Effects of a rapidly receding ice edge on the abundance, age structure and feeding of three dominant calanoid copepods in The Weddell Sea, Antarctica

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Effects of a rapidly receding ice edge on the abundance, age structure and feeding of three dominant calanoid copepods in The Weddell Sea, Antarctica

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Title:
Effects of a rapidly receding ice edge on the abundance, age structure and feeding of three dominant calanoid copepods in The Weddell Sea, Antarctica
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Burghart, Scott E.
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Tampa, Florida
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University of South Florida
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English
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v, 35 leaves : ill. ; 29 cm.

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Calanoida ( lcsh )
Copepoda -- Antarctica -- Weddell Sea ( lcsh )
Dissertations, Academic -- Marine Science -- Masters -- USF ( FTS )

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General Note:
Thesis (M.S.)--University of South Florida, 1998. Includes bibliographical references (leaves 32-35).

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University of South Florida
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Universtity of South Florida
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025841572 ( ALEPH )
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F51-00134 ( USFLDC DOI )
f51.134 ( USFLDC Handle )

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Open water, marginal ice and in-ice zone s were sampled in the Weddell Sea during November and December 1993 in an effort to examine the influence of the early spring bloom on the diet and population structure of the three biomass dominant copepods: Metridia gerlachei, Calanus propinquus, and Calanoides acutus The abundance of all three species in the upper 200m was highest at stations in the open water, but individually, each species displayed a unique trend. Metridia gerlachei which showed the least variability, was more abundant at open water stations compared to the marginal ice zone (MIZ). The abundance of Calanus propinquus was higher in open water than in the MIZ or in the ice Calanoides acutus displayed the highest variability, with significant differences between all three ice cover zones. Diet analysis revealed no significant differences in the number of diet items within each ice cover zone and diatoms were the predominant item identified in the guts of all three species There were dramatic differences in the age composition of the species between the zones Early copepodite stages of all three species predominated at the ice edge and in open water Numbers of Metridia gerlachei adult females were roughly equivalent in all three zones while Calanoides a c utus and Calanus propinquus adult females comprised a higher fraction of the total population within the ice. These results compare well with life history data compiled by other authors and reinforce the importance of the ice edge to bloomdependent Antarctic zooplankton. Abstract Approved: Major Professor: Thomas L. Hopkins Ph D Professor, Department of Marine Science Date Approved :

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EFFECTS OF A RAPIDLY RECEDING ICE EDGE ON THE ABUNDANCE, AGE STRUCTURE AND FEEDING OF THREE DOMINANT CALANOID COPEPODS IN THE WEDDELL SEA, ANTARCTICA by E. BURGHART A thesis submitted in partial fulfillment of the requirements for the degree of Master" of Science Department of Marine Science Univer si ty of South Florida December 1998 Major Profe ssor : Thoma s L. Hopkins

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Graduate School University of South Florida Tampa, Florida CERTIFICATE OF APPROVAL Master's Thesis This is to certify that the Master's Thesis of SCOTT E. BURGHART with a major in Marine Sciences has been approved by the Examining Committee on September 18, 1998 as satisfactory for the thesis requirement for the Master of Science degree Examining Committee : Major Thomas -L. Ph.D. J. Torres, Ph.D. 7 Member: Gabriel Vargph.D.

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ACKNOWLEDGMENTS I wish to thank my committee, Thomas L. Hopkins, Joseph J. Torres and Gabriel Vargo for help with the manuscript. Thanks to all those who aided the collection of this data set: Tracey Sutton, Joe Donnelly, Renee Bishop, Steve Geiger, Helena KawaU and Liz Clarke, as well as the crew of the RIV Polar Duke and all those at AS A. Thanks also go to Bob Muller for statistical advice and to my colleagues Tracey Sutton and Andrew Remsen for all their help in data processing and manuscript production. Finally, a very special thanks to my wife Terri Lynn for all her support and to my whole family for all their encouragement. This research was supported by NSF award number OPP 9220493 to T L. Hopkins.

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LIST OFT ABLES LIST OF FIGURES ABSTRACT INTRODUCTION METHODS RESULTS TABLE OF CONTENTS Species Biomass and Abundance Comparisons Metridia gerlachei Calanus propinquus Calanoides acutus DISCUSSION REFERENCES u lll IV 4 7 7 18 19 20 25 32

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LIST OFT ABLES Table 1 : A bu n dance of copepods at each stat ion in t h e upper 200 m 11 Tab l e 2 : Results from multiple comparison test of naupliar abundance by ice cover zone 12 Table 3 : A b undance of copepods at each station from 0-1000 m 12 Table 4: Categories of diet items and their frequency of occurrence wi t hin each ice cover zone 21 Table 5 : Resu l ts of Kruska l Wallis analysis on n u mber of diet items within each ice cover zone 21 11

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LIST OF FIGURES Figure 1: Sampling locations 6 Figure 2: NorthSouth SIGMA-t transect 9 Figure 3: NorthSouth chlorophyll transect 10 Figure 4 : Abundance of the three calanoid copepods in the upper 200 m by ice cover zone 13 Figure 5: Biomass of the three copepods in the upper 200 m by ice cover zone 14 Figure 6: Vertical distribution of Metridia gerlachei life stages by ice cover 15 Figure 7: Vertical distribution of Calanus propinquus life stages by ice cover 16 Figure 8: Vertical distribution of Calanoides acutus life stages by ice cover 17 Figure 9 : Life stage distribution of Metridia gerlachei by ice cover 22 Figure 10 : Life stage distribution of Calanus propinquus by ice cover 23 Figure 11: Life stage distribution of Calanoides acutus by ice cover 24 111

