The composition,abundance,biomassand diversity of the epipelagiczooplanktoncommunitiesof the southernBellingshausen Sea(Antarctic)with special referenceto krill and salps Volker Siegeland Urte Harm From tbe Federal ResearchCentrefor Fisberies,Hambwrg, FRG
Commwnirated by W. Arntz Receir.,ed:19 May 1995 Accepted:10Janwary 1996
Abstract Zooplankton was sampledin the southern BellingshausenSeawith RMT 1+8 gearduring australsummer 1994.Atotal of 1,21zooplanktonspecieswerefound. Although zooplankton diversity was high in oceanicand neritic waters,abundancesand biomasswere among the lowest recordedfor Antarctic epipelagiczooplankton. Copepodsand chaetognathsdominated numerically, while chaetognathsand krill Ewpbawsiasuperba dominated in biomass wet weight. Salpathompsonioccurredin low densities(median0.1to 0.4/1000m3),although during the sameperiod a massdevelopmentwas recordedfrom the South ShetlandIsland region. Density valuesfor Ewphawsiacrystalloropbiaswere in the samerange as reported from the southern Veddell Sea.Krill, Eupbausiasuperba,biomasswas lower than generally found in the Antarctic Peninsulaand ElephantIslandregion.A distinct spatialseparationfor size groups was observed for salps, Eupbausia crystallorophias and krill. Small salp size groups dominated in the East \flind Drift zone and larger ones further north under the influence of Vest Vind Drift waters. Larger size classeso{ E. crystallorophiasconcentrated in nearshoreareas.Krill was smallerin neritc and larger in oceanicwaters.The overall krill length frequency distribution was similar to that reported from the South ShetlandIsland region for the sameperiod. The recruitment index for E. crystallorophias and Thysanoessa nlacrura agegroup 1+ indicateda very successful year-clas s 7992/93in the region (Rr = 0.412 and Rt = 0.609,respectively),while krill showed the opposite,a poor recruitment of the 1992/93year class(Rr = 0.076).Spawning was late during the 1994seasonfor E. crystallorophias and E. swperba;no larvae were found in the area.These findings are discussedin the light of recentlydescribedcorrelationsbetweenwinter sea-iceconditionsand krill spawning and recruitment success.and lead to the conclusion that recruitment of the 1993/94 krill year-classwill be poor.
Arcb. Fisb.Mar. Res.44(1/2),1996,115-139,6figwres,5 tables
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V. Siegel and [J' Harm
Kurzfassung Diversität der- epipelagischen Die zusammensetzung, Abundanz, Biomasse und (Antarktis) unter besonBellingshausensee sudiichen Zooplanktong.rrr.i.,r.ho"fr;J; Salpen' und Krill dere^rBerückiichtigung von inder südlichenBellingshause5t,jll In RMT 1+g Netzfängenwurden im Sommer 1994 im Plankton der ozeanischenund Diversität die Zooplankton Arten gefunden.Obwohl Biomasseim unterstenBereichder und c.samtabundanz i"g." neritischenZone hoch *r., Copepoda-undChaetognathawaren numerisch bisher in der Antarktis ;;;:;;""'Werte. superüa nach Biomassedomia.. ril\'t'uphysia ch:;;;;;;;i;'""J ä;;t;;;äü;;i bi,s0,4Individuen (Median1ert.o,1 auf nierten.Salpatbomps";;;;;:;;;1;ge.i'g",rDichten im Gebiet der Süd ShetlandInseln eine rälbe,izeitraum im Ärt it" i,.iiäö;ir;,äü*äfri tM;;"-;.;;#kl";; wies Dichteanzahlen auf' wie sie aus crystallo.ropblas ;.;;. . irpuort* von Euphawsiasuperbawar ge-ringer, Biomasse bie v.aj.rr i?. r.r.ä""isind. ä;;,;;ilh;" und ,t- ElephantIsland gefunden Halbinsel Ä","rf.tischen J.. GJie, als sie allgemeinim wurde für Salpen'E' c"ry*iJ. gl"? a.utliche g..g."pftit.fte Trennungvon Längengruppen in dei Ostwindäominietten g.dtt.iC.ißä den f.il ü.oba.htet.SalpÄ "dr;;;;[;;;ä'*"Jai. stallorophiasund aufder\(/estwinddrift Einflußbereich iti Längengrupp-." giOn..en Küstenin überwieqend wurden traten.Die großen LängEngrupp"n uJn ELgrlitallorophas
ffi";if;.3'fä'ik1i:,;;nb5':J.-
Größealiin ozeanischen i.r'.ri"ä" J""tli.h geringerer
Überzeigten,eine Die Gesamtli.g."".rr"if"ng für Krili;nJ d.t'n.t,indsaufbau. Bereichen. InShetland die Süd um ,*ir.t.,'r äi? B.lliürh.;ten Seeund dem Seegebiet ;;;;#;"g Tb2sangeyl und aystalloropbias E' von I seln. Der Rekrutierungrli.tdo dt? Alttt'g..,ppe hin (Rt = 0,412 nlacruradeutetauf "iÄ r;l;;;folgreichäJ aLrgangrgg2/93i1i:^+egion Rekruschwache *?tt ft'fLitit d?n,J""htg'ng199?l9.1eine bzw. Rr = 0,609).O.-g;ffiU;t untersuchin der fand Krill und "." r. crystoltärop|ras tierung auf (Rr = 0,076):6;;il;.ft". wurden in Verbindung,mit ten Saison 1994 sehr "pä, ;;;. öi" .rorli"gÄden Ergebnisse und dem Krill\fiiter-Eisverhältnissen ,*lt.tt." gigf..it"., kürzlich beschriebene"tÄfrrt laichensowie Rekrutierungserfolgdiskutiert' R6sum6
