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Ault, Danylle N.
Temporal and spatial distribution of Chlorophyll on the West Florida shelf
h [electronic resource] /
by Danylle N. Ault.
[Tampa, Fla] :
b University of South Florida,
ABSTRACT: The West Florida Shelf (WFS), typically characterized as being oligotrophic, is one of the most productive continental shelves in the United States. In addition to supporting a large fishing industry, the WFS also supports high biomass blooms of the toxic dinoflagellate Karenia brevis. Because of the large ecological and economic impacts these blooms have on the area, the ECOHAB: Florida program was developed to gain a better understanding of red tides and their initiation, maintenance, and dispersal. This interdisciplinary program consisted of monthly cruises from June 1998 through December 2001, with a hiatus from January through March of 2001. Hydrography, nutrients, chlorophyll a, phaeopigments, and a wide variety of other factors were measured during the cruises. In this paper chlorophyll a and phaeopigment concentration, nutrients, and hydrographic data were examined to explain the temporal and spatial distribution of chlorophyll on the shelf. Average surface c hlorophyll values were 0.55 mg/m 3 with near bottom values averaging 0.85 mg/m 3. Chlorophyll was found to be highest near the estuaries of Tampa Bay and Charlotte Harbor with a decreasing gradient seaward. Near bottom chlorophyll values were generally two to fourfold greater than surface values. Midshelf stations (35-50 m) were characterized by high near bottom chlorophyll, whereas the offshore stations (86-200 m) were characterized by a subsurface chlorophyll maximum ranging between 40 to 80 m deep. Nutrients were generally low across the shelf except for 1998 when a subsurface intrusion of nutrient rich slope water reached to the 20 m isobath. Temperatures ranged from 14.00Â¨ C to 31.47Â¨ C. Salinity ranged from 30.5 to 37.50 in the study area.Four blooms of Karenia brevis, lasting several months, contributed to the high chlorophyll concentrations along the inner shelf. Maximum chlorophyll concentrations of 27.10 mg/m 3 were a result of the October 1999 to March 2000 red tide.
Thesis (M.A.)--University of South Florida, 2006.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
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Adviser: Gabriel A. Vargo, Ph.D.
x Marine Science
t USF Electronic Theses and Dissertations.
Temporal and Spatial Distribution of Chlorophyll on the West Florida Shelf by Danylle N. Ault A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science College of Marine Science University of South Florida Major Professor: Gabriel A. Vargo, Ph.D. Karen A. Steidinger, Ph.D. John J. Walsh, Ph.D. Date of Approval: April 5, 2006 Keywords: Algal Biomass, Chlorophyll a Pigments, Phytoplankton, Variability Copyright 2006, Danylle N. Ault
ACKNOWLEDGEMENTS There are many people I would like to th ank for their support, encouragement, and friendship during my many years at the College of Ma rine Science. First and foremost, I would like to thank my advi sor Dr. Gabriel Vargo for his support and encouragement over the years, for sharing his passion for oceanography and birds of prey, and for giving me the opportunity to study and experience oceanography. I would also like to thank my committee members, Dr. Karen Steidinger and Dr. John Walsh, for their support and guidance. Special thanks to Dr. Ted Van Vleet for the gentle nudges of encouragement to get my degree finished. The love, patience, and encouragement th at my parents provided helped me stay focused to persevere and finish my degree. I am deeply grateful to my husband Damon for his patience, love and unwavering confidence in me. Many thanks to Merrie Beth Neely who trained me in the lab, collected and analyzed data for this thesis, gave construc tive criticism for the written and oral parts of this thesis, and provided her friendship. Th anks to Sue Murasko for working the night shift on the ECOHAB cruises with me and for her friendship. Also thanks to Dr. Cynthia Heil for her help and advice ove r the years. Thanks to Je nnifer Cannizarro for always being willing to help me calibrate the fluorom eters. Many thanks to all of the graduate students, researchers, and technicians that he lped collect and analyze all of the ECOHAB data. Last but not least, thanks to the crews of the RV Suncoaster and RV Bellows.
i TABLE OF CONTENTS LIST OF TABLES ........................................................................................................ii LIST OF FIGURES .......................................................................................................iii ABSTRACT.....................................................................................................................vi i 1. INTRODUCTION........................................................................................................1 1.1 Background on the Gulf of Mexico and the West Florida Shelf....................1 1.2 Previous Studies on the West Florida Shelf....................................................2 1.3 Reviews on Phytoplankton Distribution and Productivity..............................3 1.4 Karenia brevis on the West Florida Shelf.......................................................7 1.5 Nutrient Sources for Karenia brevis Blooms..................................................8 1.6 Biomass Trends from Southwes t Florida Shelf Ecosystem Program.............9 1.7 Relevance ......................................................................................................10 1.8 Study Objectives...........................................................................................11 2. METHODS.................................................................................................................12 2.1 Sample Collection.........................................................................................12 2.2 Chlorophyll Sampling and Analysis.............................................................14 2.3 Nutrient Sampling and Analysis...................................................................15 2.4 CTD Data ......................................................................................................16 3. RESULTS AND DISCUSSION.................................................................................17 3.1 Phytoplankton Pigment Concentrat ions and Spatial Distribution................17 3.2 Phytoplankton Blooms and Spatial Distribution...........................................52 3.3 Seasonal Trends............................................................................................66 3.4 Isobath/Transect Comparison.......................................................................71 3.4.1 10 Meter Isobath............................................................................71 3.4.2 35 Meter Isobath............................................................................78 3.4.3 50 Meter Isobath............................................................................83 3.4.4 86 and 200 Meter Isobaths.............................................................85 4. CONCLUSIONS.........................................................................................................95 REFERENCES................................................................................................................97 APPENDIX....................................................................................................................109 Appendix A: ECOHAB: Fl orida Sampling Schedule..................................................110
ii LIST OF TABLES Table 1 ECOHAB: Florida st ation sampling schedule......................................110 Table 2 Phytoplankton blooms in th e ECOHAB: Florida study area between June 1998 and December 2001..................................................53 Table 3 Average surface concentrations of chlorophyll a and the observed range of concentrations (mg/m3) along each transect..............................67 Table 4 Average near bottom concentrations of chlorophyll a and the observed range of concentrations (mg/m3) along each transect...............69 Table 5 Phaeopigment:Chlorophyll Statistics.......................................................90
iii LIST OF FIGURES Figure 1. ECOHAB: Florida study area...................................................................13 Figure 2. Spatial distribution of average total surface chlorophyll a (mg/m3) on the WFS from June 1998 through December 2001..............................18 Figure 3. Spatial distribution of av erage total near bottom chlorophyll a (mg/m3) on the WFS from June 1998 through December 2001..............19 Figure 4. Average monthly rainfall (inches) for Tampa, Bradenton, and Ft. Myers, Florida....................................................................................20 Figure 5. Spatial distribution of surface chlorophyll a (mg/m3) for June 1998 through December 1998. No cruise in October 1998.............................21 Figure 6. Spatial distribution of surface chlorophyll a (mg/m3) for January 1999 through June 1999...........................................................................22 Figure 7. Spatial distribution of surface chlorophyll a (mg/m3) for July 1999 through December 1999...........................................................................23 Figure 8. Spatial distribution of surface chlorophyll a (mg/m3) for January 2000 through June 2000. No cruise in February 2000............................24 Figure 9. Spatial distribution of surface chlorophyll a (mg/m3) for July 2000 through November 2000. No cruise in December 2000.........................25 Figure 10. Spatial distribution of surface chlorophyll a (mg/m3) for April 2001 through June 2001. No cruise s in January, February, or March 2001..............................................................................................26 Figure 11. Spatial distribution of surface chlorophyll a (mg/m3) for July 2001 through December 2001. No cruise in October 2001.............................27 Figure 12. Spatial distributi on of near bottom chlorophyll a (mg/m3) for June 1998 through December 1998. No cruise in October 1998............28 Figure 13. Spatial distributi on of near bottom chlorophyll a (mg/m3) for January 1999 through June 1999.............................................................29
iv Figure 14. Spatial distributi on of near bottom chlorophyll a (mg/m3) for July 1999 through December 1999..........................................................30 Figure 15. Spatial distributi on of near bottom chlorophyll a (mg/m3) for January 2000 through June 2000. No cruise in February 2000..............31 Figure 16. Spatial distributi on of near bottom chlorophyll a (mg/m3) for July 2000 through November 2000. No cruise in December 2000.........32 Figure 17. Spatial distributi on of near bottom chlorophyll a (mg/m3) for April 2001 through June 2001. No cr uises in January, February, or March 2001..........................................................................................33 Figure 18. Spatial distributi on of near bottom chlorophyll a (mg/m3) for July 2001 through December 2001. No cruise in October 2001.............34 Figure 19. Cross shelf chlorophyll a (mg/m3) profiles for February 1999................35 Figure 20. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for June 1998 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................39 Figure 21. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for July 1998 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................40 Figure 22. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for February 1999 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, University of South Florida website..........................................41 Figure 23. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for March 2000 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................42 Figure 24. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for June 2000 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................43
v Figure 25. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for July 2000 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................44 Figure 26. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for August 2000 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................45 Figure 27. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for May 2001 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................46 Figure 28. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for July 2001 on the Sarasota transect. Taken from Ocean Circulation Group College of Marine Science, University of South Florida website........................................................47 Figure 29. Cross shelf chlorophyll a (mg/m3) profiles for June 1998.......................48 Figure 30. Cross shelf chlorophyll a (mg/m3) profiles for July 1998.......................49 Figure 31. Surface Trichodesmium spp concentrations (colonies/liter) from January 1999 to June 1999.......................................................................55 Figure 32. Surface Trichodesmium spp concentrations (colonies/liter) from July 1999 to December 1999...................................................................56 Figure 33. Surface Trichodesmium spp concentrations (colonies/liter) from January 2000 to July 2000. No cruise in February 2000........................57 Figure 34. Surface Trichodesmium spp concentrations (colonies/liter) from August 2000 to November 2000, and April to May 2001.......................58 Figure 35. Surface Trichodesmium spp concentrations (colonies/liter) from June 2001 to December 2001...................................................................59 Figure 36. Surface Karenia brevis concentrations (cells/liter) for the November 1998 to February 1999 bloom................................................61 Figure 37. Surface Karenia brevis concentrations (cells/liter) for the October 1999 to March 2000 bloom........................................................62
vi Figure 38. Surface Karenia brevis concentrations (cells/liter) for the October 2000 to November 2000 bloom.................................................63 Figure 39. Surface Karenia brevis concentrations (cells/liter) for the October 2001 to December 2001 bloom..................................................64 Figure 40. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 10 meter isobath..................72 Figure 41. River discharge (cubic fee t/second) into Tampa Bay and Charlotte Harbor......................................................................................................76 Figure 42. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 35 meter isobath..................79 Figure 43. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 50 meter isobath..................84 Figure 44. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 86 and 200 meter isobaths....................................................................................................86 Figure 45. Cross shelf phaeopigmen t profile for September 1999 on the Sarasota transect.......................................................................................93
vii TEMPORAL AND SPATIAL DISTRIBU TION OF CHLOROPHYLL ON THE WEST FLORIDA SHELF Danylle N. Ault ABSTRACT The West Florida Shelf (WFS), typically ch aracterized as bein g oligotrophic, is one of the most productive continental shel ves in the United States. In addition to supporting a large fishing indus try, the WFS also supports high biomass blooms of the toxic dinoflagellate Karenia brevis. Because of the large ecological and economic impacts these blooms have on the area, the ECOHAB: Florida program was developed to gain a better understanding of re d tides and their initiation, ma intenance, and dispersal. This interdisciplinary program consisted of monthly cruises from June 1998 through December 2001, with a hiatus from January through March of 2001. Hydrography, nutrients, chlorophyll a phaeopigments, and a wide va riety of other factors were measured during the cruises. In this paper chlorophyll a and phaeopigment concentration, nutrients, and hydr ographic data were examined to explain the temporal and spatial distribution of chlorophyll on the shelf. Average surface chlorophyll values were 0.55 mg/m3 with near bottom values averaging 0.85 mg/m3. Chlorophyll was found to be highe st near the estuaries of Tampa Bay and Charlotte Harbor with a decreasing gradient seawar d. Near bottom chlorophyll values were generally two to fourfold greater than surface values. Midshelf stations (3550 m) were characterized by high near bottom chlorophyll, whereas the offshore stations
viii (86-200 m) were characterized by a subsur face chlorophyll maximum ranging between 40 to 80 m deep. Nutrients were generall y low across the shelf except for 1998 when a subsurface intrusion of nutrient rich sl ope water reached to the 20 m isobath. Temperatures ranged from 14.00 C to 31.47 C. Salinity ranged from 30.5 to 37.50 in the study area. Four blooms of Karenia brevis lasting several months, contributed to the high chlorophyll concentrations al ong the inner shelf. Maximu m chlorophyll concentrations of 27.10 mg/m3 were a result of the October 1999 to March 2000 red tide. Blooms of Trichodesmium and diatoms also were contributors to patterns seen on the shelf. Maximum chlorophyll values were generally hi ghest in the late summer and fall except for offshore values which showed little to no seasonality. Inshore of the 50 m isobath, average phaeopigments comprised from 43 to 68 percent of the measured Chl a, while offshore values were from 68 to over 100 percent. Inshore chlorophyll distributions were attrib uted to riverine and estuarine flux of nutrients, localized upwelli ng, and recycling of nutrients aided by salinity and temperature fronts. Midshelf distributions were attributed to the movement of biologically important material through th e bottom Ekman layer from offshore to the inshore regions of the shelf. Offshore dist ributions were attributed to Loop Current upwelling and synoptic scale processes associat ed with seasonal meteorological forcing.