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EFFECTS OF A RAPIDLY RECEDING ICE EDGE ON THE ABUNDANCE, AGE STRUCTURE AND FEEDING OF THREE DOMINANT CALANOID COPEPODS IN THE WEDDELL SEA, ANTARCTICA by SCOTT E. BURGHART An Abstract Of a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Marine Science University of South Florida December 1998 Major Professor: Thomas L. Hopkins, Ph.D. iv

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INTRODUCTION During the austral spring and summer the ecology of the seasonal ice zone in the Southern Ocean is dominated by the retreat of the pack ice, and the oceanic system experiences a rolling wave of increased primary productivity associated with the ice edge (Smith and Nelson 1985, 1990; Sullivan eta/. 1988). The result is a frontal feature that poses a significant problem to the pelagic community in that primary consumers have a limited time during which food is abundant. The dynamics of the marginal ice zone (MIZ) affect microplankton community structure (Robins eta/. 1995), mesozooplankton vertical distributions (Hopkins and Torres, 1988; Robins et al. 1995), and trophic structure (Hopkins and Torres, 1989) The retreat of the ice edge, along with its associated bloom, is a pivotal event in the life cycles of some of the Antarctic copepods. The three dominant copepods south of the Antarctic Convergence in terms of biomass are Calanoides acutus, Calanus propinquus, and Metridia ger/achei. Hopkins and Torres (1988) found that they represented 47-56% of the mesozooplankton biomass in the MIZ of the western Weddell Sea. Each of the three has distinctly different foraging and life history strategies. Ca/anoides acutus exhibits the most extreme adaptation to a polar environment in that late copepodite stages overwinter at depths of 500 m or more, remaining trophically inactive during this period Molting to adults and mating occur at diapause depth Females then ascend into the upper 100m to spawn, allowing development of younger stages to take place at the place and time ofhighest

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productivity (Andrews, 1966; Marin 1988; Hopkins eta/. 1993; Bathmann eta/., 1993 ; Lopez eta/., 1993 ; Schnack-Schiel and Hagen 1994, 1995) Calanus propinquus has an intermediate life cycle in which a portion of the population remains in surface waters throughout the year. Feeding continues during winter but metazoans and microheterotrophs appear to comprise a greater portion of the diet. Mating and spawning occur during the productive period, but for a more protracted period than in C. acutus (Marin, 1988; Hopkins eta/., 1993a, 1993b ; Bathmann, 1993; Schnack-Schiel and Hagen, 1994 ; Hagen and Schnack-Schiel 1996). Of the three species, Metridia ger/achei depends least on the spring boom Ontogenetic vertical migration is relatively weak, feeding occurs throughout the year and the diet tends to be diverse throughout the year Mating and spawning appear to be less closely coupled with the bloom and it is possible that more than one generation is produced per year (Huntley and Escritor, 1992 ; Hopkins eta/., 1992, 1993; Schnack-Schiel and Hagen, 1994 ; Metz and Schnack-Schiel, 1995). Life cycle data on C. propinquus and M ger/achei show less clear patterns than that of C. a c utus In all three species, it is unclear exactly how tightly coupled the timing is between reproduction and arrival of the bloom. 2

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The research presented here was based upon a cruise of the RN Polar Duke to the western Weddell Sea in November/December of 1993. The overall purpose of the cruise was to elucidate the effect of the receding ice-edge on the entire pelagic community. The present study targeted the reproductive and trophic ecology of the three biomass dominant copepods discussed above. Specifically, we address the following questions: How clear was the change in population and trophic structure across the ice edge? Is the ice-edge bloom a significant influence on species that do not rely solely on herbivory as a means of nutrition? 3

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METHODS The cruise plan consisted of transects along and across the marginal ice zone (Figure 1). In addition, a series of stations were occupied in each of three distinct zones of ice cover (open water, ice edge, and within ice) for a period of several days. At each station during the transects, the water column was profiled with a CTD rosette for salinity, temperature, and chlorophyll data. During the periods when residence was maintained in the distinct ice cover zones, plankton was sampled with a plummet net and 30 liter Niskin bottles. The plummet net is an opening/closing net deployed vertically, allowing for discrete depth sampling of the water column even within heavy ice cover. The mouth area was I m2 and the net mesh 162 Jlm. Volumes filtered were calculated as the cross sectional area of the net multiplied by the vertical distance sampled assuming 100% filtration efficiency Within each ice cover zone, vertical sampling divided the water column into 6 discrete depth zones (0-40, 40-100, 100-200,200-400,400-600 and 600-1000 meters). In addition, several deployments sampled the entire upper 200 m of the water column The 0-200 m tows were primarily to collect material for physiological experiments on zooplankton, but data from those samples are included here. All plummet net samples were initially fixed in 5-7% buffered formalin and subsequently transferred to 50% isopropyl alcohol for storage and analysis Plummet net data were used to obtain the abundance of the three biomass dominant copepods: Calanoides acutus, Ca/anus 4