de zooplancton 6pipelagiquede Composition, abondanceet biomassedescommunaut6s tenu desconditions particuliöresdu la Mer Bellingshaus." a" i"a (Antarctique) compte krill et du salPe 6t6 recueilli ä l'aide d'un chalut en Au cours de l'6t6 1994,l2l espöcesde zooplanctonsont entre les communaut6sde lavari6t6 RMT 1+8 au Sud a" irü".i.llingshausen. Bien que l'abondanceg6n6raleet la n6ritiques, ät plancton soit nombreusedans le, ät oc6aniques du niveaudesvaleursrelev6es biomass.dansla r6gion la plus bassesetrorrrrriänte' d.sroui 6taienten nombre domiiusqu,äpr6sentdansf n"i''.o;q"". Les Copepodaet Chaetognatha en biomasse'La dSmllaient superba Euphausia ., lek til ä'1";Jiö;l;;äh;;;sn.,'hr o'l ä 0,4 individus de ("tl.u. allant '','oy.n.," Salpatbompsor, rpp"äri1'T;;ä;t;t];fG en masse pour la m6mep6riodeun d6veloppement o"l i ooo-31, bi.r, qu. i'..pö.. connaisse proche,de ät"tit6 ttt 'nt montrdt au Sud des Iles Shetland. LesEupbawsiacrystalloropbiis lVeddei.La biomassedela Ewphausiasuperba.6t1i!.plYt celleconnue dans Ie S"äa. i--fvf.r
..J;;;;;" quecelles faibles
desIlesA*T^iT-tt:'*"LlT gc"c'liJt* la.r6gion
res de tarllecnez '1: nette Par.,groupes i;ifepi,.'",. On distinguaune co-upureg6ographique les eaux dans dominent petites'tailläs salpes,E. crystalloropEio-"rrn"rle'krill.ieJs"lp.t'd. apparaissentplutöt dans les eaux de courants ä venrs Bri'tu"dir que les g.orrp.. pl,r, gios
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1996 Arch. Fisb.Mar Res.44(1/2)'
Epipelagiczooplanctoncommwnitiesof tbe sowthernBellingshausen Sea
influenc6espar les courantsde vents Ouest. Les groupesplus importantsdesE. crystalloropbias ont 6t6 rencontr6sessentiellement proche des cötes.Le krill 6tait de taille nettement plus petite que dans les eaux oc6aniques.La r6partition de taille du krill et la structuredes quantit6smontraientune similitude entrela Mer de Bellingshausen et deseauxaux alentours du Sud des Iles Shetland.Lindex de recrutementpar groupe d'äge 1 desE. crystalloropbias et Thysanoessa macrurA indique l'ann6e 1992-1993comme une ann6etrös bonne pour la r6gion.(de R1= 0,412ä.Rt= 0,609).Lann6e 1992/936tait au contrairepour le crill une ann6e trös modeste(Rr= 0,076).Le frai desE. crystallorophi.as et du krill a eu lieu trös tard pendant la saison1994.Lesr6sultatspr6sent6sont 6t6analys6sen mettanten relationlesddpendances entre l'6tat de la glaceen hiver et le frai du krill ainsi que le succösde la recrue. Introduction The Bellingshausen Sea remains one of the poorly known regions of the Southern Ocean. Long seasonalice cover * even during summer the pack ice does not disappear from the shelf areas- make accessto this remote region difficult. Relatively few data exist on the composition of the zooplankton in the BellingshausenSeaand even less is known about quantitative aspectsof the plankton community. However, this kind of information is essentialfor the discussionof the implications within the pelagic system and for higher trophic levels. First basic geographical descriptions of the BellingshausenSea were given by Cook (1903) on the drift of the 'Belgica' in 1898/99 south of 69o S. From this cruise we also gain the first information on the occurrence of ice algae and large quantities of krill feeding on diatoms under the ice flows as well as the occurrenceof krill in crabeaterseal stomachsin the southern BellingshausenSea(Hansen 1908).Decadeslater, studies centered around the Antarctic Peninsulasampledlimited areasof the easternBellingshausen 'Villiam 'Discovery Sea.During 1929-31 British expeditions with Scoresby' and II' conducted plankton net sampling on stations running north of the ice edge westwards from Adelaide Island to a point beyond Peter I Island. Mackintosh (1.934)reported sparsezooplankton in the waters of the easternBellingshausenSea,while it was numerically richer to the west. He also mentioned the scarcity of euphausiids in the area.In his ^Bellingshausen later publication Mackintosh (tlZ:) found evidence for a krill stock, becauseof a higher krill density in the areabetween 72" and 97'W. Former Soviet Union cruisesworking in the Peninsula areaalso collected data from the open waters of the easternBellingshausenSea,describing krill size composition (Makarov 1979),oceanographicconditions and geostrophic currents (Makarov et al. 1982)as well as krill biomass and distribution (Latogursky er al. lllO). During austral summer 1978/79 a Polish research cruise covered the area along the Antarctic Peninsulaand partly investigatedthe easternBellingshausenSea.Resultswere published on phytoplankton and krill (\fitek et al. 1982), occurrence of krill larvae (Vitek et al. l98Q) and acoustic observations on krill aggregations(Kalinowski and \fitek 1980). The investigation of macroplankton samplescollected during an AngloGerman expedition along the Antarctic Peninsula in February 1982 resulted in the description of different communities which also extended into the northern Bellings-