1 1. INTRODUCTION 1.1 Background on the Gulf of Mexi co and the West Florida Shelf The Gulf of Mexico (GOM) is a semi-e nclosed deep marginal sea encompassing a broad spectrum of productivity from eutrophic coastal waters to oligotrophic deep ocean conditions (Lohrenz et al., 1999). Bordered to the north and eas t by the continental United States, to the south by Cuba, and to the south and west by Mexico, it covers an area of 1,507,639 km2, has an average depth of 1615 m (maximum depth 3850 m), and a volume of 2,434 (1000 km3). Water is carried into the Gulf through the Yucatan Channel from the Caribbean Sea and ex its the Gulf through the Strait s of Florida into the North Atlantic Ocean. Thirty-five percent of the ar ea of the GOM are continental shelves. The West Florida Shelf (WFS) accounts for a pproximately 75% percent of the total continental shelf area border ing the USA (Anonymous, 1994). The WFS is the second largest continental sh elf in the United States after the shelf off Alaska (Anonymous, 1994) and is located al ong the eastern margin of the Gulf of Mexico. This carbonate shelf is broad, up to 220 to 275 km wide, gentle sloping, and has low-relief shore parallel topography related to positions of former shorelines (Roberts, et al., 1999). Although the Gulf of Mexico is usua lly thought of as an oligotrophic sea, the WFS supports one of the richest fisherie s in the U.S. both commercially and recreationally (Anonymous, 1994). There are many characterisctics which enable the WFS to support such a rich fish ery. These include: 1) the br oad continental shelf that has light penetration to greater than 100 m, 2) its waters are warm temperate to tropical, 3) it
2 has marsh, mangrove, and seagrass communities which act as nurseries for various fish species, and 4) DOC/DOM is transported to the shelf a nd is produced on the shelf (Steidinger, per. comm..). In addition, the shelf also has a three dimensional structure related to density gradients, upwelling, downwelling, patch reefs, and biological communities that are in the wa ter column and on the bottom along the shelf (Steidinger, per. comm.). In spite of its vast size and pr oductivity, it is one of the least studied areas in the Gulf of Mexico. 1.2 Previous Studies on the West Florida Shelf Most of the large scale st udies on the WFS, funded by the U.S. Department of the Interior, Bureau of Land Management (BLM) or Minerals Management Service (MMS), are related to potential oil and gas explora tion and production on the shelf. The main objectives of these studies were to obtain en vironmental data on the impacts of petroleum exploration and production activ ities on the outer continenta l shelf (OCS) and provide relevant information to support manage ment decisions concerning OCS leasing (Environmental Science and Engineering, In c., et al., 1987). The BLM/MMS spent $35 million dollars on environmental studies on the WFS in the period between 1972 to 1987 (Anonymous, 1994). Several smaller regional studies on th e WFS have focused on a variety of different aspects of the shelf ecosystem. The National Marine Fisheries Service funded the Southeast Monitoring and Assessment Program (SEAMAP) and the State of Florida funded two programs, the Hourglass crui ses (1965-1967) and th e Coastal Production Program (1991-1994). The Hourglass cruise s studied hydrography and plankton using
3 trawls, dredges and handline surveys between Tampa Bay and Charlotte Harbor (Joyce and Williams, 1969). The Coastal Production Program studied phytoplankton production and general ecology including nutrients, hydr ography, and selected ichthyoplankton and zooplankton ecology on the WFS from Ceda r Key to the Dry Tortugas (Anonymous, 1994). Other studies have focused on the physical oceanography and modeling of the WFS (Hsueh, 1982; Cooper, 1987; Yang et al., 1999; Liu et al., 2005; Yang and Weisberg, 1999). One such program was the West Central Florida Coastal Studies Project started in 1994 by the Department of Marine Science at the University of South Florida and the U.S. Geological Survey. This program was designed to study circulation and its effects upon WFS coastal processes. Though these studies are a rich source of information on the WFS, data are still lack ing on the biological o ceanography of the area and how it interacts with the physical a nd chemical dynamics of the system. 1.3 Reviews on Phytoplankton Di stribution and Productivity One area that still lacks informa tion and understanding is that of the phytoplankton dynamics on the WFS and how th ey interact with the physical and chemical dynamics of the system. There have been several reviews of the distribution and primary productivity of phytoplankton in the GOM. Investigations of phytoplankton productivity and ecology in the coastal areas of northern and eastern GOM were discussed by Davis (1954). The distribution of dinoflagellates was reviewed by Graham (1954) and Steidinger (1972) Lasker and Smith (1954) and Rounsefell and Nelson (1966) reviewed red tides. Conger (1954) and Saunders and Fryxell (1972) reviewed diatoms. El-Sayed (1972) summarized data on phytoplankton productivity and
4 chlorophyll concentration throughout the GOM obtained between 1964 to1971. Based on mean values for productivity (109 g C m-2 y-1) and for chlorophyll (0.2 mg/m3) El-Sayed concluded that the GOM was ve ry oligotrophic. His study al so showed that the inshore standing crop was almost twi ce as high as offshore. In 1973, Steidinger discussed the di stribution, productivit y, and ecology of phytoplankton in the eastern GOM. Problem s in plankton methodology were outlined and their implications for data interpretation were discussed. One problem was the use of chlorophyll a (Chl a ) for determining standing crop. St anding crop is defined as the amount of viable phytoplankton (mg Carbon or mg Chl a ) per m3 or underneath 1 m2 of sea surface at any given time. The problem with using chlorophyll as an indicator of biomass is that its degradation products ( phaeopigments) are ofte n not accounted for when calculating the standing crop and ma y constitute a large portion of the Chl a signal. This leads to an overestimation of the standi ng crop. This aspect is very important in areas with large amounts of plant detritus, su ch as coastal and estuarine areas, and in areas of heavy grazing. Chlorophyll a is considered a relative index of biomass by several researchers (Wood and Corcoran, 1966; Steidinger and Williams, 1970; Tappan and Loeblich, 1971). Differences between spec ies and the physiological state of the phytoplankton population make it difficult to equate Chl a values to available carbon. Despite these concerns, Chl a Â’s adsorption and fluorescence pr operties make it relatively easy to measure bio-optically thus providi ng a reasonably good index of seasonal and regional variations in phytoplan kton abundance and bloom dynamics. The majority of the earlier GOM Chl a data is total Chl a including phaeopigments. This makes it difficult to co mpare live standing crop between areas. It
5 also makes it difficult to determine live seas onal peaks and vertical distributions of the standing crop in the GOM. Steele (1964) noted Chl a maxima in the Gulf at depths between 50 and 150 m. Because earlier data did not take into account phaeopigments, it was not known whether this represented live bi omass or detrital material accumulating at a particular boundary. El-SayedÂ’s data (1972), which are Chl a minus phaeopigments, showed the Chl a maximum in Gulf waters to be at 50 to 200 m, thus SteeleÂ’s observation must have been live biomass. El-Sayed noted that these depths coincided with the lower limit of the euphotic zone. In SteidingerÂ’s (1973) review, several characteristics/patter ns of eastern GOM phytoplankton were noted. In general, estuarin e waters are more productive than coastal waters, and coastal waters are more produc tive than open Gulf waters. Certain continental shelf waters are more productive than others due to suspected upwelling and land runoff. These areas include the Missi ssippi delta, Campeche Bank, Northeastern Gulf, Yucatan Peninsula, slope and lower shel f of southwestern Flor ida and the edge of the Loop Current. Four broad types of phytoplankton assemblages were identified: 1)estuarine, 2)estuarine and coastal, 3) coastal and open Gulf, and 4)open Gulf. Representative species were listed for each group. The diversity of phytoplankton increases from inshore to offshore. Diat om species diversity and abundance dominate inshore coastal areas while dinoflagellate di versity often dominates open Gulf waters. Even though not as identifiable, microflagella tes (5-15 ) numerically dominate eastern Gulf coastal and estuarine environments. Eastern Gulf waters are populated with cosmopolitan coastal species that form the resident popula tion, but upwelling of deeper water and eddies from the Loop Current can in troduce Â“visitorsÂ”. Phytoplankton peaks as
6 measured by cell numbers, Chl a or primary production were in spring and late summer/fall for the FloridaÂ’s west coast estu arine and inshore waters (Saunders et al., 1967; Steidinger et al., 1966). The Naples coast had peaks in the spring and summer (Dragovich, 1963). It was speculated that a winter phytoplankton peak occurred offshore in deeper continental shelf waters and the open Gulf, because El-SayedÂ’s (1972) data showed high Chl a and high silicates for surface waters in the winter. Also Saunders and GlennÂ’s (1969) most seaward station on the We st Florida coast had great diatom diversity and abundance peaks in winter. Steidinger ( 1973) noted that seasonal peaks varied from year to year and area to area. Primary productivity and Chl a maxima in shelf waters and the open Gulf are typically subsurface. It was noted that most of the data for this area was derived from stations occupied too in frequently or only once and that synoptic sampling is needed for this area to better understand its dynamics. In a review by Iverson and Hopkins ( 1981), they noted that chlorophyll values were low throughout the year at BLM-Mississi ppi/Alabama/Florida (MAFLA) stations in the Eastern Gulf of Mexico. On the transect off Tampa Bay, the average summer, fall, and winter surface Chl a values were 0.21 mg/m3, 0.47 mg/m3, and 0.37 mg/m3, respectively. The average summer, fall, and winter bottom Chl a values were 0.57 mg/m3, 1.90 mg/m3, and 0.45 mg/m3, respectively. Chlorophyll a values collected during the Hourglass cruises indicated that there was considerable mont hly variation in values for inshore stations. They also mentione d that offshore phytoplankton abundance was greatest in January and February. St eidinger and Williams (1970) found that dinoflagellate abundance occasionally reached co ncentrations offshore similar to those at inshore stations.