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propinquus, and Metridia ger/achei Samples were subdivided in a Motoda box splitter to a fraction of 114 to 118 and the total nwnbers of each of the three species were enumerated by copepodite stage Stage was determined by recording size distribution data for each species while performing total counts, with life stages being determined from distinct size peaks in these data. The mean life stage of samples was calculated using the following equation from Marin (1987): MLS = (N1 *S 1) + (N2*S2) + (N3*S3) + (N4*S4) + (N5*S5) + (N6*S6) N1 + N2 + N3 + N4 + N5 + N6 Where N1 to N6 is the number of individuals in copepodite stages 1 to 6, and S 1 to S6 is the stage number Size was converted to biomass using the length:biomass equations for each species presented in Fransz (1988). All abundance and biomass data were tested using an ANOVA procedure (general linear models) The 30 1 bottles were used for one vertical series in each ice cover zone; samples were collected at depths of 0, 10, 20 40, 80, 100, 150, 200, 400, 600, 800 and 1000 m. Niskin samples were filtered through 28 J..Lm gauze and used for counts of copepod nauplii. Diet of each species was analyzed by light microscopy. Individuals were measured, their life stage determined, then guts were then dissected out. Gut contents were spread on glass slides in a mixture of glycerol, water, and Fucsin acid stain. Food items were identified to lowest possible taxonomic level and counted at 400X magnification using Nomarski DIC optics on a research-grade compound microscope Each slide was transected 20 times, with the nwnber of food items standardized to total coverslip area to give a measure of relative gut fullness between slides. No attempt was made to convert food items to biomass. 5

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90 W -10 Ice Edge (11124/93) --------48... -4"4_ _____ I ce Edge (12/20/93) A Figure 1. Sampling l ocat i ons. Open water sta tion s ( ), ice edge stations (.a. ), pack ic e stat i ons ( ) 6

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RESULTS CTD data from the North-South transect showed a meltwater lens 27.3) between 59 and 61 S associated with the marginal ice zone (Figure 2; Vargo unpubl.) Peak chlorophyll biomass (2.5 Jlg 1"1 113 mg m-2 integrated 0-40 m) was associated with this lens (Figure 3; Vargo, unpubl.). Chlorophyll within the pack ice was lowest of the three zones (15-17 mg m-2), while open water stations were intermediate (mean= 69 mg m-2), indicating our study area conformed to the classical model of primary productiv i ty following a receding ice edge (Sullivan et a/., 1988). Thus, data presented here as trends in copepod abundance and population structure from within the ice into the open water can be considered not only as traversing a geographical area, but compressing a period of time as well Species Biomass and Abundance Comparisons Combined, the abundance of the three species in the upper 200 m of the water column increased from within the ice to open water (Table 1 and Figure 4). Individually, Ca/anoides acutus was most abundant with an average of 4.0 ind m-3 for all three ice cover zones. Ca/anus propinquus was least abundant (2.1 ind m-3 ) and Metridia gerlachei was intermediate (3 7 ind. m-3). Among the three species, C. acutus had the highest biomass (P > 0.0001), averaging 1.2 mg m-3 (DW) per station in the upper 200m. The average biomass per station of C. propinquus and M gerlachei were 0.5 and 0.3 mg 7

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m-3 respectively, but this difference was not statistically significant (Table 1 and Figure 5). Naupliar counts were highest in the MIZ (2 2 ind L -1 ) while values were intermediate within the ice (1.8 L-1) and lowest in the open water (0 9) These numbers were subjected to a Kruskall-Wallis test followed by a multiple comparison (Zar, 1984). This analysis revealed the naupliar abundance in the MIZ was higher than that in the open water, but not statistically separable from in-ice abundance s (Table 2) Analysis of abundance and biomass was limited primarily to the upper 200 m because this is where the majority of all three copepod populations reside during the productive season. It should be noted the biomass and abundance numbers for M gerlachei in the upper 200 m might be somewhat misleading due to the vertical distribution of this species. The fraction of the population sampled below 200 m was always at least 30% (30% in ice, 34% MIZ, 42% open water; Figure 6). In contrast, only 4-16% of C. propinquus and 3-15% of C. acutus individuals were sampled below 200 m (Figures 7 and 8 respectively) In fact, when only the 0-1000 m plummet net series are considered, M gerlachei becomes the numerical dominant (Table 3), although C. acutus remains the biomass dominant. 8

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.......... E .._... I I-0.. w 0 0 34 68 102 135 169 203 237 270 27.3----27 .36-------27 3 1 '36 27 36 '2. 2 7 .4 2-------27 .. 42 27.42 27 48----v27.48------------27 54 __./ .---/'7 6 27. 54-D . -----1 -------27.66 :t1'l: 27.72-----------304 338 371 405 58.00 58.33 58.67 59.00 59.33 59.67 60.00 60.33 60.67 61.00 61.33 61.67 62.00 LATITUDE (south) Figure 2. North-South SIGMA-t transect 9

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______ 2.6 _/'"2.2 -J -2.2 .4 ----.______2.6_./ ------.1 2 2 1 8 18 r-=::1.4 50 t-----1:4=---"'--1.8 1.4 -1 --,-----1 ........._1__; 25 75 --------0.6-----}0. 6 1-----0.6-----.......... E -I 100 1--0... w 0 125 -------0. 2 t------0.2 150 175 200 58.00 58.33 58.67 59.00 59 .33 59 .67 60.00 60 .33 60.67 61.00 61.33 61.67 62.00 LATITUDE (south) Figure 3. NorthSouth chlorophyll transect 10