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in hausen Sea (Piatkowski 1989). An acoustic krill biomass survey was carried out 'W 85" around to 69o S 65o S between cruise British duringthe 1992 Nov/Dec JGOFS just north of the marginal ice edge zone (Murray et al. tsvs). Almost each of rhese cruises concentrated on the northern part of the Bellingshausen Seanorth of 69o S, i.e. outside the pack-ice zone and beyond the continental shelf of the Bellingshausen Sea.However, data of Murray et al. (in press), Makarov (1979), and Latogirsky et al. (tWO) show that the northern distribution limit of krill is gradually -orrirg ,outh to 67o S when moving west from the Peninsulato Peter I Island. For this 'Polarstern' cruise to extend the studiesfrom the r.."ror,]it was the intention of the 1994 ice covered shelf areas,to describe permanently open water areasinto the poorly known data on zooplankton density and quantitative support the zooplankton composiiion, ro krill and salpswith survey data collected of composition biomasi, and compa.e the stock north. to the further simultaneously in regions
Material and Methods 'Polarstern' from Zooplankton was sampled onboard RV Jantaty 25 to March 11,1994 on2^2stationssourh of 66" S (Fig.1). Details on generalscientific objectivesof the cruise, starion lists and cruise rracks are summarizedby Miller (in press). Zooplankton was collected using the RMT 1+8 (single RectangularMidwater Trawl). A detailed descrip-
Fipure 1; Geographicallocation of RMT samplingstations,geostrophiccurrentsafter Ma'Belfica' in 1898/1899,thefast straightdrift to the ka"rover al. 06sf), and BD = drift of the \Teddell Sea,P: Peter Sea,_W': BS:Bellingshausen months. west occurredduring the summer I Island, E: Elephanilsland, S: South ShetlandIslands,B: BransfieldStrait,SH: shelf break, I: ice edgeinJanuary-March7994
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Epipelagiczoopkncton communitiesof tbe soutbernBellingsbausen Sea
tion of the net system is given by Baker et al. (1973).Standardoblique tows were made for the depth stratum 0 to 200 m. Filtered water volumes were calculatedconsidering net speed,net angle and flow data according to Pommer^nz et al. (lsAz}. The net was deployed in open water and in leads within in pack-ice zone. \When sampling in ice a sufficient sized areaof free water was formed behind the ship by propeller action to allow deployment and retrieval of the net from the stern. 'When the net reacheda depth of 20 m during hauling, the propeller was stopped to minimize the effects on the net operations and to avoid damageof the samples. Immediately after each haul samples were stored in 4%" buffered formalin seawater solution. Sorting, counting and measuringwas carried out a few days later onboard ship. Taxa were identified to the specieslevel, adult fish were excluded from the analysis, becausetheir abundance is strongly affected by net avoidance. Speciesbiomass and abundanceswere standardizedto grams or numbers per 1000m3.\flet weight was generally measured from formalin preserved material in the lab after the cruise, while salps were weighed onboard immediately after the haul. The RMT 1 and RMT 8 nets (mesh sizes 320 p,m and 4.5 mm, respectively) collect a broad size range of zooplankton and samplesfrom both nets were analyzedfor this study. Standardizedabundancevalues of each specieswere compared for both nets, and in each case the higher standardized abundancevalue was considered to minimise effects of net selection. Length frequency distributions were produced for euphausiids,measuredastotal length from the anterior margin of the eye to the dp of the telson for krill and from the tip of the rostrum to tip of the telson for the other species.Length measurementsfor salpsrefer to body length according to Foxton (1966).Maturity stageswere determined following the classificationof Makarov and Denys (1981). Severaldiversity indices were calculatedto characterizethe zooplankton community in different subareas.The most simple and unambiguous index is speciesrichnessS, which givesthe total number of speciesin a sample.The Shannon-\[iener index H'is maximum when all speciesare representedby the same number of individuals. H' is zero if there is only one speciesin the sample.The N2 index measuresthe number of very abundant speciesand is based on Simpson's index. For the evennessindex we used the modified Hill's ratio E5, becausethis index is relatively unaffectedby the number of speciesin the sample and tends to be independent of sample size. E5 approaches 1 as number of individuals become more and more evenly distributed among speciesor zero as a single speciesbecomes dominant. Further details on indices and formulas as well as the software to calculatethe indices are given by Ludwig and Reynolds (198S). The MIX programme of Macdonald and Pitcher (1979)was applied to length frequency data for the distribution mixture analysis of size groups. Variables of the age class 1 component were calculatedby stepwise optimization, i.e. proportion, mean length and sigma and their standard errors. The recruitment index R1 was determined according to de la Mare (1994). All other statitistical procedures were carried out using the software packageCSS Statistica.