7 1.4 Karenia brevis on the West Florida Shelf The species that is usually responsible for these large offshore dinoflagellate populations is Karenia brevis ( K. brevis) Dinoflagellate populations of K. brevis are a unique feature on the WFS. Though they ha ve impacted all the states surrounding the Gulf of Mexico, the area along the WFS fr om Tarpon Springs/Clearwater to Sanibel Island has the greatest frequency of K. brevis red tides of any other area in the Gulf of Mexico (Steidinger et al., 1998). This t oxic, unarmored dinofla gellate is commonly found at background concentra tions of 1 to1000 cells L-1 (Dragovich & Kelly, 1966; Steidinger, 1975b; Geesey & Kelly, 1993), but can reach concentrations that are high enough to cause severe ecological and economi cal impacts. The potent polyether toxin that K. brevis releases at high Â“bloomÂ” concentrations (>105 cells L-1) can kill fish, birds, and marine mammals, cause closure of she llfish harvesting areas, produce respiratory irritation in residents and tourists on s hore, and cause economic losses to local communities that depend on tourism, water related recreational activities, and fisheries for their livelihood. The init iation phase of blooms occurs in oligotrophic mid-shelf waters 18 to 74 km offshore in the late summer or fall in conjunction with fronts associated with Gulf Loop current intrusi ons on the outer contin ental shelf (Dragovich and Kelly, 1966; Steidinger, 1975b; Steidinger and Haddad, 1981; Tester and Steidinger, 1997). These initial populations are then transported inshore via winds and tidal currents (Steidinger and Haddad, 1981). Once inshore, cells are concentrated along thermal and salinity fronts that act as bot h barriers and transport mech anisms (Vargo et al., 2001). Karenia brevis has many adaptive strategies th at make it successful in the oligotrophic waters of the WFS. These st rategies include: 1) being adapted to high
8 salinity water with wide temperature ranges, 2) being efficient at us ing inorganic nitrogen (N) and phosphorus (P), 3) being able to use organic N and P, 4) being photosynthetically efficient over varying light le vels, 5) being protected by it s photobiology and behavior as it concentrates and disperses into surface wate rs in daylight, 6) being protected from certain zooplankton predators by its morphology and toxins, a nd 7) being able to out compete other faster growing plankton to form nearly monospecific blooms that can last for months (Tester and Steidinger, 1997; Steidinger et al., 1998). 1.5 Nutrient Sources for Karenia brevis Blooms Despite all these strate gies, identifying nutrient s ources that can support high biomass blooms of K. brevis (> 106 cells L-1) that last for months remains difficult. Possible nutrient sources that have been identified include nitrogen input from N2 fixation by Trichodesmium blooms (Lenes et al., 2001; Walsh and Steidinger, 2001), remineralization of the near bottom diatom bloom which is fueled by shelf break upwelling (Walsh et al., 2003), remineraliza tion of fish killed by brevetoxins during blooms (Walsh et al., 2003), benthic flux, zooplankton excret ion, atmospheric deposition (Vargo et al., in revision), and estuarine flux of N and P from Ta mpa Bay and Charlotte Harbor (Vargo et al., in revision). Vargo et al. (in revision) exam ined the magnitude of these sources to determine the amount of N and P potentially avai lable to support large blooms. They concluded that atmo spheric deposition, benthic flux, and N2 fixation were minor contributors to the flux required to s upport growth of populati ons greater than 2.6 x 104 cells L-1. N and P from decaying fish could maintain moderate populations of K. brevis, but there was insufficient data on the fl ux and mixing rates of decaying fish to
9 calculate average valu es. Zooplankton excretion rate s could supply all the N and P required to support populations greater than106 cells L-1, but confirmation of zooplankton excretion rates found in the literature is requ ired. Estuarine flux of N and P can meet the requirements of larger K. brevis blooms only if the populat ions are located in the immediate vicinity of the estuaries, since coastal nutrient inputs do not extend beyond a 1 to 3 km coastal zone (Steidinger et al., 1998). 1.6 Biomass Trends from the Southw est Florida Shelf Ecosystem Program A final piece of historical data on th e WFS was found in an executive summary written for MMS for the 6 year Southwest Fl orida Shelf Ecosystem Program started in 1980 (Environmental Science and Engineeri ng, Inc. et al., 1987). It was an interdisciplinary study designed to determine the potential impact of OCS oil and gas offshore activities on live-bottom habitats and communities on the southwest Florida shelf. The study area was from 27 N latitude southward to th e Florida Keys and seaward from the west coast of Fl orida to the 200 m isobath. Woodward Clyde Consultants (1983) found chlorophyll values in this ar ea to range from less than 0.1 to 1.5 mg/m3. They found no apparent geographical or seas onal trends with regard to chlorophyll distribution. However, the highest overall ch lorophyll concentrations occurred during the fall. The lowest chlorophyll values were reco rded in the spring and were comparable to summer values inshore of the 100 m isobath. For both seasons, chlorophyll values ranged from 0.1 to 0.5 mg/m3. Because the spring and summer values were so low, it was suggested that a phytoplankton bloom ha d been missed either in the spring or summer. It was also suggested that the maximum values reported by Woodward Clyde
10 Consultants and Skidway Institute of O ceanography (1983) were low and should be considered conservative when estima ting the productivity of the shelf. 1.7 Relevance Even with all this information availa ble on the WFS, it has not been integrated into a comprehensive and usable format fo r understanding the processes, dynamics, and driving forces that maintaining the natural physical, chemical, and bi ological components of the WFS or how these processe s interact with the rest of the Gulf of Mexico ecosystem as a whole. Understanding these proce sses will allow managers to make better management decisions, will reveal significant information gaps providing useful focal points to researchers for further study, and will help physical and biological oceanographers build better models of the ecosystem. These, in turn, will provide managers new insight to deve lop better resource management techniques to protect the Gulf ecosystem. TodayÂ’s society has become very intere sted in changes in the environment and how they affect the local economy and community life. A large segment of the population resides along the west coast of Florida. Tourism, wetlands, recreational fishing, artificial reefs, seafood production, boating, marinas, beaches, marine transportation, oil and gas production, and urban use add up to billions of dollars for the communities neighboring the WFS. This make s the WFS a focal point for the impacts and consequences of many upland, waterfront, and offshore activities. Coastal resource overexploitation, habitat loss caused by increasing coasta l development, and increasing pollution associated with i ndustrial/domestic development and high population densities
11 (Anonymous, 1994) have increased the need for comprehensive, interdisciplinary, integrated, long-term studies for the WFS. 1.8 Study Objectives In recent years, severa l programs have collected in situ data over limited spatial areas along the WFS. One of these programs is the ECOHAB: Florida program (Ecology and Oceanography of Harmful Algal Blooms). The focus of this program was to gain a better understanding of red tides and th eir initiation, maintena nce, and dispersal. Monthly near-synoptic cruises were c onducted from June 1998 through December 2001 along an established grid of stations from Tampa Bay to Ft. Myers and offshore to the 200 m isobath. These cruises produced a very comprehensive collection of biological, chemical, and physical oceanographic data for the ECOHAB region on the WFS. Since very little is known about how chlorophyll a varies on the WFS, I investigated the spa tial and temporal distribution of chlorophyll a on the WFS using the June 1998 to December 2001 ECOHAB data set. Since this unique data set has good temporal, depth, and spatial coverage over a large area, I was able to examine 1) the seasonal cycle of chlorophyll a on the shelf, 2) the temporal and spatial distribution of chlorophyll a on the shelf, and 3) the physical/che mical factors that correspond to the observed spatial and temporal patter ns of chlorophyll a on the shelf.
12 2. METHODS 2.1 Sample Collection Samples were collected monthly along the WFS from June 1998 through December 2001 as part of the ECOHAB: Fl orida program. Weather problems forced cancellation of cruises in October 19 98, February 2000, December 2000, and October 2001. No cruises were scheduled for Ja nuary through March 2001. Sampling was conducted along an hourglass shaped, near synop tic series of transects between Tampa Bay on the north and Charlotte Harbor on the south (Fig. 1). Sampling stations were spaced at approximately five nautical mile in tervals along each transect At each station, a vertical profile of temperature, salinit y, sigma-theta, and chlorophyll fluorescence was taken with a Seabird SBE CTD (conductivity, te mperature, density) attached to a rosette sampler, equipped with twelve, eight liter Ni skin bottles. At every other station, water samples were taken at predetermined depths from the Niskin bottles for chlorophyll a ; dissolved inorganic nu trients (nitrate(NO3), nitrite (NO2), phosphate (PO4), and silicate (SiO4); dissolved organic nitr ogen (DON) and phosphorus ( DOP); and particulate carbon (C), nitrogen (N), and phosphorus (P). In addition to these samples, continuous underway measurements of surface temperat ure, salinity, and chlorophyll fluorescence were measured using a Falmouth Scientific CTD. Surface seawater from the shipÂ’s seawater system was continuously pumped through a container housing the CTD and measurements were taken at two second inte rvals throughout the duration of the cruise.
13 Figure 1. ECOHAB: Florida study area.
14 For this thesis only chlorophyll a nutrient, and CTD data from the three cross shelf transects were used. 2.2 Chlorophyll Sampling and Analysis Chlorophyll a samples were taken at predeter mined stations and depths along each transect (Appendix A). At each dept h, replicate 285 ml water samples were collected in dark Nalgene bottles, filtered th rough Whatman GF/F glass fiber filters (25 mm), and placed in polycarbonate test tubes wi th 10 ml of 100% methanol. The test tubes were capped, mixed on a vortex mixer, wr apped in aluminum foil, and frozen at 20 C for the duration of the cruise. Upon return to the laboratory, typically within two to five days, pigments were analyzed on tw o Turner Design (TD) 10AU fluorometers using the methods of Holm-Hansen and Riemann (1978) and Welschmeyer (1994). The acid fluorescence method of Holm-Hansen et al. (1965), in which the fluorescence of extracted Chl a is determined prior to and after ac idification with broad-banded excitation and emission filters, is routinely used due to its convenience, sensitivity, and provision of both Chl a and phaeopigments concentrations. This method has long been known to be inaccurate when either Chl b or phaeopigments are presen t because the wavelengths of phaeopigment fluorescence overlap with those of Chl a in non-acidified samples and the acidification step of the Holm-Hansen et al. (1965) method results in a reduction in the wavelength of Chl b to near that of Chl a (Mantoura et al., 1997) This interference results in an underestimate of Chl a concentration and an overestimation of phaeopigment concentration (Gibbs, 1979; Lorenzen, 1981; Trees, 1985; Neveau et al., 1990; Welschmeyer, 1994). Welschmeyer (1994) pr oposed using a new combination of narrow
15 band optical filters and a different lamp that selectively measures Chl a in the presence of both Chl b and phaeopigments. This allows for a more accurate measurement of Chl a because it minimizes the fluorescence overlap between Chl a and b This method does not, however, measure phaeopigment con centrations. For th is study, chlorophyll a and phaeopigment concentrations determined by the Holm-Hansen and Riemann method were used exclusively for data analysis except for September and November 1998 when only Welschmeyer data were available. Chlorophyll a and phaeopigments were calculated using standard formulas based on the calibration of the fluorometers with a Sigma chlorophyll a standard. Fluorometers were calibrated every 12 months with interim checks using the TD solid state standard. Replicate Chl a and phaeopigment values for each depth were averaged and plotted for each cross shelf tran sect to create vertical cross shelf profiles and other plots. In order to compare pigments on differen t transects over the same depth range, Chl a and phaeopigments were depth integrated over 5 m, 30 m, 45 m, 80 m, and 185 m using the SimpsonÂ’s (trapezoidal) Rule. 2.3 Nutrient Sampling and Analysis Inorganic nutrient samples for NO3, NO2, PO4, and SiO4 were taken at the same locations and depths as the chlorophyll a samples. One 30 ml unfiltered water sample was collected in a high density polyethylene (HDPE) bottle from each Niskin bottle then frozen upright at -20 C until analyzed. Concentrations of each nutrient were determined using an Alpkem RFA II segmented flow nut rient analyzer from June 1998 through May 2000. From April 2000 through December 2001, an Astoria Pacific Autoanalyzer was
16 used to determine the concentration of each nut rient. All nutrients were analyzed using standard methods as described in Gor don et al. (1993) by the Oceanic Nutrients Laboratory, College of Marine Science, Univer sity of South Florid a under the direction of Dr. Kent Fanning. Nutrient data for NO3, NO2, PO4, and SiO4 were averaged for the 10 m, 25 m, and 50 m isobaths. Figures were modified from Vargo et al. (in revision). 2.4 CTD Data CTD data were binned at one meter interv als. Cross shelf vertical profiles for each transect were plotted for each cruise by the Ocean Circulation Group, College of Marine Science, University of South Florida under the directio n of Dr. Robert Weisberg.
17 3. RESULTS AND DISCUSSION 3.1 Phytoplankton Pigment Concentr ations and Spatial Distribution The average surface and bottom distribution of chlorophyll a on the WFS between Tampa Bay and the Charlotte Harbor region di splays an estuarine signature; i.e. higher concentrations are typically found off the mout hs of the two estuaries (Figs. 2 and 3). Distinct seasonal variations w ith elevated chlorophyll conc entration off the mouths of each estuary during the wet season (Fig. 4) can be seen in the surface estuarine signature (see Figs. 5 to 11), whereas the bottom signature is not as seasonally di stinct (see Figs. 12 to 18). Surface chlorophyll a concentrations for all locati ons and depths between Tampa Bay and Charlotte Harbor averaged 0.55 mg/m3, but ranged from 0.01 to 27.10 mg/m3 (Fig. 2); whereas near bottom Chl a concentrations averaged 0.85 mg/m3, but ranged from below detection limits (B LD) (detection limit, 0.01 mg/m3) to 16.80 mg/m3(Fig. 3). Both the surface and bottom maximum valu es occurred in October 1999 at the 8 m isobath on the Sarasota transect (St. 32, see Figs. 1, 7, and 14). Overall, average near bottom concentrations of Chl a on the shelf were greater than surface concentrations from the 20 m isoba th seaward (compare Figs. 2 and 3). At nearshore locations inside the 20 m isobath, vertical mixing typically led to vertical isopynals and homogeneous chlorophyll conc entrations. An example of nearshore vertical chlorophyll isopleths and offshore s ubsurface maxima can be seen in Fig. 19. Subsurface maxima along the Sarasota transe ct were found at depths between 40 and 80 m.
18 Figure 2. Spatial distribution of average total surface chlorophyll a (mg/m3) on the WFS from June 1998 through December 2001.
19 Figure 3. Spatial distribution of average total near bottom chlorophyll a (mg/m3) on the WFS from June 1998 through December 2001.
20 Figure 4. Average monthly rainfall (inches) for Tampa, Br adenton, and Ft. Myers, Florida. Tampa Precipitation Data Lat. 27.7 Long. -82.40 2 4 6 8 10 12 14 16 18 20 Jan FebMarAprMayJunJulAugSepOctNovDec MonthsInche s 1998 1999 2000 2001 Bradenton Precipitation Lat. 27.45 Long. -82.47 0 2 4 6 8 10 12 14 16 18 20 Jan FebMarAprMayJunJulAugSepOctNovDec MonthInche s 1998 1999 2000 2001 Ft. Meyers Precipitation Lat. 26.6 Long. -81.870 2 4 6 8 10 12 14 16 18 20 Jan FebMarAprMayJunJulAugSepOctNovDec MonthInche s 1998 1999 2000 2001
21 Figure 5. Spatial distribution of surface chlorophyll a (mg/m3) for June 1998 through December 1998. No cruise in October 1998.