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Table 1: Abundance of copepods at each station in the upper 200m. lee Cover Zone Stat i on Numbef Metridia gerlac hei Ca/anus propinquus Calanoides acutus individuals I mgDW 1m3 individuals I mgDW / m 3 individuals I m 3 mgDW / m 3 m3 m3 Ice ---------24 4.9 0.5 0.8 0.8 2 9 2.9 26 4 5 0.5 1.1 1.0 5.1 4.3 28 2.7 0.3 0.6 0.6 4.2 2.9 zone mean 4.0 0.4 0.8 0.8 4.1 3.4 MIZ 8 0.4 + 0 1 + 0.1 + 10 1.4 0.2 0.4 + 0 3 0.3 16 7.2 0 .1 1.4 0 2 1.1 0.1 21 1.0 + 0.3 + 0.3 0.1 44 2.3 0 1 0.6 + 0.8 0.2 48 2.2 0.1 0.5 0.1 1.6 0.3 51 2.4 0.3 2.3 0.5 2.8 0.4 zone mean 2.4 0.1 0.8 0.1 1.0 0 2 Open Water 11 0.6 0 .1 7.0 1.7 10 0.3 14 11 0 8 4.1 0 7 7.0 1.2 62 4.4 0 2 4.6 0.8 10 2.0 64 6.6 0.4 5.4 0.7 10 1.9 zone mean 5 7 0.3 5.3 0 9 9.3 1.3 Overail Mean 3 7 0.3 2.1 0 5 4 0 1.2

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Table 2: Results from multiple comparison test ofnaupliar abundance by ice cover zone. (OW= open water) Compar is on Diff SE q q0.os,a,3 MIZ vOW 53.5 1 2.47 3 4.2893* 3.314 MIZ vice 21.5 12.473 1.7237 3.314 ice vOW 32 12.473 2.5656 3 .314 indi cates s i gn i ficant difference Ta bl e 3: Abundance ofcopepods at each station from 0-1000 m lee Cover St a t i on Metri d ia gerl a c he i Ca/anu s pro pinquu s Calanoid es a c utu s Zone Number individuals mgDW / m3 individuals mgDW / m3 individuals mgDW / m3 I m3 / m l / m l tv Ice 24 1.4 0 1 0 2 0 2 0 7 0.7 26 2 3 0 3 0.4 0 3 2.3 1.7 28 1.6 0.1 0 2 0 2 1.4 0 9 zone mean 1.8 0 2 0 3 0 2 1.5 1.1 MIZ 4 8 1.2 0 1 0.3 0 1 0.4 0.1 51 1.2 0 1 0 7 0 1 0 8 0.1 zone mean 1.2 0.1 0.5 0.1 0.6 0 1 Open 6 2 2.3 0.2 1.0 0.2 2. 1 0 5 64 3.5 0.3 1.4 0 2 2.4 0 5 zone mean 2.9 0 3 1.2 0.2 2.3 0.5 Overall Mean 1.9 0 2 0 8 0.2 1.4 0 6

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t: rll = "Q .... -> ..... ... "Q = .... o Metridia gerlachei o Calanus propinquus Calanoides acutus 2500 2000 I 1500 1000 500 0 +--------'-------L-----1.-.J Ice Edge Open Figure 4: Abundance of the three calanoid cope pods in the upper 200 m by ice cover zone

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o Metridia gerlachei o Calanus propinquus Calanoides acutus 900-,-------------------800 700 600 +-------1 .,..= 5oo OJ) 400 +----------1 e 300 +--------1 200 +--------+-1 100 l 0 u Ice Edge Open Figure 5: Biomass of the three copepods in the upper 200 m by ice cover zone

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Ice MIZ Open -40 40 40 !:'-) 100 1.20 100 OA7 100 ..... 0.46 200 200 200 = "0 400 400 400 1.38 < 600 600 600 1000 1000 1000 40 40 40 ;;;... 100 100 100 I c: ;;;... Q 200 200 200 N VI u ..c: ..... 400 400 4oo o.67 Q. 600 600 600 ll] Q 1000 1000 1000 40 40 40 100 100 0 .63 100 I 200 0.39 200 200 I 12.98 u 400 400 400 D 600 600 600 1000 1000 1000 0 1 2 3 1 2 3 1 2 3 Individuals I m3 Figure 6: Vertical distribution of Metridia gerlachei life stages by ice cover zone

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Ice MIZ Open .. 40 40 40 riJ 100 0.84 100 0.17 100 ..... 200 200 200 = "C 400 400 400 < 600 600 600 1000 1000 1000 40 40 40 0.70 = 100 100 100 Q I N 200 200 200 ,.... ..Q 400 400 400 0'> u ..... Q., 600 600 600 I[) Q 1000 1000 1000 40 40 0.48 40 ,.... 100 0.12 100 OA9 100 ,.... ,.... 200 200 200 I ,.... u 400 400 400 600 600 600 D 1000 1000 1000 0 0.5 1 0.5 1 2 4 6 Individuals I m3 Figure 7: Vertical distribution of Calanus propinquus life stages by ice cover zone

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Ice MIZ Open 40 4.7 40 0 .10 40 !i 100 100 100 200 200 200 = "0 400 400 400 < 600 600 600 1000 1000 1000 40 40 H 0.25 40 = 100 100 100 ;;;... 0 I N 200 200 200 -;;;... -= 400 -...) """' .... 400 400 u Q. 600 600 600 [ill Q 1000 1000 1000 40 40 1 .72 40 -11::::::=------...18.9 """' """' 100 100 100 """' 200 200 200 I """' u 400 400 400 600 600 600 D 1000 1000 1000 0 1 2 3 4 5 1 2 3 4 5 5 10 Individuals I m3 Figure 8: Vertical distribution of Calanoides acutus life stages by ice cover zone