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Results A total of tZt zooplankton specieswere counted from 22 samples in the epipelagic water layer. The largest taxonomic grouP was represented by copepods which consisted of 31 species.Ranking second was the group of pelagic fish larvae, with 23 species.Hyperiids and polychaeteseachcontributed 11 species.A complete specieslist is presentedin Table l. Diversity of zooplankton Speciesrichness,diversity and evennesswere calculatedseparatelyfor shelf and oceanic siations. A Mann-Whitney U test was carried out between the two groups for each of the variables.None of the variables showed a significant difference between shelf and p-level = 0.060;N2: oceanicareas(Richn ess;Z = -0.791,p-level = O.429tH';Z = -1'.91'2, -0.396,p-level = -1..583,p-level = reasondata from all = For this 0.692). 0.1,14;E5:z z = with results 2 together srarionsweie pooled and recalculated.Results are listed in Table Veddell Seain 1988 from a study in oceanicwaters of the southern Scotia Sea/northern For a direct were applied. (Siegel et al. t}lZl for which precisely the same methods layer 200 m water for the data set comparison indiceswere recalculatedfrom the original of the northern \Teddell Sea. Mean speciesrichness was higher in the BellingshausenSea(S = 35) than in the open warer a;d the pack-ice zone (S = 31) of the Veddell Sea.The mean partly reflects the difference in total number of speciesbetween the regions, which was evidently higher in the BellingshausenSea(120 versus 88 speciesin the \üeddell Sea).This difference can mostly be explainedby the higher number of copepod (7 species)and the larvaeof neritic fish species(14 species)in the BellingshausenSea. The diversity H'for the Bellingshausensamplesis very similar to the results found in rhe northern Weddell Sea.If only the upper 60 m beneath the closed ice cover is considered, then the diversity is much lower in the northern Veddell Sea.The N2 index is heavily weighted toward the most abundant speciesin the samples while being less sensiti.,reto speciesrichness.A low N2 value means a high dominance of few species. The relatively low N2 diversity index indicatesa dominance of few speciesin the catches from the Bellingshausenand Veddell Seas.Interestingly the index was highest in the transition belt, the marginal ice zone of the \fleddell Sea,demonstrating a more uniform speciescomposition. In generalthe majority of specieswere rare' The rank abundanceplot is one graphical method of presenting speciesabundancedata in which percentageabundanceis plotted againstspeciesrank (seeMagurran 1988).The plot for the Bellingshausencommunity illustrates the typical shape of the_log series irr*", while the speciesabundancesfrom the closedpack-ice zone of the northern \fleddell Seafollows the geometric seriesmodel (Fig.2). In generalthe figure shows that the majority of speciesin the BellingshausenSeaare rare. IJnder the closed pack-ice of the Weddell Seathe situation shifts to an extreme with very few abundant and some rare species.
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Epipelagic zooplancton commwnities of the soutbern Bellingshausen Sea
Table I: Species list. Asterisks indicate specieswhich occurred only once in the samples. Hydrozoa
E uch ire lla rostrornagnd
Calycopsis borch greainhi 't'Haliscera conica
Gaidius intermed.ius
':'HaLiscera racooitzae 't'Ru ssellia rnirab ilis '?Solm un d.ella b it ent dcu lat a Scyphozoa Peripbylla peripbylk Siphonophora Dimophyes arctica
Gaidius tenuispinus Heterorbabdus fanani H eterorbab dus papilliger 'rH eterorh ab dus p ustulifer Lucicutin flaoicornis Lucicutia uolfendeni Metridia gerlachei
Dipbyes antarctica
Metridia lucens 't-M io ocalan us p y gtn aeu s
't Marras antarcttcas
Parae u ch a eta dn tdr ctica
Pyrostepb os vanb oeffeni 'tVogtia serrata
Paraeucbaeta biloba 't'Paraeucb aeta farrani
Ctenophora '?Beroecucumis
Paraeuchaeta rasa 't'Ple uromamma ro b usta