22 Figure 6. Spatial distribution of surface chlorophyll a (mg/m3) for January 1999 through June 1999.
23 Figure 7. Spat ial distribution of surface chlorophyll a (mg/m3) for July 1999 through December 1999.
24 Figure 8. Spatial distribution of surface chlorophyll a (mg/m3) for January 2000 through June 2000. No cruise in February 2000.
25 Figure 9. Spatial distribution of surface chlorophyll a (mg/m3) for July 2000 through November 2000. No cruise in December 2000.
26 Figure 10. Spatia l distribution of surface chlorophyll a (mg/m3) for April 2001 through June 2001. No cr uises in January, February, or March 2001.
27 Figure 11. Spatial distribution of surface chlorophyll a (mg/m3) for July 2001 through December 2001. No cruise in October 2001.
28 Figure 12. Spatial di stribution of near bottom chlorophyll a (mg/m3) for June 1998 through December 1998. No cruise in October 1998.
29 Figure 13. Spatial di stribution of near bottom chlorophyll a (mg/m3) for January 1999 through June 1999.
30 Figure 14. Spatial di stribution of near bottom chlorophyll a (mg/m3) for July 1999 through December 1999.
31 Figure 15. Spatial di stribution of near bottom chlorophyll a (mg/m3) for January 2000 through June 2000. No cruise in February 2000.
32 Figure 16. Spatial distribution of near bottom chlorophyll a (mg/m3) for July 2000 through November 2000. No cruise in December 2000.
33 Figure 17. Spatial di stribution of near bottom chlorophyll a (mg/m3) for April 2001 through June 2001. No cruises in January, February, or March 2001.
34 Figure 18. Spatial di stribution of near bottom chlorophyll a (mg/m3) for July 2001 through December 2001. No cruise in October 2001.
35 Figure 19. Cross shelf chlorophyll a (mg/m3) profiles for February 1999.
36 Surface chlorophyll a concentrations 0.50 mg/m3 generally were found inshore of the 35 m isobath. However, high surface concentrations were found outside the 35 m isobath in June 1998 (Fig.5), January 1999 (Fig. 6), November 1999 (Fig. 7) May 2001(Fig. 10), and November 2001 (Fig. 11). Since near bottom Chl a concentrations can be two to four fold greater than the surface values, sufficient light must be available to support these populations. Ten percent of surface photosynthetically active radi ation (PAR) is found at depths of ~ 30 m on the WFS resulting in Chl a concentrations within the upper 1.0 cm of sediment that are two to four fold those of the overlying water column (Darrow, 2003; G. Vargo per. comm.). Mller-Karger et al. (1991) stated that there is adequate illumination in the mixed layer all year long. Sylvia Earle (p er. comm. with Humm, 1973) noted that water clarity in the region has been so high that attached macroalgae have been observed and collected at depths beyond th e 200 m shelf break. More rece nt data suggests that enough light reaches 75 m depths to enable photosynt hesis of WFS benthic microflora (Okey et al., 2004). Recent studies have demonstrated that microphytobenthos communities can contribute a considerable portion of overal l continental shelf primary productivity on tropical and subtropical shelves where overlying waters are rela tively clear (Colijn and de Jonge, 1984; Cahoon and Cooke, 1992; MacIntyr e and Cullen, 1995; MacIntyre et al., 1996; and Nelson et al., 1999). The distributi on of living chlorophy ll associated with benthic microalgal communities out to the 100 m isobath on the WFS indicates that sufficient light penetrates to the bottom to maintain elevated near bottom populations of water column phytoplankton.
37 Unlike the high surface Chl a concentrations ( 0.50 mg/m3) generally only found inshore of the 35 m isobath, similar concentrations of near bottom Chl a were seen as far out on the shelf as the ~80 m isobath (Fi g. 15, May 2000). Frequent ly, near bottom Chl a concentrations along th e ~50 m isobath are 0.40 mg/m3 as seen in all months of 1998 (Fig. 12); January, February, May, (Fig. 13) and July through December 1999 (Fig. 14); January, March, and June through November of 2000 (Figs. 15-16); and all months of 2001 (Figs. 17-18). Elevated chlorophyll leve ls in the area between the 30 and 50 m isobaths on all three transect s during the February 1999 crui se (Fig. 19) are commonly found along the shelf throughout the year (see Figs 12 to 18). It is thought that these near bottom populations are supported by nutrient rich slope waters upwelled onto the shelf by Loop Current intrusions (Walsh et al., 2003). The Loop Current is present 30-35% of th e time at 27 N on the edge of the WFS south of Tampa Bay (Vukovich and Hamilton, 1989) Examples of the cold nitrogen (N), phosphorus (P), and silica (Si) enriched slope waters upwel led onto the shelf seaward of the 30 m isobath can be seen in cross shelf transects found in Heil et al., 2001; Vargo et al., 2001; and Walsh et al., 2003 and Figs. 20 an d 21. Cross shelf profiles of temperature for 1999 show no Loop Current intrusion for February 1999 (Fig 22). However, there was a Loop Current intrusion on the shelf in 1998 and at the shelf break in June 2000. Cross shelf temperature profiles for June and July 1998 (Fig. 20 and 21) show colder (20C) upwelled water on the shelf as far in shore as the 20 m isobath. Walsh et al. (2003) showed that the near bottom isopleth of 1 umol NO3 kg-1 associated with the cold upwelled water had penetrated to the 20 m isobath by May 1998 in the Panhandle, Big Bend, and Southeastern regions of the WFS. In May 1999, 2000, and 2001, concentrations of 1 umol NO3 kg-1 were only found at the ~65 m isobath indicating weaker upwelling of slope waters for these years (Walsh et al., 2003). These weaker upwelling episodes can be seen in Figs. 23 th rough 28 as domed cold water isotherms at
38 the shelf break. Phytoplankton populations a ssociated with these weaker upwelling episodes at the shelf break showed high ph aeopigment/chlorophyll ratios, usually 1.0 or higher. The interannual differences in slope water nutrient supply lasted until the fall (Walsh et al, in review). Figures 29 and 30 show the near bottom chlorophyll associated with the 1998 intrusion. Thes e slope water nutrients are ut ilized by both summer and fall diatom blooms (Walsh et al., in review). In November 1998 along the 10-30 m isobaths, a near bottom chlorophy ll biomass of 3-4 mg/m3 was produced by diatom communities of Rhizosolenia and Chaetoceros spp. (Walsh et al., 2003) In strong upwelling years, such as 1998, mid-shelf near bottom nut rients in November were 1.02 mol NO3 kg-1, 0.15 mol PO4 kg-1, and 3.74 mol SiO4 kg-1, with a N/P ratio of 6.8 (Walsh et al., in revision). In contrast, w eak upwelling years of 1999-2001 yielded smaller November nutrient stocks of : 0.04 mol NO3 kg-1, 0.01 mol PO4 kg-1 for 1999, 1.16mol SiO4 kg1; 0.23 mol NO3 kg-1, 0.08 mol PO4 kg-1, 1.04 mol SiO4 kg-1 for 2000; 0.05 mol NO3 kg-1, 0.01 mol PO4 kg-1, and 1.29 mol SiO4 kg-1 for 2001, with N/P ratios of 3-5 (Walsh et al., in revision). These low disso lved N:P ratios at the 60 m isobath in weak upwelling years reflect continue d recycling of meager upwelle d slope waters (Walsh et al., in revision). Though Loop Current intrusions appear to be relatively rare (Weisberg and He, 2003), these interactions have important effect s on the distribution of material over the WFS. The Loop Current influence on the shel f leads to a stronger on-shore transport of material within the bottom Ekman layer by intensifying the mid-sh elf currents (He and Weisberg, 2003). The Loop Current is instrume ntal in causing cold, nutrient rich waters of deep origin to be transported between the shelf break and the inner shelf (He and Weisberg, 2003). This increased bottom Ekman transport of nutrients is responsible for the elevated biomass, as seen on the shelf in 1998 (Walsh et al., 2003).
39 Figure 20. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for June 1998 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
40 Figure 21. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for July 1998 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
41 Figure 22. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for February 1999 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
42 Figure 23. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for March 2000 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Sc ience, University of South Florida website.
43 Figure 24. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for June 2000 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
44 Figure 25. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for July 2000 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
45 Figure 26. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for August 2000 on the Sarasota tr ansect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
46 Figure 27. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for May 2001 on the Sarasota tr ansect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
47 Figure 28. Cross shelf pr ofiles of temperature ( C), salinity, sigma-theta (kg/m3), and fluorometry for July 2001 on the Sarasota transect. Taken from Ocean Circulation Group, College of Marine Science, Un iversity of South Florida website.