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Metridia gerlachei When subjected to multiple comparison analysis Metridia gerlachei showed a sign i ficant difference in abundance (0-200 m) only between the MIZ and the open water (Figure 4). There were no significant differences in biomass between the three zones in the upper 200m (Figure 5). However, the age composition did change with respect to ice cover The mean life stage MLS, averaged 4.3 at pack ice stations but dropped to 3.4 in the MIZ, and 3.1 in open water. This was entirely due to the increase in early copepodite stages (CI-111) within the samples. While the percentage of adults remained between 11% and 18%, the percentage of CI-III s rose from 21% within the ice to 69% in the MIZ and 74% in open water (Figure 9). The vertical distribution of M gerlachei changed as well This species actually appeared to descend in the water column moving from within the ice to open water For example, the abundance mode for adults and late copepodite stages (CIV-V) was in the 40-100 m depth zone within the ice, but in the 200-400 depth zone in the open water (Figure 6). It does appear that the early copepodite stages (CI-111) remain a depth zone above the middle copepodites and adults. Diet analysis revealed diatoms as the most commonly identified food item (Table 4). Other items such as tintinnids and silicoflagellates occurred only rarely Metazoan material was found in the guts of this species, but there was no trend in its occurrence in diets between ice cover zones Empty guts were infrequent, and only occurred within the ice or MIZ. A Kruskal-Wallis test failed to fmd any differences in the number of diet items between the different ice cover zones (Table 5) 18

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Ca/anus propinquus This species was most abundant in open water when compared to within the ice and the MIZ (Figme 4), while biomass was significantly lower at the MIZ than either within the ice or open water (Figure 5). Ca/anus propinquus showed a stronger trend of decreasing age of the population between ice cover zones than M ger/achei. The MLS was 4.7 within the ice and only 2.6 and 2.4 in the MIZ and open water respectively. Unlike M ger/achei, adults of C. propinquus comprised significantly less of the MIZ and open water samples While adults were 46% of the individuals within the ice they were less than 5% in the MIZ and open water stations (Figme 1 0) Meanwhile, the percentage of CI-111 individuals increased from 19% to 84% The drastic change in the age structure of this species made it difficult to compare vertical distributions between the different life stages. However, both early and late copepodite stages were absent from the upper 40 m within the ice (Figure 7). In the MIZ the abundance modes for both groups were still found from 40-1 00 m. but individuals were present in the upper 40 m At open water stations, the abundance modes had moved into the upper 40 m. Diatoms were the most commonly identified diet item (Table 4). This species also had metazoan material and there was no trend in its occurrence based on ice cover Within the ice, 20% of the stomachs were empty, compared to none in the MIZ and 4% in open water. Despite this, there was no significant difference in the number of diet items between ice cover zones (Table 5) 19

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Calanoides acutus This copepod exhibited the most distinct pattern in abundance with all three zones significantly different from each other (Figure 4). MIZ stations had the lowest abundance and open water stations the highest. Biomass was also significantly different between each of the three zones and, like the abundance data, lowest values were in the MIZ. However, contrary to the abundance pattern, the highest biomass values were seen within the ice. This discrepancy between the abundance and biomass data resulted from the age structure of the population. The MLS consistently decreased moving from ice to MIZ to open water (5.2, 3.2 and 2 6 respectively). Adults declined from 37% of the samples within the ice to 1% in the open water while CI-111 rose from only 2% within the ice to 79% in the MIZ and open water (Figure 11 ). The vertical distribution data for C. acutus showed middle copepodite stages tended to be somewhat deeper than adults under the ice (Figure 8) Note the mode for CIV-V is in the 40-100 m depth interval while that for adults is in the 0-40 m range. Additionally, 53% of the CIV-V's were found below 100 m as opposed to only 3 7% of the adults. Diatoms were the most commonly identified diet item for this species as well, however, unlike M gerlachei and C. propinquus, there was no occurrence of metazoan material (Table 4). Empty guts were found only within the ice samples, however, the Kruskal-Wallis test did not indicate any significant differences in number of diet items between the zones (Table 5). 2 0

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Ta bl e 4 : Categories of diet items an d their freq u ency of occurrence within each ice cover zone (OW= open water). N Empty DiaDinoSilicoTint inUnidentiMetazoan toms flagellates flagellates nids tied material Eukaryotes Metridia Ice 25 3 2 1 9 5 gerlac hei MIZ 25 4 21 6 5 ow 25 25 2 5 4 Calanus Ice 25 5 20 2 3 13 4 propinquus MIZ 25 25 3 9 3 ow 25 24 1 4 5 Calanoides Ice 25 6 19 2 9 acutus MIZ 25 25 4 ow 25 25 3 T abl e 5: Results ofKruskal-Wallis analysis on number of diet items within each ice cover zone. Mean Rank H e v p Ice MIZ Open Metridia gerlachei 34.72 34 .9 4 44 .3 4 5.4185 2 0.05 < p < 0. 1 0 Calanus propinquus 29.94 40 38 43.68 3.1746 2 0 1 0 < p < 0.25 Ca l a n oides acutus 32.2 42 82 38 98 3.0458 2 0. 1 0 < p < 0 25 2 1

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101 Q. c:IJ 'S ... = 101 N N 101 Q. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 ci 1:1 cii Iilli ciii civ cv D females Ice Edge Open Figure 4: Life Stage Distribution of Metridia gerlachei by Ice Cover Zone

