10 em 5 em 0 em blade u. 3 em 2 em )"' t"t:J t"t:J tr.1 a H :>< tv n 0 :::J n J-' :::J (!J 0.. f-' tv w


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