48 Figure 29. Cross shelf chlorophyll a (mg/m3) profiles for June 1998.
49 Figure 30. Cross shelf chlorophyll a (mg/m3) profiles for July 1998.
50 Another period of upwelli ng was in June 2000. Figure 24 shows the steeply doming isobaths observed at the shelf break (~80 m isobath). Very cold, upwelled water of 16C is seen at the shelf break. These steeply sloping isotherm s at the shelf break suggest a strong southward baroclinic cu rrent (He and Weisberg, 2003). Sea surface temperature (SST) images show a well defined frontal feature associated with the Loop Current (see He and Weisberg, 2003). Locat ed south of 28N, relatively cold water looped around anticyclonically and struck the WFS between the 200 m and 75 m isobaths (see He and Weisberg, 2003). Currents at these isobaths were exceptionally large peaking in July. Maxima near surface and near bottom flows at the 150 m isobath were 1.0 m s-1 and 0.4 m s-1, while on the 75 m isobath they were 0.3 m s-1 and 0.25 m s-1 respectively, whereas typical speeds are about 0.1-0.2 m s-1 (He and Weisberg, 2003). Chlorophyll fluorescence (Fig. 24) is seen at the shelf break extending inshore along the bottom of the shelf. He and Weisberg ( 2003) explained the links between chlorophyll fluorescence patterns and across sh elf movement of material: Â“Chlorophyll fluorescence re quires two ingredients: nutrients and light. The upwelled water provides nutrients and the shallow depths provide for the light. Nutrient concentrations ma y also be elevated near-shore due to land drainage through Tampa Bay and Ch arlotte Harbor estuaries. These two sources of nutrients (shelf-break and near-shore), both with available light, are connected through the bo ttom Ekman layer. Thus, and especially under stratified conditions (Weisberg et al, 2001), the bottom Ekman layer provides an effective ac ross-shelf conduit for the delivery of biologically important materials. Â…t he inner shelf circulation in June
51 2000 was primarily wind-induced downwe lling type. This was reflected in the changes of the temperature, density, and fluorescence isolines between June 6 and June 28 transects, attesting to the bottom Ekman layer playing a critical part in th e WFS biological productivity.Â” Even in years of weak upwelling and low near bottom nutrients, near bottom phytoplankton populations can still be 0.40 mg/m3. Another explanation for the high near bottom chlorophyll could be that the phytoplankton have increased thei r cellular photosynthe tic pigment content as a result of living at lower light levels. Most phyt oplankton cannot remain fixed in space with respect to a light field, because they are at th e mercy of the motion in the water column to remain in the euphotic zone, and as a result, ex perience large variations in light intensities throughout the course of the day (Falkowsk i, 1980). Therefore, many species of phytoplankton maintain a remarkable degree of physiological plasticity allowing them to respond to wide variations in light inte nsity (Steeman-Nielsen, 1975). Light-shade responses are generally charac terized by one or more of the following: 1) changes in photosynthetic pigment concentrations, 2) changes in the ratios of photosynthetic pigments, 3) modification of photosynthesis-irradiance profiles, 4) changes in enzyme activity, especially those associated with carbon fixation, and 5) changes in cell volume, respiration rates, and chemical compositi on (Falkowski, 1980). Johnsen and Sakshaug (1993) found that Chl a per cell was 1.1 to 2.6 times higher in shade adapted than in light adapted cells. However, light-shade adap tation responses are species specific. For example, the chlorophyll conten t of the marine chlorophyte Dunaliella tertiolecta can vary by a factor of about fi ve between 30 and 600 Ein m-2 s-1, whereas the neritic diatom Skeletonema costatum grown under similar light intensities had a cellular chlorophyll content that varied only about twofold (Falkowski, 1980). Shade adapted
52 algae are often capable of utilizing low light intensities with a higher photosynthetic efficiency (on a per cell basi s) than light adapted cells (Falkowski, 1980), thus making them perfectly suited to near bottom envir onments with naturally lower ambient light levels. Also, some of these near bottom phytoplankt ers may be living heterotrophically in deeper waters. El-Sayed (1972) recorded high Chl a values below the euphotic zone with no corresponding uptake of 14C. Riley and Chester (1971) have expressed similar phenomenon for other oceanic areas. Vargo et al. (in revision) also mentioned that the benthic community of the WFS supports a diverse autotrophic and heterotrophic benthic community. 3.2 Phytoplankton Blooms and Spatial Distribution A feature seen in both the surface and near bottom Chl a distributions was the presence of phytoplankton blooms. These are te mporally and spatially discrete events of high phytoplankton populations and were seen on the shelf as localized patches of high Chl a concentrations. Direct live counts ex amined onboard the ship at the time of sampling identified the presence and abundance of the different phytoplankton populations that comprised each of these bloom s. Table 2 is a list of the known blooms seen during the study, when they occurred, and what type(s) of phytoplankton were associated with each bloom. Blooms consisting of various species of diatoms are common on the WFS, especially inshore near the mouths of Tampa Bay and Charlotte Harbor. Common species seen include: Skeletonema costatum, Coscindodiscus spp. Rhizosolenia spp. Thalassiosira spp. Chaetoceros spp. and Guinardia flaccida. All of these species were listed in SteidingerÂ’s review (1973) as estuarine/coastal types. Guinardia flaccida was listed in the coastal/open Gulf assemblage. Diatom dominated blooms occurred during
53 Table 2. Phytoplankton blooms in the ECOHAB: Florida study area Karenia brevis blooms Maximum Chlorophyll a ( mg/m3) Comments November 1998 Â– February 1999 5.03 October 1999 Â– March 2000 27.1 October 2000 Â– Novemver 2000 4.64 October 2001 Â– December 2001 4.59 Diatom blooms Maximum Chlorophyll a ( mg/m3) Comments July 1999 8.68 Rhizosolenia spp. dominated bloom September 2000 0.85 Guinardia flaccida bloom October 2000 4.63 S. Coscindodiscus spp., Rhisosolenia spp. April 2001 1.97 May 2001 4.19 Dinoflagellate bloom Maximum Chlorophyll a ( mg/m3) Comments August 1999 0.24 5000 cells/liter at offshore stations 13, 15 April 2000 1.91 inshore station 51 Trichodesmium bloom Maximum Colonies L-1 Comments February 1999 6500 August 2000 433 September 2000 61 October 2000 120
54 the summer rainy season (July 1999, Septem ber 2000), early fall (October 2000) and in the spring (April and May 2001) and led to the estuarine signature in the surface chlorophyll isopleths seen in Figs. 7, 9, and 10. A Karenia brevis bloom in October 2000 (Table 2) also contributed to the elevated chlorophyll in nearshor e waters during that month (Fig. 7). Another potential source of phytoplankton populations th at contribute to the spatial distribution of surface chlorophyll is the Loop Current transport of seed populations of Trichodesmium erythraeum from the Caribbean. King (1950) reported that this N2-fixing cyanobacterium forms dense blooms in the eastern Gulf of Mexico from February to August. During my study, background concentrations of T. erythraeum could be found in all months sampled in 1999 and 2000 as well as July, September, November, and December of 2001. Compared to background concentrations of 0.75 colonies L-1, bloom concentrations were seen in Fe bruary 1999 at station 41 in excess of 6500 colonies L-1 (Fig. 31). Summer concentrations averaged 20 colonies L-1 (Fig. 32 and Lenes et al., 2001), but March (Fig. 31) and December 1999 (Fig. 32) also had similar concentrations. Low concentrations from background to ~ 5 colonies L-1 were seen across the entire study ar ea in the first four months of 2000 (Fig. 33). August 2000 had concentrations as high as 400 colonies L-1, decreasing to 55 colonies L-1 in September, and then increasi ng slightly to 110 colonies L-1 in October (Fig. 34). Summer and fall of 2001 had low concentrations of Trichodesmium spp no higher than 5 colonies L-1 (Fig. 35).
55 Figure 31. Surface Trichodesmium spp. concentrations (colonies/liter) from January 1999 to June 1999.
56 Figure 32. Surface Trichodesmium spp. concentrations (colonies/liter) from July 1999 to December 1999.
57 Figure 33. Surface Trichodesmium spp. concentrations (colonies/liter) from January 2000 to July 2000. No cruise in February 2000.
58 Figure 34. Surface Trichodesmium spp. concentrations (colonies/liter) from August 2000 to November 2000, and April to May 2001.
59 Figure 35. Surface Trichodesmium spp. concentrations (colonies/liter) from June 2001 to December 2001.
60 During the summer months, iron laden Saharan dust is carri ed across the Atlantic Ocean by the prevailing winds. This iron is deposited on the oligotrophic WFS and is utilized by Trichodesmium for nitrogen fixation and growth (Lenes et al., 2001). Subsequently, Trichodesmium releases ammonium, amino acids, and other dissolved organic nitrogen (DON) which may fuel initial populati on increases of Karenia brevis (Walsh and Steidinger, 2001; Lenes et al., 2001). It has been observed that large K. brevis blooms frequently co-occur or occur subsequent to blooms of Trichodesmium spp (Walsh and Steidinger, 2001). Four large Karenia brevis blooms occurred during the study period and also contributed to the spatial heterogeneity of chlorophyll concentration in nearshore waters between Tampa Bay and Charlotte Harbor. A ll of these blooms lasted several months and were spatially extensive (Vargo et al., 2004). Table 2 gives the dates and maximum chlorophyll a concentrations for each bloom, while Figures 36 through 39 show the locations and concentrations (cells L-1). The 1998-1999 bloom started offshore of Charlotte Harbor in November 1998 (Fig. 36) wh ere it persisted before being transported north in February 1999. Vargo et al. (2001) determined that northward flowing currents could have transported the bloom from Char lotte Harbor to Tampa Bay within the one month time frame. Highest K. brevis populations occurred in the October 1999 to March 2000 bloom (Fig. 37), reaching populations of >5 million cells L-1 and Chl a concentrations of 27.10 mg/m3 at station 32 on the Sarasota transe ct. By January 2000, this bloom covered the area between the two estuaries out to ~ 30 m isobath. Karenia brevis populations decreased by March 2000 when the remnants of the bloom were last seen off Charlotte Harbor (Fig. 37). Later in 2000 a second bloom was detected just north of Charlotte Harbor along the 8 m isobath, which spread north and south covering the area between the two estuaries by November 2000 (Fig. 38). A unique feature of this bloom was a
61 Figure 36. Surface Karenia brevis concentrations (cells/liter) for the November 1998 to February 1999 bloom.
62 Figure 37. Surface Karenia brevis concentrations (cells/liter) for the October 1999 to March 2000 bloom.
63 Figure 38. Surface Karenia brevis concentrations (cells/liter) for the October 2000 to November 2000 bloom.
64 Figure 39. Surface Karenia brevis concentrations (cells/liter) for the October 2001 to December 2001 bloom.
65 second patch which developed along the Tamp a Bay transect at the 30 m isobath (Fig. 38). This patch was totally isolated from th e nearshore bloom and was associated with a salinity front (Vargo et al., 2004). The bloom, which started in October 2001 off the mouth of Tampa Bay (Fig. 39), later spread throughout the area wi th populations > 1.5 million cells L-1. This bloom exhibited a highly patchy dist ribution along the coast out to the 30 m isobath (Fig. 39). As previously noted, Trichodesmium spp. may play a key role in the development of K. brevis blooms, since it could supply the nitroge n required to support high biomass if sufficient phosphorus is available. Lene s et al. (2001) estimated that 8.4 mol kg-1 of new total nitrogen could have been available to K. brevis populations during the summer of 1999. It followed the b acterial degradation of Trichodesmium released DON and photolysis of the intact Trichodesmium population, thus providing for such a large red tide in the fall. Examination of the spatial distribution of Trichodesmium, prior to and during the K. brevis blooms described above, suggests th at there is not a high degree of coherence between the two species. Trichodesmium data are not available for the 19981999 bloom, but abundant populations of Trichodesmium in October and November 2000 (Fig. 34) coincide with the distribution of K. brevis during those months (see Fig. 38). However, Trichodesmium populations were low prior to and during the intense 2001 bloom (see Fig. 35) although popul ations did co-occur with K. brevis (see Fig. 39) Walsh and Steidinger (2001) concluded in a study of four sets of time series taken from 42 years of K. brevis red tide data that the likeli hood of a large, long red tide at the shoreline emerges from a sequence of events that include the following: 1) summer Saharan dust events, 2) sufficient rain fall, 3) dissolution of aeolian iron, 4) seed stocks of both Trichodesmium and K. brevis, 5) Trichodesmium release of DON to all dinoflagellate competitors, 6) selective grazing stress on faster growing, non-toxic dinoflagellates and diatoms, and 7) down welling-favorable, onshore winds and flow
66 fields that allow for landward transport of blooms to convergent fronts of estuarine phosphorus supplies. Interannual variations in the size of red tides are modulated by diatom raised zooplankton during years of strong slope water intrusions of nitrate (Walsh et al., in review). Decay and remineraliza tion of the near bottom diatom populations found seaward of the 20 m isobath (Heil et al., 2001), combined with breakdown of thermal stratification in fall by vertical mi xing, may be another s ource of N for bloom development and growth (Vargo et al., in revision). Modeled benthic flux values indicated N flux from remineralization of benthic communities meet the growth requirements of moderate K. brevis populations up to approximately 2.6x 104 cells L-1 and that this flux may be sufficient to mainta in standing stocks of inorganic N (Vargo et al., in revision). Vargo et al (in revision), after analyzi ng many other possible nutrient sources that could support large blooms, concluded 1) N and P from decaying fish theoretically could maintain populations at moderate concentrations, 2) zooplankton excretion of ammonia was only sufficient to main tain populations of no more than 104 cells L-1 although phosphorus excretion could supply all of the P required for 106 cells L-1, and 3) estuarine flux of N and P can meet high biomass bloom requirements only if the K. brevis populations were located in the im mediate vicinity of the estuary. 3.3 Seasonal Trends The average surface and near bot tom concentrations of Chl a and their observed range for each transect for each of the four seasons can be found in Tables 3 and 4. Tampa, Sarasota, and Ft. Myers transect data are for the stations to the 50 m isobath. This represents the complete Tampa and Ft. Myers transects and stations 23 to 32 on the Sarasota transect. Offshore Sara sota represents stations on th e Sarasota transect that are seaward of the 50 m isobath out to the 200 m isobath (stations 11-21).