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100% 90% 80% 70% Q. 60% ""' Q .... SO% = N Vi c. 40% 30% 20% 10% 0% D ci !Zl cii [ill) ciii civ cv 0 females Ice Edge ....... . . . ....... ........ . . . . Open Figure 5: Life Stage Distribution of Calanus propinquus by Ice Cover Zone

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100% 90% 80% 70% "5. i ell 60% ""' 0 SO% = N "" c. 40% 30% 20% 10% 0% Ice D ci 1Z1 cii Iilli ciii civ cv D females ........ . . . Edge . ...:::::.::::::::::::::::::: . ............... . . . . ........ 0 . . . . . . . . ........ . . . . . . . 0 Open Figure 6: Life Stage Distribution of Calanoides acutus by Ice Cover Zone

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DISCUSSION Metridia ger/achei has the least specialized life cycle of the three copepods considered here It is, therefore, not surprising that the data for M ger/achei show the least amount of variability with respect to ice cover. The abundance of M ger/achei in open water was higher than that at the MIZ, but, the difference was not significant between the pack ice and open water stations. The greatest difference was in the amount of early copepodite stages (CI-III) found in the pack ice and in open water (29% and 74% respectively) Atkinson and Shreeve (1996) saw a similar shift in population structure across an ice edge in the Bellingshausen Sea, except that adults dominated their ice bound stations while late copepodite stages dominated ours. In the Bransfield Strait, Huntley and Escritor (1992) found a similar shift in age structure over a very similar time period. Schnack-Schiel and Hagen (1994, 1995) reported age structure of this species in the summer as being dominated by CI's and later stages (CIV-adults) in the southeastern Weddell Sea. Furthermore by autumn, early copepodite stages comprised a vast majority ( of their samples. The November/December life stage data from our samples within the ice most closely resemble their October/November samples while within the MIZ and open water we obtained very similar percentages of early copepodite stages (about 70 % ) to what they reported in summer and autumn (Schnack Schiel and Hagen, 1994, 1995). Therefore, the change in the age structure ofthe population we observed appeared to be compressed relative to their data. This is not suprising as theirs 25

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was a seasonal study while we attempted the equivalent of compressing the seasons by sampling across a rapidly receding ice edge The predominance of diatoms in the diet of M gerlachei has been r eported previously In fact, an investigation in the western Weddell Sea in autumn (Hopkins and Torres, 1989) placed M ger/achei in a feeding guild characterized by 97% of the diet being comprised of phytoplankton The results presented here certainly don't contradict that assignment. It should be noted, however, the type of diet analysis used in the present study did not detect small non-thecate diet items This may be especially important in the case of M ger/achei as Atkinson ( 1995) and Atkinson et a/. ( 1996 ) suggested such motile organisms are important in the diet of this species Cal anus propinquus resembles M ger/achei in that a portion of the population remains in surface waters and stays trophically active in winter (Bathrnann, 1993; Hopkins eta/., 1993; Spiridonov and Kosobokova, 1997). However, there were some differences in the present data to suggest that the seasonal surge in primary productivity was more important to C. propinquus than M ger/ach e i. First, stations in the open water had a higher abundance of C. propinquus than stations within the ice or MIZ. The copepodite stage data indicated this was due to the addition of younger individuals to the population Second, the life stage percentages showed some differences For example, while the percentage of adults of M ger/achei varied little, that of C. propinquus dropped dramatically. Within the ice adult C. propinquus made up 46% of the samples but the percentage dropped to 3% at the MIZ and remained low at open water stations Data from Atkinson and Shreeve (1996) showed adults were never abundan t and early 2 6

PAGE 35

copepodite stages were always prevalent regardless of ice cover in the Bellingshausen Sea. Compared to Schnack-Schiel and Hagen ( 1994, 1995) the age structure within the ice most closely resembles their October/November data, while the MIZ and open water samples most closely resemble their January/February data. There was no difference in gut fullness based on food item counts for C. propinquus among the different zones It is worth noting that the majority of empty stomachs were found in the pack ice ( 5 versus only 1 between the other two zones). Perhaps more significant than the gut fullness data was the lack of a trend in the amount of metazoan material in the guts like that found by Hopkins and Torres (1989). Of the three species, C. acutus displayed the most apparent trends. The low abundance of copepodites and adults at the ice edge was counterintuitive. However, the fact that naupliar counts were higher at the MIZ than within the other two ice regimes may provide an explanation for this observation. In other words, C. acutus may have been present at the ice edge in high numbers, but as naupliar stages rather than copepodite stages. The dramatic increase in abundance in the open water was due to the massive addition of younger individuals into the population. The presence of copepodite stages IV, V and adults of this species in the upper 200m even within the ice is significant as it indicates the summer ascension of this seasonal vertical migrant has already taken place. Based on the lack of males occurring in surface waters, Marin (1988) proposed a scenario in which C. acutus females mate at depth and then ascend into surface waters in the spring In the southeastern Weddell Sea, Schnack-Schiel eta/. ( 1991) primarily found mature females made the vertical ascent, which agrees with Marin's model. Therefore, the large numbers ofCIV and CV individuals in the upper 27