Beroe forskalii Cephalopoda A llur ot eut h is antar cticu s Gastropoda Clio pyrarnidata Clione limacina Limacina belicina Spon giob r an cb a ea au stra lis Polychaeta 'tBylgides pekgica " Lopadorrbynchus appendiculatus Maupasia graciLis Pelagobia longicrrdtd Rbyncb onereella bongraini SagiteLlahowalepsbii Tomopteris carPenteri 'r Torn opt e ris sePt ent rio n a lis '?Trav i siopsi s conicePs 't'Trav isioPsislev inseni Vanad.isantarctica Copepoda Aetideopsis minor 'tBathycalanws brad'yi Bathycalanus princePs
Pseudocbirelh mawsoni Rbincahnus gigas Scapbocalan us antarcticu s 't-Scaph ocaknu s p arantarcticus Scole cit h r icella d ent iPes Scole cit h ric elk rninor Ostracoda Alacia belgicae Ahcia bettacra '?Boroecia antiPoda 't-M etacon cb oecia isocb eira M etaconchoecia shogsbergi Euphausiacea
Primno macropa 'tScina antarctica Tbemisto gaudicbaudü Vibiliz antarctica Gammaridea Epimeriella tnacronyx Eusirus antarcticws Eusirus microps E usir ws p r opep er d.entat u s Orchornene plebs 't'Orchomenerossi Chaetognatha Euhrohnia hamata Sagitta gazellae Sagitta tnarri Sagitta maxirna Tunicata Salpa tbompsoni Ihlea racovitzai Pisceslarvae 'tArtedidraco loennbergi 'tArte d id.raco orianae 'tArtedidraco shottsbergi 'tB at hy dr aco a.ntarcticus 't'C h aenodraco uiLsoni
E upb awsia cry stalloroPb ias
C h iono draco rastro sPinosus 't Cry odr a co antar cticus
Euphausia superba
't'Dacodraco bunteri
Euphausia triacantha ThysanoessaTndcrilrd Decapoda larvae Acanthephyra pelagica C h orism us a.ntarctict'ts 't'N oto cran gon antar cticu s Mysidacea 'tAntar ctomy sis rnax im a 't'An t ar ctirny sis ob lini ':'E uch aetomera zr.trstrassent
Calanoides acutus Calanus propinquws
Hyperiidea
Candacia maxima Ctenocalanus citer
Cyllop us rnagellanic us 'rHyperia antarctica
E ua ugaptilu s antar cticu s 'tE ua ugap t il u s p ar ant ar cticu s
Hyperiella nxacronyx 't'Hyperocb e me dusarum
Cyllopas lucasü
Electrona antarctica Lepidonotothen hempi '?Lepidonotothen larseni 't'M uraenolepis rnicr ops Notolepis annulata Notolepis coatsi 't-Pagetop sis macr oPt er u s PIeuragramma an t drcticum 't'P ogon opb ry n e 7nar rn or at a 't'Prionodraco evansü "Psilodraco breoiceps '?Ra cov it z ia glacia lis
Hyperia mdcrocepbala
't'7i em at om u s lep id or h in us 't Tr erna t otn u s n eutn esi
Hyperielh dilatata
'rTiematomus scotti
Arch. Fish.Mar. Res 44(1/2),1995
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Table 2: Mean speciesrichness,diversity and evenness for zooplanktonin the Bellingshausen Sea.Resultsfrom a study in the oceanicregion of the northern \(eddell Sea(from Siegelet al. 7992and 'i recalculatedfrom original datafor 0 to 200m depth) arelistedfor comparison. O\7 = open water, TRANS = outer and inner marginalpack-icezone,CP = closedpack-ice zofle.
BellingshausenSea Sea 0-200m
Index
RichnessS Diversity H' Diversity N2 EvenessE5
o\r 0-60m
35.2 1.997 5.41 0.58
26.4 1.743 4.71, 0.64
O\7* 0-200m
31, 1.923 4.95 0.56
Veddell Sea TRANS CP 0-200m 0-60 m 27.8 2.128 6.1,2 0.63
i,7.4 1,.435 3.56 0.57
CPX 0-200m 31 1.985 5.25 0.58
D istribution, dominance and abundance In the BellingshausenSea 46 of the recorded specieswere only found at one or two stations and iherefore have a constancy of lessihan tO % . Thäse are marked with an asterisk in Table 1. Only 8 specieshad a very wide distribution and were observed at more than 90 "Ä of all stations.Thesewere the copepods Calanoidesacutus,Rhincalanws gigas, Calanas propinquus, Metridia gerlachei, the chaetognaths Ewb,rohnia bamata, Sagitta gazellae, the euphau säd Thysanoessan'tacrura and the siphonophor e Dipbyes antarctica. Except for the siphonophore these specieswere also the ones with the highest abundances. Several investigations identified tbe existenceof a neritic and an oceanicplanh.ton community in Antarctic rpaters(e.g.Boysen-Ennen and Piatkowski 1988;Piatkowski 1989; Siegeland Piatkowski 1990; Hosie 1994).Ve calculatedthe abundancesseparatelyfor the shelf and oceanic stations. Due to the non-normal frequencv distribution of the speciesabundance data we preferred the median instead of the mean abundance.Zero
10 b! O C
o ! c l
B e l l i n g s h o u s eSne o
_o
0.01
n o r t h e r n W e d d e l lS e o
closed pock-ice
zone
40 Snpriec
122
60
80
100
Figure 2: Speciesrank abundanceplot for datafrom the Bellingshausen Sea(present study) and the closedpackice zone of the northern Veddell Sea(datafrom Siegel et al. 1992).