67 Table 3. Average surface concentrations of chlorophyll a and the observed range of concentrations (mg/m3) along each transect. Year/Season Tampa Sarasota Ft. Myers Offshore Sarasota 1998 Spring ND ND ND ND (Range) Summer 0.58 0.48 0.47 0.21 (Range) (0.25-2.28) (0.17-0.98) (0.15-1.11) (0.07-0.42) Fall 1.13 0.80 1.05 0.12 (Range) (0.10-5.03) (0.25-3.14) (0.29-5.70) (0.06-0.20) Winter 0.46 0.46 0.94 0.22 (Range) (0.12-2.04) (0.15-1.48) 0.16-3.56) (0.02-0.57) 1999 Spring 0.44 0.27 0.34 0.11 (Range) (0.11-2.62) (0.12-0.81) (0.10-1.12) (0.07-0.27) Summer 0.46 0.32 0.53 0.11 (Range) (0.06-2.87) (0.06-1.39) (0.07-3.05) (0.01-0.24) Fall 1.54 1.61 1.02 0.18 (Range) (0.19-8.91) (0.20-10.62) (0.16-3.19) (0.12-0.30) Winter 0.32 0.36 0.59 0.16 (Range) (0.14-1.01) (0.20-0.83) (0.22-1.73) (0.13-0.20)
68 Table 3 continued. 2000 Spring 0.30 0.18 0.39 0.09 (Range) (0.06-1.52) (0.07-0.43) (0.06-1.91) (0.05-0.14) Summer 0.42 0.45 0.33 0.11 (Range) (0.06-1.69) (0.07-2.51) (0.08-1.73) (0.04-0.21) Fall 0.80 0.51 0.76 0.12 (Range) (0.10-4.63) (0.16-2.29) (0.07-4.37) (0.09-0.18) Winter ND ND ND ND (Range) 2001 Spring 0.63 0.35 1.23 0.13 (Range) (0.13-3.34) (0.12-1.06) (0.15-4.19) (0.09-0.22) Summer 0.58 0.37 0.88 0.10 (Range) (0.10-3.41) (0.08-1.47) (0.08-3.86) (0.06-0.20) Fall 1.17 0.86 1.27 0.14 (Range) (0.10-5.76) (0.10-5.22) (0.08-3.28) (0.08-0.20) Winter ND ND 0.63 ND (Range) (0.19-4.59) ND No Data Spring March-May Summer June-August Fall -September-November Winter December February
69 Table 4. Average near bottom concentrations of chlorophyll a and the observed range of concentrations (mg/m3) along each transect. Year/Season Tampa Sarasota Ft. Myers Offshore Sarasota 1998 Spring ND ND ND ND (Range) Summer 1.48 1.67 1.14 0.23 (Range) (0.46-3.35) (0.50-5.45) (0.48-4.17) (BDL-0.56) Fall 2.07 1.62 1.49 0.21 (Range) (0.81-4.85) (0.27-3.72) (0.41-5.55) (0.02-0.64) Winter 1.03 0.95 0.97 0.19 (Range) (0.38-2.32) (0.44-2.06) (0.32-2.07) (BDL-0.65) 1999 Spring 0.55 0.42 0.43 0.23 (Range) (0.22-2.79) (0.23-0.88) (0.21-1.18) (0.01-0.88) Summer 0.76 0.65 1.01 0.25 (Range) (0.26-2.96) (0.09-1.29) (0.07-5.11) (0.01-1.87) Fall 1.74 2.01 1.30 0.19 (Range) (0.37-7.75) (0.36-16.80) (0.23-5.08) (0.01-0.41) Winter 0.57 0.50 0.63 0.15 (Range) (0.36-0.93) (0.28-0.81) (0.38-1.41) (0.01-0.47)
70 Table 4 continued. 2000 Spring 0.45 0.32 0.46 0.29 (Range) (0.12-1.55) (0.21-0.46) (0.22-2.13) (0.01-0.78) Summer 0.81 0.77 0.49 0.21 (Range) (0.23-1.66) (0.37-2.60) (0.14-1.63) (0.01-0.86) Fall 0.89 0.90 1.19 0.19 (Range) (0.16-4.60) (0.29-3.81) (0.25-6.31) (0.01-0.49) Winter ND ND ND ND (Range) 2001 Spring 0.87 0.85 1.35 0.34 (Range) (0.26-2.14) (0.33-2.88) (0.27-3.66) (0.01-1.24) Summer 0.99 0.61 1.18 0.23 (Range) (0.29-3.76) (0.28-1.57) (0.27-4.20) (0.01-0.68) Fall 1.19 1.13 2.21 0.28 (Range) (0.39-4.21) (0.42-4.32) (0.30-8.74) (0.02-0.59) Winter ND ND 1.73 ND (Range) (0.64-4.25) BDL Below Detection Limits ND No Data Spring March-May Summer June-August Fall -September-November Winter December February
71 The typical seasonal pattern of chlorophyll concentrations in coastal waters of the WFS can be characterized by somewhat elevated concentrations in fall relative to other seasons when increased rainfall fueling diatom blooms and red tide blooms yield approximately 1.5 to 2-fold increases. Averag e values for spring, summer, and winter are typically <1.5 mg/m3 out to the 50 m isobath with offshore Sarasota values showing no seasonal pattern and concentrations which rarely exceed 0.3 mg/m3. 3.4 Isobath/Transect Comparison 3.4.1 10 Meter Isobath Along the 10 m isobath, depth integrated Chl a values ranged between 1.05 and 70.4 mg/m2 but most values were between 1.05 and 11.95 mg/m2 (Fig. 40). The overall trend on the 10 m isobath was ch aracterized by increasing Chl a in late summer with the highest values in the fall (September October, and November). Several Karenia brevis blooms on this isobath contributed to the hi gher values in the fa ll/winter of 1999, fall of 2000, and fall/winter of 2001. Diatom blooms at station 51 on the Ft. Myers transect were reflected in elevated chlor ophyll concentrati ons (20.15 and 20.4 mg/m2) in the summer of 1999 and late spring of 2001. Similarly, a Guinardia flaccida bloom in the fall of 2000, also at station 51 on the Ft. My ers transect, yielded values of 26.7 mg/m2. The nearshore station on the Ft. Myers transe ct (station 51, Fig. 40) typically had values higher than similar stations on the Tampa or Sara sota transects. This is most likely due to the influence of Charlotte Harbor and the Ca loosahatchee River on the coastal waters off of Ft. Myers. Nutrient flux from the estu aries and the rivers undoubt edly contributes to maintenance of higher phytoplankton biomass in nearby coastal waters (Vargo et al., in revision). Rivers of different sizes, nutrient loading, and flow rate s interface with the coastal waters of the WFS. These rive r systems drain a wide variety of watersheds with differing
72 Figure 40. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 10 meter isobath.
73 land uses, including agri culture, ranching, and urban development (Heil et al., submitted). The Peace River, which drains into Charlotte Harbor, lies within a watershed containing the Hawthorne phosphatic deposits, which have been mined since the 1880Â’s (Pittman, 1990). This watershed also cont ains considerable citrus grove s as well as cattle ranches (Heil et al., submitted). Increasing developm ent and urban pressures along the coast in the Charlotte Harbor region are also leading to increased sewage loadings into receiving waters (Heil et al., submitted). The Caloos ahatchee River system is heavily impacted by nutrient inputs from the sugar and citrus i ndustries within the Everglades Agricultural Area (Heil et al., submitted). Heil et al. (submitted), in a study of th e coastal area between Tampa Bay and the western Florida Keys, found that PO4 (max 1.5 M) and DOP (max 5.4 M) concentrations were highest in coastal areas adjacent to outflows from Charlotte Harbor and Tampa Bay. Inorganic N concentrations were low, while DON distributions ranged from 10.8-30.0 M nearshore along the entire region decreasing seaward to 1.9-8.0 M. Ammonium (NH4 +) was the dominant form of inorganic N ranging from 0.2 to 2.7 M N. DON concentrations were approx imately an order of magnit ude greater than dissolved inorganic nitrogen (DIN) concentrations. Silica values were between 5 and 23 M. Nutrient ratios showed the area between Tampa Bay and Sanibel to be N limited. The phytoplankton community composition in the vi cinity of Charlotte Harbor and the Caloosahatchee River showed a mixture of zeaxanthin containing cyanobacteria and peridinin and gyroxanthin di-e ster containing flagellates (Heil et al., submitted). According to Heil et al. (submitted), Â“the data suggest that inorganic and organic N and P nutrient fractions in the near shore region reflect the longit udinal gradients in watershed
74 characteristics within the region, and that va riations in nutrient form in turn drive phytoplankton community composition in these coastal watersÂ”. Vargo et al. (in revision) al so found N concentrations in coastal water to be low (<0.5 M) and that concentrations varied little s easonally or with distance offshore. It was noted that Tampa Bay also lies in th e Hawthorn phosphatic depos its and is highly P enriched. Thus, both estuarie s are typically N limited and show very low DIN:DIP ratios (Vargo et al., 2001; Heil et al., 2001). Vargo et al. (in revisi on) also found that values of P and Si are elevated at the 10 m isobath and display a distinct seas onal pattern occurring in late summer and fall in relation to the rai ny season (Fig. 4). DIP and DOP also peak in late summer and fall and have similar ranges of concentrations in relation to each other. When typical nutrient concen trations found at the mouth of Tampa Bay and Charlotte Harbor are compared to nutri ents along the 10 m isobath, Va rgo et al. (in revision) found reduction of almost 70%. They suggest that a combination of dilution and biological uptake reduces the estuarine concentrations. Therefore estuarine nutrients are used within 1-3 km off the estuary and will have little impact in offshore waters. Chlorophyll values from the stations on the Tampa and Sarasota transects followed similar patterns. These stations are geographically close and Tampa Bay has less of an influence on coastal waters compared to the larger estuary of Charlotte Harbor. Tampa Bay receives drainage from a waters hed of 2,235 square miles compared to 2,657 for Charlotte Harbor (Ross, 1973) and the calc ulated average daily volume of water into Tampa Bay from all tr ibutaries is 1.32 x 1010 L compared to 2.85 x 1010 L for Charlotte Harbor (Vargo et al., in revision ). This difference can also be seen in the average mean monthly river discharge to each estuary (Fig. 41).
75 The 10 m isobath is influenced mostly by local scale processes such as river and estuarine outflow, wave effects, and near shore circulation (Lohrenz et al., 1999). Nearshore coastal areas tend to exhibit high chlorophyll levels due to increased nutrient inputs from land runoff, riverine and estuar ine flux, resuspension of sediments, pore water nutrients, and recycling of nutrients. Nu trient loads of terrestrial origin to the
76 Figure 41. River discharge (cubic feet/second) into Tamp a Bay and Charlotte Harbor. Monthly Mean Streamflow into Tampa Bay0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 JanFebMarAprMayJunJulAugSeptOctNovDecMonthFlow (cu. ft/s) 1998 1999 2000 2001 50 Year AVG*Summ of flow from USGS gage stations Alafia at Lithia, Little Manatee at Wimauma, Hillsbourough River at Tampa, FL. Monthly Mean Streamflow into Charlotte Harbor0 1000 2000 3000 4000 5000 6000 7000 8000 9000 JanFebMarAprMayJunJulAugSeptOctNovDecMonthFlow (cu. ft/s) 1998 1999 2000 2001 50 Year AVG*Summ of flow from USGS gage stations Peace River at Arcadia, Myakka River near Sarasota, FL.
77 nearshore waters of the WFS have increased by an order of magnitude (Okey et al., 2004). This is most likely due to the increa se urbanization and ag ricultural use in the watersheds of the study area (Heil et al., submitt ed; Vargo et al., in re vision). Bissett et al. (2005) noted that terrestrial c oncentrations of DON, S, DOP, NO3, orthophosphate, and colored dissolved organic matter (CDOM) were greater in rivers and estuaries flowing onto the WFS than concentrations of these materials at the offshore boundary. Bissett et al. (2005) also noted that slight elevation in near shore satellite derived CDOM signals, Chl a and backscatter are indicative of rive rine sources. In 1998, fall peaks in nutrient concentrations were co rrelated with peaks in discharges released from the Peace River and elevated Chl a concentrations were co-localized with regions of lower salinity found at the mouth of Charlotte Harbor (Bissett et al., 2005) These large freshwater flows from Charlotte Harbor in the fall were the result of the passage of Hurricanes Georges (September 16-19) and Mitch (October 22-Nov 5). These large freshwater flows can also set up salinity fronts, which may ha ve the ability to i nhibit the cross-shelf exchange of dissolved and suspended material s (i.e. Blanton, 1981), thereby affecting the residence time of nutrients and phytoplankton on the inner shelf (Yoder, 1985). Longer residence times favor the recycling of nutrients. Nearshore circulation has a large impact on the distribution of nutrients and other dissolved and particulate materi al in the nearshore zone. Northerly winds cause surface and mid level currents near Tampa Bay to move offshore, while bottom currents move onshore. The strong coastal jet is evident in the shelf circul ation, particularly near Tampa Bay (Yang et al., 1999). Convergent coasta l geometry and the bottom topography from south of Tampa Bay strengthens the coastal je t and the bottom and su rface transport, and induces a maximum local upwelling near Tamp a Bay (Yang et al., 1999). This localized upwelling could further enhance biomass incr eases by adding additional nutrients from the shelf with those of estuarine origin from Tampa Bay. Vargo et al. (in revision) noted
78 that except for DIN, the three year average of N and P species at the mouths of each estuary were higher in Tampa Bay than in Ch arlotte Harbor. Greater urbanization in the St. Petersburg area was suggested to explain the difference, bu t the upwelling in this area also may also contribute to the increased values. Other local scale processes, such as tida l and wave induced turbulence and their effects on phytoplankton biomass, are poorly un derstood (Lohrenz et al., 1999). Vertical mixing in the shallow coastal zones can also affect phytoplankton by limiting the available light due to sediment resuspension (L ohrenz et al., 1999). In the coastal waters off of Georgia, Oertel and Dunstan (1981) showed that areal pr oduction was highest in the turbid zone off the coast of Georgia in spite of the very shallow compensation depth (1% light level at 1 m). Oe rtel and Dunstan (1981) expl ain that the phytoplankton are constantly moving in and out of the surface where light is sufficient for photosynthesis, thus allowing them to overcome light limita tion. Also, highly turbid waters introduce regenerated nutrients from sedi ment into the water column. Phaeopigment concentrations showed sim ilar seasonal patterns as for chlorophyll and were typically about 50% of the chlo rophyll concentration as indicated by the phaeopigment:chlorophyll (P:Chl) ratio (Fig. 40). 3.4.2 35 Meter Isobath Along the 35 m isobath, integrated ch lorophyll values ranged between 3.3 and 41.57 mg/m2, but most values were between 3.3 and 20 mg/m2 (Fig. 42). Chlorophyll values increase late in the summer with fall maxima. In July of 1998, the effect of upwelling is seen as an increase in standing st ock on the Sarasota transect (Fig. 42). In June 1998, cold (~20C) water was upwelled ont o the shelf as far inshore as the 20 m isobath on the Sarasota transect (Fig. 20). By July, the waters were still cool on the shelf and a bolus of the cold ~20C water could be seen at the 40 m isobath on the Sarasota
79 Figure 42. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 35 meter isobath.
80 transect (Fig. 21). Associat ed with this cold water bol us was a large phytoplankton population with maximum chlo rophyll values of 5.91 mg/m3 (Fig. 30). Karenia brevis and diatom blooms contribute to chlorophyll stocks at this isobath. Karenia brevis blooms occurred in the winter of 1999 and fall of 2001, while diatom blooms were extant in the summer 1999, summer/fall of 2000, and the spring of 2001. Generally, chlorophyll values seen on the 35 m isobath are influenced by the near bottom chlorophyll values. Average surface values at this isobath range between 0.20-0.40 mg/m3 (Fig. 2), whereas average near bottom values range between 0.60-0.80 mg/m3 (Fig. 3). Vargo et al. (in revi sion) noted concentrations of Si at the 25 m isobath were occasionally higher than those seen at the 50 m isobath, indicating estuarine influence and nutrient flux may extend to the 25 m isobath Examination of Si concentrations at the 35 m isobath reveal that they are very sim ilar to those seen at the 50 m isobath. Thus, estuarine input is probably not an important regulator of phytoplankton biomass these locations. Mesoscale processes (10 to 300 km ), which may influence phytoplankton dynamics at this isobath and contribute to the patterns seen, could include: wind induced upwelling, meteorological forcing, regional ci rculation, internal waves, topographic effects, fronts, and Loop Current circulation (Lohrenz et al., 1999) Liu and Weisberg (2005) noted that the inner, middle, and outer shelf regions are c ontrolled by different dynamical forces. Li and Weis berg (1999b) found that the i nner shelf is the region of transition from a near shore balance between surface and bottom stress to a mid shelf balance between surface stress and Coriolis force (Ekman balance), while outer shelf variability is influence by deep ocean forci ng along with local winds (Liu and Weisberg, 2005). Wind forcing is largely responsible fo r inner shelf (50 m to shore) circulation patterns (He and Weisberg, 2003).