PAGE 36

200m in our study was not expected. In the Gerlache Strait, Lopez eta/. (1993) found immature individuals above diapause depth, but suggested the depth of the water column may have played a key role in their results. As the depth of our sampling area was always at least 1000 m, we cannot invoke that explanation. Since no males were present in our samples there was no evidence to suggest mating was taking place in the upper 200m. One possible explanation is the suggestion that C. acutus may, at times, have a two year life cycle (Atkinson, 1991) In this scenario, the late copepodite stages found in surface waters would have overwintered at an earlier stage, and therefore not been ready to spawn at the onset of spring. Such individuals would presumably overwinter twice before reproducing. Other studies have found C. acutus concentrating in the upper 200 m at least a full month later than our data indicate (Schnack-Schiel eta/. 1991; Schnack Schiel and Hagen, 1994, 1995), a discrepancy possibly due to the difference in latitude between their sampling locations and ours (72-73 S versus 58-62 S) Life stage data for C. acutus was similar to that of C. propinquus The percentage of adults plummeted from 37% within the ice to 1% in the open water. The dramatic difference in the age structure of the samples explains the higher biomass of C. acutus within the ice despite a higher numerical abundance at open water stations There was a greater percentage of young (CI-111) C. propinquus than C. acutus within the ice, indicating the species may have started breeding earlier than C. acutus Lack of a trend in diet composition across ice cover zones was expected for C. acutus. Previous studies have shown this species remains trophically inactive in winter rather than showing a shift in diet (Schnack-Schiel eta/., 1991; Hopkins eta/., 1993a; Atkinson, 1995). The most important result of the diet study was the fact that individuals 28

PAGE 37

were feeding within the ice at all. Despite the proportionally higher occurrence of empty stomachs within the pack ice, and the total lack of empty stomachs in the MIZ or in open water, there was no trend in gut fullness for C. acutus across the three ice zones. This indicated trophically it had already switched to spring/summer mode, and that "grazable" phytoplankton is available in the water column under the ice. This is not suprising considering that chlorophyll values within the ice were higher than those typically reported in winter (Garrison eta/., 1993; Scharek eta/. 1994), and around the range suggested to be at the threshold for egg production (Lopez eta/., 1993). The primary hypothesis of the present study was that the rapidly receding ice edge was a vital event in the life cycles of the three species considered It was expected that copepod populations would display the most winter-like characteristics within the ice and the most summer-like characteristics in the open water. Therefore we looked for the following changes moving along the space/time transect of Ice => MIZ => Open water : 1) dietary shifts in C. propinquus and M ger/achei (less omnivorous at the MIZ and open water stations), as well as C. acutus (trophically inactive within the ice). 2) change in the vertical distribution of C. acutus (ascent of the population). 3) increased abundance of all three species (due to increase in juveniles in the population) 4) proportionally more young life stages present. The diet data were of little help in highlighting the importance of the receding ice edge Although there appeared to be a trend regarding the number of empty stomachs in the ice for C. acutus and C. propinquus there was no significant trend in the number of identifiable diet items for any species Additionally, M ger/achei and C. propinquus 29

PAGE 38

didn't display any compositional trends (e g more metazoan material within the ice compared to the edge and open water) The type of diet analysis performed for this study requires caution to be used in making any statements regarding diet changes related to ice cover, especially if there are no apparent trends in the readily observable (e.g. diatoms) food items. There is a continually growing data base regarding the importance of small soft-bodied forms of eukaryote plankton in the diets of copepods Such items usually cannot be effectively detected by microscopal examination of gut contents. However, the higher occurrence of empty stomachs within the ice for C. propinquus and C. acutus is worth noting Vertical distribution data also were of little help in the case of C. acutus, the strongest seasonal migrant, as this species was in the epipelagial in all three ice cover zones Abundance data favored accepting the basic hypothesis. The two species which are affected most by season, C. propinquus and C. acutus, showed trends of increasing abundance from within the ice to open water. Metridia gerlachei, which has shown the least variability in seasonal studies, did not show as pronounced a change in abundance with ice cover (within ice and open water values not significantly different). Without question, the strongest evidence for importance of the receding ice edge came from the life stage data Adult females of all three species were well represented within the ice. The change in age structure of the C. propinquus and C. acutus samples across the MIZ was dramatic, with adults of both species all but disappearing. Even the M gerlachei samples got younger across the ice cover cline Other studies have shown a similar shift in age structure, but generally over a longer period of time 30

PAGE 39

In conclusion, the data support the idea the receding ice edge is an important feature in the life cycles of these organisms Despite the fact our ice stations obviously didn't represent winter conditions we were still able to detect changes in the abundance and age structure of the populations These changes also seemed to show a gradation of being strongest in C. acutus and weakest in C. propinquus Our data fit in very well with previous work on the life cycles of these copepods, and further demonstrate the response to the seasonal production pulse may occur over fairly small time and space scales 3 1

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REFERENCES Andrews, K. J. H. ( 1966) The distribution and life-history of Calanoides acutus (Giesbrecht). Discovery Rep. 34 : 117 162. Atkinson, A. (1991) Life cycles ofCa/anoides acutus Calanus simillimus, and Rhinca/anus gigas (Copepoda: Calanoida) within the Scotia Sea. Mar. Bioi. 109: 79-91 Atkinson, A ( 1995) Omnivory and feeding selectivity in five copepod species during spring in the Bellingshausen Sea, Antarctica ICES J mar. Sci. 52 : 385-396. Atkinson, A. and R S. Shreeve (1996) Response of the copepo d community to a spring bloom in the Bellingshausen Sea. Deep-Sea Res. 42: 1291-1311. Atkinson, A., R. S Shreeve, E. A. Pakhomov, J. Priddle S P. Blight, P. Ward (1996) Zooplankton response to a phytoplankton bloom near South Georgia, Antarctica Mar Ecol. Prog. Ser. 144 : 195-210. Bathmann, U. V., R. R. Makarov, V. A. Spiridonov and G. Rohardt (1993) Winter distribution and overwintering strategies of common Antarctic copepod species (Crustacea, Calanoida) in the Weddell Sea. Polar Bioi. 13: 333-346 Fransz, H G ( 1988) Vernal abundance structure and development of epipelagic copepod populations of the eastern Weddell Sea (Antarctica). Polar Bioi. 9 : 107114 32