Arch. Fish.Mar. Rel44(1/2), 1996
Sea Epipelagiczooplanctoncornmunitiesof tbe sowthernBellingsbausen
carcheswere included in the calculation of the median. Results for the more abundant we found four copepod speciesare listed in Table 3. Among the six dominant specie_s ".rd t*o chaetognath speciescontributing 80"/" of the total number of zooplankton specimens,with-Calanoides acutus (37.6%) dominating numerically in the neritic zone. fbyronorrro n'racruftt(2.6%) was ranked seventh,krill twelvth and Ewpbawsiacrystalloiopbias fourteenth. Eupbawsia crystalloropbias was clearly restricted to neritic waters' ", *".. larvae of almost all fish species.However, even theseand other neritic indicator species(Antarctomyslsspp, Gammaridea,Pleuragramma antarcticum) occurred in very läw densities,or rhe distribution was extremely patchy resulting in a low median abundance as for Eupbawsia crystalloropbias. for the dominant speciesin oceanicand neritic Table 3: Median abundances(in n/1OOOm3) watersof the BellingshausenSea.Resultsfrom a study in the oceanicregion of the northern \Teddell Sea(from Si"geler al. 1992)are listed for comparison,abbreviationsseeTabIe2.
Species
Sea Bellingshausen Oceanic Neritic
38.5 28.0 45.6 2.9 9.1, 16.0 2.5 0.0 2.1, 0.6 1.5 0.2 0.0 0.0 1.1 0.2 2.3 0.2
O\T 0-60m
VeddellSea CP TRANS 0-200m 0-60m 45.5 40.9 7.5 37.9 10.9 24.7 36.7 0.0 1.4 0.0 0.0 2.6 0.0 0.0 0.0 0.5 0.0 18.8
101.0 1,6.3
2.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0
245.3
180.6
66.4 23.1 15.4 14.5 1,1,.7 9.9
Salpatbompsoni Hyperiella dilatata Eupbawsiafrigida Vibilin antarctica Tomopt eris sept entrionalis Pleurornamma robusta
0.4 0.0
n l
0.0 0.0 0.0
0.0 0.0 0.0
30.5 34.1,
176.8 576.8
155.1 981.3
5277.7
Median total Mean total
1996 Arch. Fisb.Mar Res.44(1/2),
t.o
4.3 4.2 3.4 2.9 2.7 2.5 2.1. 2.0 1.6 1..4 1.4
0.1
11.0 36.1.
947.6 459.8 271.0 31.8 3.7 2988.9 1,44.r 0.0 0.0 0.0 0.0 0.9 0.0 0.0 0.0 11 3 . 8 0.0 40.8
Calanoidesacutus Calanuspropinquus Metridia gerlachei Eukrohnia harnata Sagitta gazellae Rhincalanusgigas Thysanoessamacrura Lucicutia llaaicornis Alacia belgicae Dimophyes arctica Par aewchaet a ant ar ctic4 Euphausiasuperba Scolecithricelladentipes E uph ausia crystalloroPh ias Pyrostephosaanhoeffeni Limacina helicina Heterorhabdus farrani Alacia bettacra
+J.Z
I O.a
t.u
4.1 22.4 38.8 0.0 0.0 0.0 0.0 oz.+
0.0 0.0 0.0 0.2 0.0 0.2
r23
V.Siegeland U. Harm
The abundance of the dominant speciesdid not differ noticably between shelf and oceanicstations.Metridia gerlacbei (29.4%) replaced Calanoidesacutus (24.8"Ä) as the dominant speciesin the oceanic region. Chaetognaths were less abundant in oceanic waters, while Rhincalanus gigas occurred in higher densitiesfurther to the north. Thysanoessa. mAcrLtrA,a common speciesaround the Antarctic continent, occurred everywhere in the study area,but only in relatively small quantities.Ewpbausiatriacantbawas found at only three of the northernmost oceanicstations. Comparison with the results from a cruise to the oceanicsouthern Scotia Sea/northern Veddell Seain spring 1988/89 clearly demonstratesthe overall low total abundanceof zooplankton in the BellingshausenSea(Table 3). Even the maxima for single stations in the Bellingshausen Sea (577 plankton specimens/lOOOm3at one shelf station and 981/1000m3 for an oceanicstation) were one order of magnitude lower than the overall median abundancefound in the open water of the Scotia/\Teddell Sea area(5277 specimens/1000m3).The BellingshausenSeaplankton reachedonly the abundancelevel of the very poor plankton community under the pack ice of the northern \WeddellSea. Table 4: Median wet weight (in g/1000m3) for the dominant speciesin oceanicand neritic watersof the BellingshausenSea.Resultsfrom a study in the oceanicregion of the northern Veddell Sea(from Siegeler al. 1992)are listed for comparison.O\ü = open water, TRANS = outer and inner marginalpack-icezone, CP = closedpack-icezone.