81 The bottom Ekman layer is a major conveya nce for the across shelf transport of material on the WFS (Weisberg et al., 2001; Weisberg and He, 2003). Materials are transported from the shelf break to the mi d shelf regions by Loop Current induced flows and bottom Ekman layer responses. Locally driven flows and their bottom Ekman layer responses (amplified by the Loop Current effect ) then take over to transport materials nearshore (He and Weisberg, 2003). Modeled tr ajectory tend to intercept the nearshore region between Tampa Bay and Charlotte Harb or, consistent with the local upwelling maximum argument advanced by Weisberg et al.(2000). This may be one reason K. brevis blooms are found in this area more of ten then any other area in the Gulf. Similarly, the dominant process controlli ng the supply of Â“new Â” nutrients to the middle and outer portions of the Southeastern Continental Shelf (SEC) is upwelling at the shelf break in response to eddi es and meanders in the Gulf Stream front (Yoder, 1985). The distance that upwelled waters penetr ate across the SEC shelf depends on wind velocity, local topography, and the density of resident shel f waters (Atkinson, 1985). The dynamics of production on the middle shelf are principally controlled by processes that transport nutrients across the inner and outer shelf zones (Yod er, 1985). It is very likely that the middle WFS chlorophyll distributi ons are also being controlled by local processes that transport nutrients acro ss the inner and outer shelf zones. Hurricanes and tropical storms have the poten tial to significantly increase vertical advection of nutrients into surface waters, thereby causing an increase in phytoplankton biomass (Lohrenz et al., 1999). These large, pow erful storm systems also have the ability to affect shallow coastal and shelf waters with 1) increased loadings of terrestrial material due to land run-off, 2) sediment resuspensi on, this in turn, affect s nutrients and light availability, 3)the creation of frontal zones due to large fr esh water inputs and, 4)wind induced upwelling/downwelling of waters associated with the high winds of the system.
82 Effects on primary production have been re ported by Franceschini and El-Sayed, 1968 and Iverson, 1977. The Atlantic hurricane season lasts from the first of June through the end of November. Several hurricanes went near or through the study area during the hurricane seasons of 1998-2001. The 1998 hurricanes in cluded Earl (August 31-September 3), Georges (September 16-29), and Mitch (Oct ober 22 to November 5). Large fall fresh water flows from Charlotte Harbor were a ssociated with the passages of Hurricanes Georges and Mitch (Bissett et al., 2005). Tr opical Storm Harvey occurred in September 19-22, 1999. In September 14-18, 2000, Hurricane Gordon came within ~165 nautical miles southwest of Tampa and caused power outages and minor structural damage along the coastal areas of the study area. Hurricane Gabrielle occurred September 11-19, 2001. This hurricane and its associ ated rainfall caused major floodi ng of several rivers that empty into Tampa Bay and Charlotte Ha rbors (NOAA/National Weather ServiceNational Hurricane Center data). Very high river discharges into Tampa Bay (2948 cubic feet s-1) and Charlotte Harbor (8361 cubic feet s-1) during September 2001 can be seen in Fig. 41. Because hurricanes are infrequent a nd happen over restricted spatial extents, their integrated impact over longer tempor al and spatial scales is probably minor (Lohrenz et al., 1999). The passage of meteorologi cal fronts can have an impact on phytoplankton and primary productivity by assisting with the brea k down of shelf stratif ication, resuspension of regenerated nutrients from bottom waters, and by deepening the mixed layer. Passage of meteorological fronts on the Louisiana Shelf have resulted in break down of stratification and ventilation of oxygen depl eted shelf bottom waters during spring and summer (Wiseman et al., 1986, 1992). Dagg (1988) observed mixing of low nutrient shelf water into the surface laye r after the passage of a front. In the open Gulf, Ortner et al. (1984) observed a deepening of the mixed layer during the passage of a winter front.
83 In addition, they noticed a shoaling of th e nutricline and an increase in primary production. Modeling the effects of the passage of a hurricane, Iverson (1977) concluded that the associated deepening of the mixed layer c ould result in a two to threefold increase in nitrate in the euphotic zone over an approximate 100 km wide track. Phaeopigment concentrations varied little between transects and tracked chlorophyll concentrations with similar seasonal patterns. The P:Chl ratio was elevated relative to the 10 m isobath values with most values above 0.5. 3.4.3 50 Meter Isobath Along the 50 m isobath, Chl a values integrated over 45 m ranged between 5.2 and 57.68 mg/m2, but most values were between 5.2 and 20 mg/m2 (Fig. 43). However, there is a distinct lack of seasonality as would be expected in a typical tropical oligotrophic area. There is greater similarity between the stations of all three transects along this isobath compared to the 10 and 35 m isobaths. Elevated near bottom chlorophyll concentrations al ong the shelf drive the patter ns seen in Figure 43. Mesoscale processes, as mentioned above on the 30 m isobath, are most likely controlling these patterns. One mesoscale process not mentioned previ ously is the effect of eddies. Studies have shown their impacts on primary pr oduction rates (Biggs, 1992) and chlorophyll concentrations (Biggs and Mller-Karger, 1994). Upwelling at the periphery of anticyclonic rings and the cen ter of cyclonic rings may in crease vertical inputs of nutrients that can then be utilized by phytoplankton populatio ns (Lohrenz et al., 1999). Interaction of Loop Current eddies with the continental margin may transport high chlorophyll shelf waters offshore in the form of a jet or squirt (Biggs and Mller-Karger, 1994). It is suggested that additional vertical entrainmen t of nutrients across the
84 Figure 43. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 50 meter isobath.
85 nutricline may accompany the above feature (Lohrenz et al., 1999). Also upwelling along the edge of the Loop Current is a majo r source of nutrients to the euphotic zone (Wiseman and Sturges, 1999). Walsh et al. (1 989) estimated that this process delivers three times that amount of nitrogen to the euphot ic zone as is delivered by the Mississippi River. It is suggested this upwelling causes a two to threefold increase in the annual rate of primary production in the GOM. Phaeopigment concentrations are only sli ghtly lower than chlorophyll levels at this isobath which are reflected in the elev ated P:Chl ratio compared to the 10 and 35 m isobaths (compare Figs. 40 and 42 with Fig. 43). 3.4.4 86 and 200 Meter Isobaths Station 11 (200 m) and station 17 (86 m) were located offshore on the Sarasota transect and were the only st ations at these isobaths. Th ere is much data missing from both of these stations. In th e event of bad weather and ve ry rough seas, these stations were not sampled due to the danger it posed to the crew and equipment. No seasonal pattern is seen in either of these two offshore oligotrophi c stations (Fig. 44). Depth integrated values for the 86 m station ranged from 11.95 to 43.1 mg/m2, but most values were between 11.95 and 20 mg/m2. The elevated near botto m chlorophyll concentration noted at the 5 and 30 m isobaths is lost at these deeper stations, but subsurface chlorophyll maxima were included ov er the depth of integration. The deep chlorophyll maximum (DCM) is of ten associated with the depth of the pycnocline and also coinci des with, or is centered slightly above, the zone where nutrient concentrations rapidly increas e with depth (Lohrenz et al., 1999) (Figs 19 and 22). The DCM seen on these two stations is a prevalent feature in the open waters of the Gulf (Hobson and Lorenzen, 1972). El-Sayed and Turner (1977) noted that the deep
86 Figure 44. Depth integrated chlorophyll a (a), phaeopigment (b), and Phaeopigment:Chlorophyll a (c) along the 86 and 200 meter isobaths.
87 chlorophyll maximum appears to be a regular feature with world wide distribution, appearing in high latitude to tropical waters of the world ocean. The depth of the DCM on the WFS was variable, ranging from 20 to 150 m at station 11, but averaged 40 to 80 m. This variability is likely due to differen ces in irradiance and upward nutrient transport that are a function of currents and mixing (Var ela et al., 1992). Depth integrated values for the 200 m station ranged from 10.76 to 41.66 mg/m2, but most values were between 10.76 and 27.36 mg/m2. Synoptic scale processes, such as seas onal variations in solar and atmospheric conditions and Loop Current excursions, are like ly to control the patterns in chlorophyll in these offshore oligotrophic waters. Meteorol ogical variations in mixed layer depth and large scale circulation dominated by the Loop Current affect rates of primary production in the Gulf (Lohrenz et al., 1999). Seasona l meteorological forcing affects the water column hydrography by influencing temperature, density, stratificat ion, and circulation patterns (Lohrenz et al., 1999). These, in tu rn, affect the distribution of nutrients and phytoplankton on the shelf. Mller-Karger et al. (1991), using a numerical simulation and Coastal Zone Color Scanner (CZCS) im agery, suggested an annual cycle exists related to wind-induced variati ons in mixed layer depth thro ughout the Gulf of Mexico. High pigment concentrations were found in wi nter months with low values during late spring and summer. He and Weisberg (2003) suggested a seas onal cycle in which the circulation tends toward upwelling in the winter and downwe lling in the summer. Transitions between these two phases occur in the spring and fall a nd are caused by the change in surface heat flux. Due to these seasonal heat flux cha nges, combined with shoaling topography, the
88 across shelf temperature gradient changes dire ction from seaward to landward during the spring transition and the baroclinic circula tion flows northwestward. In the fall, the across shelf temperature gradient changes di rection from landward to seaward and the baroclinic circulation flows southeastward. This baroclinic circ ulation, adding either constructively or destructively with the wi nd driven circulation, provides both season and location dependent along shelf and across shelf current distribution. This circulation may account for some of the temporal and spatia l variability and dist ribution of phytoplankton on the WFS by affecting the distribution of properties and materials important to phytoplankton growth. Phaeopigment concentrations were of the same order as chlorophyll, and in some cases, at higher concentrations as reflect ed in the P:Chl ratio, which ranged from approximately 1.0 to over 2.0. This increase in the P:Chl ratio from the 10 m to the 200 m isobath suggests that processes which cont rol the degradation of chlorophyll increase from coastal to offshore waters. Several processes, including phyt oplankton growth, ce ll sinking, cellular senescence, zooplankton gr azing, and photo-oxidation, affect the concentrations and distribution of phaeopigments in the o cean (Welschmeyer and Lorenzen, 1985). Zooplankton grazing is consid ered to be a major pathwa y for phaeopigment production (Lorenzen, 1967; Daley, 1973; Head and Harris, 1996). Phaeopigments from macrozooplankton are packaged in large, rapidly sinking faecal pellets, whereas phaeopigments from microzooplankton are packaged in small faecal pellets that remain in suspension (Soohoo and Kiefer, 1982b; Welschmeyer and Lorenzen, 1985).
89 Barlow et al. (1993) stated that the pattern of phaeopigment distribution in the water column is the net result of production through grazing and loss by photo-oxidation. On the WFS inshore of the 50 m isobat h, average phaeopigments comprised from 43 to 68 percent of the measured Chl a, while offshore values were from 68 to over 100 percent (Table 5). These high values suggest that grazing may be an important regulator of Chl a biomass in this area. Sutton et al (2001) studied the estimated zooplankton grazing impact on the Sarasota transect in September 1998. They found that zooplankton distributions showed a strong correl ation with chlorophyll distributions.