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Garrison D. L., K. R. Buck and M. M Gowing (1993) Winter plankton assemblage in the ice-edge zone of the Weddell and Scotia Seas : composition biomass and spatial distributions. Deep-Sea Res. 40: 311-338 Hagen, W and S B Schnack-Schiel (1996) Seasonal lipid dynamics in dominant Antarctic copepods: energy for overwintering or reproduction? Deep-Sea Res 43: 139-158 Hopkins T L. and J J Torres (1988) The zooplankton community in the vicinity of the ice edge, western Weddell Sea, March 1986 Polar Bioi. 9 : 79-87 Hopkins, T L. and J J Torres (1989) Midwater food web in the vicinity of a marginal ice zone in the western Weddell Sea Deep Sea Res 36 : 543-560 Hopkins, T L., T. M Lancraft, J. J Torres and J Donnelly (1993a) Community structure and trophic ecology of zooplankton in the Scotia Sea marginal i c e zone in winter (1988). Deep-Sea Res. 40: 81-105 Hopkins, T. L., D G Ainley, J J Torres, and T M Lancraft (1993b) Trophic structure in ope n waters of the marginal ice zone in the ScotiaWeddell confluence region during spring (1983) Polar Bioi. 13: 389-397 Huntley M. E and F. Escritor (1992) Ecology of Metridia ger/achei Giesbrecht in the western Bransfield Strait Antarctica Deep-Sea Res 39(6) : 1027 1055 Lopez M D G., M E Huntley and J. T Lovette (1993) Ca/anoides acutus in Gerlache Strait Antarctica I. Distribution of late copepod i te stages and reproduction during late spring Mar Ecol. Prog Ser 100 : 153-165 33

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Metz, C. and S B. Schnack-Schiel (1995) Observations on carnivorous feeding in Antarctic calanoid copepods. Mar Ecol. Prog Ser. 129 : 71-75 Marin V. (1987) The oceanographic structure of the eastern Scotia Sea-IV. Distribution of copepod species in relation to hydrography in 1981 Deep-Sea Res. 34(1): 105-121. Marin, V. (1988) Qualitative models of the life cycles of Calanoides acutus, Calanus propinquus and Rhinca/anus gigas. Polar Biol. 8 : 439-446. Robins, D B., R P Harris A. W Bedo, E Fernandez, T W Fileman, D S Harbour, and R.N. Head (1995) The relationship between suspended particulate material, phytoplankton and zooplankton during the retreat of the marginal ice zone in the Bellingshausen Sea Deep-Sea Res. 42 : 1137-1158 Scharek, R., V. Smetacek, E. Fahrbach, L. I Gordon, G Rohardt and S. Moore (1994) The transition from winter to early spring in the eastern Weddell Sea, Antarctica : plankton biomass and composition in relation to hydrography and nutrients Deep-Sea Res 41(8) : 1231-1250 Schnack-Schiel, S B W Hagen and E Mizdalski (1991) Seasonal comparison of Ca/anoides acutus and Calanus propinquus (Copepoda : Calanoida) in the southeastern Weddell Sea, Antarctica Mar Ecol. Prog Ser. 70: 17-27. Schnack-Schiel, S B and W Hagen (1994) Life cycle strategies and seasonal variations in distribution and population structure of four dominant calanoid copepod species in the eastern Weddell Sea, Antarctica J. Plank Res 16 : 1543-1566 34

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Schnack-Schiel, S. B. and W. Hagen ( 1995) Life-cycle strategies of Calanoides acutus Calanus propinquus, and Metridia gerla c hei (Copepoda: Calanoida) in the eastern Weddell Sea Antarctica. ICES J. mar. Sci. 52: 541-548 Smith W 0 Jr. and D M Nelson (1985) Phytoplankton bloom produced by a receding ice edge in the Ross Sea : spatial coherence with the density field Science 227: 163-166. Smith, W 0., Jr. and D. M. Nelson (1986) Importance of ice edge phytoplankton production in the Southern Ocean Bioscience. 36 : 251-257. Smith W 0 Jr. and D. M Nelson (1990) Phytoplankton growth and new production in the Weddell Sea marginal ice zone in the austral spring and autumn Limnol. Oceano gr 35( 4): 809-821. Spiridonov, V A. and K. N Kosobokova ( 1 997) Winter ontogenetic migration s and the onset of gonad development in large dominant calanoid copepods in the Weddell Gyre (Antarctica). Mar. Ecol. Prog. Ser. 157 : 233-246. Sullivan, C W., C. R. McClain, J C. Comiso and W 0. Smith, Jr. (1988) Phytoplankton standing crops within an Antarctic ice edge assessed by satellite remote sensing J. Geophys. Res. 93(Cl0): 12,487-12,498. Zar, J. H. (1984) Biostatistical Analysis Prentice Hall. Englewood Cliffs New Jersey. 35


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