Species
Bellingshausen Sea Oceanic
Neritic
Sagitta gazellae Euphausiaswperba Salpa tbompsoni Dybyes antarctica Dimopbyes araica Eukrohnia hamata Pyrostrepb os aanh oeffeni Calanoidesacutus Tbysanoessa macrura Calanuspropinquus E uphausia crystal I oroph ia s Rhincalanusgigas Par aewchaet a antar ctica Metridia gerlachei Sagitta maxima Sagitta mani Heterorhabdus farrani Epimeriella macron)tx Alacia belgicae Themistogaudichaudii
1.64 1.52 0.73 0.39 0.31 0.27 0.26 0.26 0.18 0.r2 0.12 0.11 0.03 0.03 0.02 0.01 0.01 0.01 0.01 0.00
1,.27 0.15 0.1,4 0.46
Median total
6.03
124
O\T 0-60m
VeddellSea TRANS CP 0-200m 0-60m
0.06 0.20 0.15 0.08 0.14 0.00 0.18 0.02 0.00 0.01 0.01 0.01 0.00 0.00 0.13
0.37 0.49 101.00 0.09 0.00 0.38 0.00 3.32 7.21, 2.53 0.00 28.39 0.00 0.30 0.00 0.04 0.00 0.00 0.00 0.11
1.09 1..43 2.00 1.26 0.00 0.46 0.00 0.16 1.83 0.23 0.00 0.23 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00
0.04 34.32 0.00 0.41 0.00 0.05 0.00 0.04 1,.94 0.20 0.00 0.21. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
3.07
r49.73
9.1,2
37.35
Arch. Fish.Mar. Rel44(1/2), 1996
Epipelagic zooplancton commwnities of the soutbern BellingshawsenSea
Biomass Median total biomass of zooplankton was extremely low with a slightly higher biomass than in the oceanicregion (3.07 g/1000m3).These in neritic areas(6.03 g/1OOOm3) 'S7eddell values were of a similar magnitude to the marginal ice zone o{ the northern Sea in 1988, but were much lower than in the open water and closed pack-ice zone (Table 4). In the open water zone of the Veddell Sea salps contributed 67.5 "/" to the total zooplankton biomass of L49.73g/1000 m3. In the closed pack-ice zone krill dominated the zooplankton biomass 6y 92 % (37.35 g/1000 m3);the remaining zooplankters contributed only 3 g/1000 mr. Krill and salps regwlarly occwrred in tbe samplesof tbe BellingsbawsenSea,and interestingly tbey followed tbe cbaetognatb Sagitta gazellae in the dominance of biomass. Flowever, dominance values for krill and salps were not substantially high in neritic warers (25.2% and l2.l o/o respectively) and even lower at oceanic stations. In the oceanic region several siphonophore and copepod speciesadded more, or at least the same,wet weight to the zooplankton standing stock (Table 4). Median biomass of Ewpbausia crystalloropbiaswas as low as 0.12 g/1000 m3, not exceeding2 o/" dominance in biomass,although this speciesregularly occurred in the shelf samples.For neritic waters again siphonophores (15.9 %) and copepods (9.3 %) contributed more to the zooplankton standing stock than this typical neritic euphausiid species,and even Tbysanoessa macrwra showed a slightly higher biomass (3 % dominance). SaIps Salps occurred in 68 %" of the samplesfrom the BellingshausenSea,but median abundance was low in offshore and shelf waters (0.1 and 0.4 salps/1000m3,respectively). Maximum salp abundancein the BellingshausenSeawas lessthan 51 salps/1000m3.This was a striking contrast to the findings at two stations further north in the Bransfield Strait (see Fig. 1), where salps showed a mass occurrence. These catchesyielded 161 (1134g/1000m3).This resultedin (358 g/1000m3)and 263 salps/1.000m3 salps/1OOOm3 anumericaldominanceoflz.g-73.2o/" orabiomassdominanceof ge.s-98.9o/" of salps in the Bransfield Strait catches. Salpbody length datawere collectedfrom both areasand length datawere submitted to a hierarchical cluster analysis using relative frequencies of length classesas station parametersto distinguish possibledifferencesin distribution patterns.The Euclidean coefficient was used as the dissimilarity coefficient and clustering was done by the minimum variancelinkage rule (\(ard's method). At a distancelevel of 0.60,the size frequency distributions of salpsclusteredinto two distinct groups of stations.There was a strong spatial coherenceof the two groups (Fig.3a),with group I located along the Antarctic Peninsula and including the northernmost oceanicstationsto the west, while group 2 was restricted ro anareasouthofalinefromAdelaideIslandtoPeterl Island.Themodalsizeofthe groups differed by approximately 24mm body length (Fig 3b). Group 1 in the northern region was the larger sized (L = 40mm). Group 2 was distinctly smaller (L = 16mm).
Arcb. Fish.Mar. Res.44(1/2),1996
1,25
V.Siegeland U. Harm
a)
Cluster1
b)
_-/
.
n Eleph.tst. * Bell.north
15
- Bell. south
I
d ro c o
= q
E
* 5
0
10 20 30 40 s0 60 70 80 90 100 Length (mm)
Figwre3: a) Spatialdistribution of sizegroupsof Salpatbompsonifrom the southem (ch.rste r 2) and northern Bellingshausen Sea/Bransfield Straitarea(cluster1) duringJanuaryto March 1994.b) Length frequencydistribution (body length). Krill and other ewpbawsiids Thysanoessamacrura was present in 91 %" of all samples.Maximum abundancein the BellingshausenSeareached47 specimens/1000m3,but the speciesshowed no preference for shelf or oceanic areas.Due to the early spawning seasonof this species,larvae had
rzo
Arcb. Fish.Mar. Rel44(1/2), 1996
Epipelagiczooplanctoncommwnitiesof tbe southernBellingsbawsen Sea
already progressedto the furcilia stage.However, larvaewere only found at three northern oceanicstationswith high densitiesbetween 867 and 991 specimens/1000ml, respectively). Length of the larvae ranged from 3 to 7 mm and this group was clearly separated from the one year old juveniles of 1Oto 16 mm length (Fig. a).