90 Table 5. Phaeopigment:C hlorophyll Statistics. Tampa Transect Station 1 3 5 7 9 10 Average Percentage of Total Phaeopigment for Station 47.52 44.04 49.81 60.89 61.76 68.43 Standard Deviation of Total Phaeopigment for Station 13.30 11.13 12.87 45.39 17.55 33.47 Average Phaeopigment:Chlorophyll Ratio 0.48 0.44 0.50 0.61 0.62 0.68 Phaeopigment:Chlorophyll Standard Deviation 0.13 0.11 0.13 0.45 0.18 0.33 Minimum Surface Phaeopigment:Chlor ophyll 0.27 0.20 0.20 0.24 0.29 0.29 Maximum Surface Phaeopigment:Chlor ophyll 0.83 0.69 0.68 0.68 0.77 0.90 Minimum Bottom Phaeopigment:Chlorophyll 0.23 0.27 0.36 0.43 0.38 0.20 Maximum Bottom Phaeopigment:Chlorophyll 0.93 0.67 1.00 1.25 1.08 1.50 Sarasota Transect Station 23 25 27 29 30 32 Average Percentage of Total Phaeopigment for Station 60.86 56.73 51.41 50.32 49.16 47.38 Standard Deviation of Total Phaeopigment for Station 18.82 18.01 14.63 16.63 14.10 10.75 Average Phaeopigment:Chlorophyll Ratio 0.61 0.57 0.51 0.50 0.49 0.47 Phaeopigment:Chlorophyll Standard Deviation 0.19 0.18 0.15 0.17 0.14 0.11 Minimum Surface Phaeopigment:Chlor ophyll 0.17 0.23 0.23 0.18 0.03 0.21 Maximum Surface Phaeopigment:Chlor ophyll 0.89 0.88 0.72 0.79 0.82 0.71 Minimum Bottom Phaeopigment:Chlorophyll 0.44 0.16 0.39 0.35 0.19 0.20 Maximum Bottom Phaeopigment:Chlorophyll 1.21 1.17 0.86 1.64 0.75 0.74 Sarasota Transect Offshore* Station 11 13 15 17 19 21 Average Percentage of Total Phaeopigment for Station 150.60 150.52 144.03 110.39 83.80 68.49 Standard Deviation of Total Phaeopigment for Station 130.73 141.55 128.76 90.20 46.49 29.38 Average Phaeopigment:Chlorophyll Ratio 1.51 1.51 1.44 1.10 0.84 0.68 Phaeopigment:Chlorophyll Standard Deviation 1.31 1.42 1.29 0.90 0.46 0.29 Minimum Surface Phaeopigment:Chlor ophyll 0.29 0.24 0.17 0.10 0.23 0.28 Maximum Surface Phaeopigment:Chlor ophyll 1.48 8.01 1.98 0.83 0.87 0.75 Minimum Bottom Phaeopigment:Chlorophyll 0.27 0.57 0.86 0.79 0.63 0.58 Maximum Bottom Phaeopigment:Chlorophyll 6.85 3.98 7.00 4.10 3.82 2.78
91 Table 5 continued. Ft. Myers Station 40 42 44 46 48 50 51 Average Percentage of Total Phaeopigment for Station 62.62 58.95 53.50 48.04 45.03 43.48 43.33 Standard Deviation of Total Phaeopigment for Station 18.44 19.20 13.08 13.42 11.50 14.13 13.97 Average Phaeopigment:Chlorophyll Ratio 0.63 0.59 0.53 0.48 0.45 0.43 0.43 Phaeopigment:Chlorophyll Standard Deviation 0.18 0.19 0.13 0.13 0.11 0.14 0.14 Minimum Surface Phaeopigme nt:Chlorophyll 0.13 0.21 0.25 0. 25 0.17 0.18 0.16 Maximum Surface Phaeopigment:Chloro phyll 0.99 0. 91 0.92 0.63 0. 81 0.87 0.76 Minimum Bottom Phaeopigment:Chlorophyll 0.30 0.46 0.33 0.21 0.17 0.16 0.22 Maximum Bottom Phaeopigment:Chlorophyll 1.54 1.20 0.83 0.68 0.72 1.06 0.73 *Offshore Stations with bottom depth greater than 50 meters
92 Three zones were identified on the transect: 1) an offshore mixed la yer zone, seaward of the 30-35 m isobath; 2) an offshore subpyc nocline zone, whose landward extent was delimited by the intersection of the pycnocline and the botto m (30-35 m isobath); and 3) an inshore mixed layer zone landward of the 30-35 m isobath characterized by a near vertical salinity gradient. Chlorophyll maxima were observed in the offshore subpycnocline zone, particularly near the inte rsection of the thermocline and the bottom, and in the inshore salinity grad ient zone, mainly in the lower half of the water column (similar to the distribution seen in Fig.19) The offshore subpycnocline zone, showed a high abundance of low intensity grazers (i .e. small poecilostom atoid, small cyclopoid copepods, ostracods), while the inshore mixe d layer zone, showed moderate abundances of high-intensity grazers (i.e larvaceans). The offshore mixed layer zone, showed low abundances of all grazers (small calanoid cope pods dominated this zone). Sixty-five percent of the total grazing pressure was in the offshore subpycnocline zone, 29% was in the inshore mixed layer zone, and only 6% wa s seen in the offshore mixed layer zone. The areas with the most grazing, the offs hore subpycnocline and inshore mixed layer zones, are the areas of higher phaeopigment concentrations as seen in September 1999 (Fig. 45). This month/year has comparable Chl a distributions as seen in the Sutton et al. (2001) study. No phaeopigment data was available for September 1998 because the Welschmeyer method was used to determine ch lorophyll concentrations. Thus, it appears that zooplankton grazing is correlated with phaeopigment concentrations in the study area.
93 Figure 45. Cross shelf phaeopigment pr ofile for September 1999 on the Sarasota transect.
94 Alternate explanations for the phaeopigmen t distributions seen in the study area include: 1) resuspension of be nthic associated pigments due to the shallow nature of the WFS, 2) flux of particles derived from sh elf and estuarine detrital seagrass and/or estuarine derived mangrove and macroalgal ma terial, 3) cellular senescence associated with declining blooms, and 4) microbial degradation. Barlow et al. (1993) suggest caution in interpreting phaeopigment concentrations measured fluorometrically. Since the advent of high performance liquid chromatography (HPLC) for measuring pigments, the fluorescence method has been shown to overestimate phaeopigment concentrations, par ticularly when chlorophyll b is present (see Methods section for discussion). Gieske s and Kraay (1986), using HPLC, measured trace levels of phaeopigments at the deep chlorophyll maximum in the tropical Atlantic where chlorophyll b concentratio ns were significant. Conven tional fluorometry indicated and abundance of phaeopigments in the same area. Similarly, in a study in the North Pacific Central Gyre, Ondrusek et al. (1991) detected low or undetectable phaeopigment levels by HPLC, where Welschmeyer and Lore nzen (1985) had previously measured high P/Chl ratios in the same area at approximately the same time of year using fluorometry. Thus, the high P/Chl ratios offshore seen in th is study may be an artifact of the method used to measure phaeopigments (i.e. fluorometry).
95 4. CONCLUSIONS Chlorophyll is highly dynamic on the WFS. Average surface chlorophyll c oncentrations are 0.55 mg/m3, while near bottom chlorophyll values average 0.85 mg/m3. The average surface and near botto m distributions of chlorophyll a display an estuarine signature with higher co ncentrations found off the mouths of Tampa Bay and Charlotte Harbor. Surface chlorophyll a concentrations 0.50 mg/m3 generally were found inshore of the 35 m isoba th decreasing seaward. There are distinct seasonal variat ions with elevated chlorophyll concentrations off the mouths of eac h estuary during the wet season (JuneSeptember). Offshore waters show ed little or no seasonality. Near bottom signatures are not as seasonally distinct. Near bottom chlorophyll is usually two to threefold greater than surface chlorophyll extending out to the shelf break. Blooms of Trichodesmium, K. brevis, diatoms contributed to the higher chlorophyll concentrations seen on the shelf. Nutrient flux from rivers and es tuaries, localized upwelling, and salinity/temperature fronts which may ai d in nutrient recycling are thought to be responsible for the di stributions seen inshore. Bottom Ekman transport (intensified by the Loop Current affect) of biologically important material across th e shelf from the shelf break to the inner shelf is thought to re gulate midshelf phytoplankton. Offshore Loop Current dynamics and s ynoptic scale processes are likely responsible for the patterns seen in chlorophyll. Average phaeopigments comprised from 43 to 68 percent of the measured Chl a inshore of the 50 m isobath, while offshore values were from 68 to over 100 percent.
96 Phaeopigment distributions are like ly due to localized grazing by zooplankton, cell senescence, sink ing, microbial degradation, photooxidation or they could be an artif act of the fluorometry method of measurement.
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110 Appendix A: ECHOHAB: Florida Sampling Schedule ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) TAMPA BAY 1 0 X X X X X X X + DMS 3 X ZOOPLANKTON X 2 CTD ONLY DEL N-15 TRICHO 3 0 X X X X X X X 5 X X 10 X X 13 X X X X X 4 CTD ONLY 5 0 X X X X X X X 10 X ZOOPLANKTON X 20 X X 25 X X X X X 6 CTD ONLY DEL N-15 TRICHO 7 0 X X X X X X X 10 X X 20 X X 30 X X X X X X X 35 X X 8 CTD ONLY 9 0 X X X X X X X 10 X X 20 X X 30 X X 40 X X
111 Appendix A continued ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) 10 0 X X X X X X X + DMS 5 X ZOOPLANKTON X 10 X DEL N-15 TRICHO X 15 X X 20 X X 25 X X 30 X X 35 X X 40m OR DCM 40 X X X X X X X DEPTH 45 X X 11 0 X X X X X X X + DMS 10 X X 20 X TRICHO X 30 X X 50 X X 75m OR DCM 75 X X X X X X 100 X X 150 X X 175 X X 185 X X X X 12 CTD ONLY DEL N-15 TRICHO
112 Appendix A continued ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) 13 0 X X X X X X X 10 X X 20 X X 30 X X X 50 X X DCM IF PRESENT 75 X X 100 X X 150 X X 155 X X 14 CTD ONLY 15 0 X X X 10 X X 20 X X 30 X X 50 X X 75 X X 100 X X 120 X X 16 CTD ONLY TRICHO 17 0 X X X X X X X 10 X X 20 X X 30 X X 40 X X 50 X X 60 X X 70 X X 80 X X
113 Appendix A continued ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) 18 CTD ONLY 19 0 X X X X X X X 10 X X 20 X X 30 X X 40 X X 50 X X 60 X X 65 X X X X X X X 20 CTD ONLY 21 0 X X X X X X X 10 X X 20 X X 30 X X 40 X X 50 X X 22 CTD ONLY DEL N-15 TRICHO 23 0 X X X X X X X 5 X X 10 X X 15 X X 20 X X 25 X X 30 X X 35 X X 40 X X X X X X X 45 X X
114 Appendix A continued ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) 24 CTD ONLY 25 0 X X X 5 X X 10 X X 15 X X 20 X X 25 X X 30 X X 35 X X 40 X X 26 CTD ONLY 27 0 X X X X X X X 5 X X 10 X TRICHO X 15 X X 20 X X 25 X X 30 X X X X X X X 28 CTD ONLY 29 0 X X X X X X X 5 X X 10 X X 15 X X 20 X X
115 Appendix A continued ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) 30 0 X X X X X X X 5 X X 10 X X 15 X X X 31 CTD ONLY DEL N-15 TRICHO END 32 0 X X X X X X X SARASOTA 5 X X TRANSECT 8 X X START 40 0 X X X X X X X + DMS SANIBEL 5 X ZOOPLANKTON X TRANSECT 10 X X 15 X X 20 X X 25 X X 30 X X 35 X X 40 X X 45 X X X X X X X 41 CTD ONLY DEL N-15 TRICHO 42 0 X X X X X 10 X X 20 X X 30 X X 35 X X 43 CTD ONLY
116 Appendix A continued ECOHAB MONTHLY CRUISES STATION LIST, SAMPLE DEPTHS AND TYPES OF SAMPLES SAMPLE TYPE TRANSECT STATION SAMPLE CHL CHN PPO4 TDP TDN COUNT NUT'S NUMBER DEPTH (KENT) 44 0 X X X X X X X 10 X X 20 X X 30 X X 45 CTD ONLY DEL N-15 TRICHO 46 0 X X X X X 10 X X 20 X X 25 X X 47 CTD ONLY 48 0 X X X X X X X 10 X X 15 X X 49 CTD ONLY DEL N-15 TRICHO 50 0 X X X X X + DMS 5 X X 10 X X END 51 0 X X X X X X X SANIBEL 5 X ZOOPLANKTON X TRANSECT