't2 11 10 I
8 e ä 7
6 6 * s
fi+ 3 2 1 0
10
1s 20 25 Length(mm)
30
35
40
macrurafrom the Bellingshausen Sea Figwre4: Length frequencydistribution of Thysanoessa during January /Mar ch 1994.
The recruitment index was calculatedfor age group 1 using the formula given by de la Mare (1994).The index can range between O(no recruitment) to 1 (stock consistsonly of recruits). For further detailson the estimation of the index seede la Mare (1994)and Siegel nlacrura with R1 = 0.609 and Loeb (1.995).Thisvalue was relatively high for Thysanoessa (Se = 0.046) indicating a very successfulyear-class 1992/93.Adult stagesdominated the stockwith am odalsizeof 20 mm.0.5 "Ä of theadultfemalesrepresentedthe recently spent maturity stage,while 93'/" had already recovered from spawning and belonged to the resrrngstage. Ewphawsiacrystalloropbiaswas absentat all oceanicstations but did occur in all but one sample from the neritic zone (92o/o constancy). However, abundance was extremely patchy. Maximum densitieswere recorded in nearshore shelf areas,e.g. exceeding310 specimens/1000m3 in Marguerite Bay (Adelaide Island), whereasouter shelf areaswere less densly populated, generally less than 3 specimens/1000m3.There are indications that larger specimensdominated in nearshorewaters (group 2,Fig5), while smaller size classeswere more abundant in the outer shelf zone (group 1, Fig.5). The overall length frequency distribution is characterizedby the strong juvenile mode around 15 mm, representingagegroup 1+ (Fig. 5). The recruitment index for the 1992/93 year-classwas relatively high, Rr = 0.412(Ss = 0.006).A second mode at26 mm consisted of subadults and adults. 32%" of the adult femaleswere carrying spermatophoresand
Arcb. Fish.Mar. Res 44(1/2),1996
t27
V Siegeland U. Harm
were sdll in the processof spawning, while 31 %" were spent. No advancedlarval stages were caught during this study.
12 10
$ a o
6 6
ElRaun N suuaa. Z,tuv 5
101520253095
j
(t
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5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 Length (mm) Figure ); Length frequency distribution of Ewphausiacrystalloropbias,frorntheshelf areasof thä BellingshäusenSia. Insertedfigure shows the differencein length frequenciesbetween Group 1 and Group 2 occurring in outer and inner shelf zones,resPectively.
The krill Eupbawsiasuperbawas present in most (56 %) of the samplesfrom oceanicand ice covered neritic waters. Maximum density was measuredat station 45 (278/1000lr:'3) in oceanic,ice-freewaters eastof Peter I Island. However, abundancewas extremely low in the region west of Peter I Island. Cluster analysison dissimilarity of length frequency distributions separatedtwo distinct groups at a distancelevel of 0.7. Group 1 was representedby stations located on the southern shelf and around the shelf break (Fig.6a). These samplesincluded a higher proportion of agegroup 1+ specimens(23.1 %) and the modal size of adults was about 45 mm. Group 2 consisted only of oceanic stations. In this cluster krill age class 1+ was scarseand adult krill dominated the offshore waters. Furthermore, this group showed a shift to larger adult animals with a modal size of 50mm (Fig.6b). The overall length frequency distribution (Fig.6c) shows the relatively low abundance of age group 1+ specimensin the region. The recruitment index was R1 = 0.076 (Sr = o/. 0.011). Mean length of one year recruits was 23.6 t 2.20 mm. Juveniles made up 9.4 of the stock, subadults47.8"/", and adults 42.8%. Most of the adult females(36.6%) belonged to the prespawning stage 3A (adult without spermatophores).Since gravid (stage3D) and recently spent females(stage3E) were absent(Fig.6d), one can conclude that the krill stock was still in an early processof spawning, which is confirmed by the absenceof any surfacelarval stagesduring the study period.
128
Arch.Fish.Mar. Res.44(l/2),1996
Epipelagic zooplancton communities of tbe soutbern BellingsbawsenSea
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Arch. Fisb.Mar. Rel44(1/2), 1996
129
V.Siegeland U. Harm
Discussion Mean speciesrichness of zooplankton was similar in the BellingshausenSeaand in the northern oceanicVeddell Sea(Siegelet al. 19921,although the total number of species was obviously higher in the BellingshausenSea.The lower number of zooplankton speciesin the \(eddell Sea area may be explained by the early spring situation when many speciesstill live in deeperwater layers and have not started their seasonalvertical migration (Mackintosh 1937,Yoronina 1,973).This is supported by the observations from the northern Veddell Sea,when deeperwater layers (down to 200 m) are included in the analysis (seeTable 2). Furthermore, summer data from the easternand southern Veddell Seashowed that the speciesrichnesswas almost as high asin the Bellingshausen Sea(Boysen-Ennenand Piatkowski 1988). Zooplankton diversity indiceswere againvery similar betweenthe two regions.Only the transitional zone (marginal pack-ice zone of the northern Veddell Sea)showed higher diversities,becausedominance of singlespecieswas lesspronounced in this zone. On the other hand, studies carried out along the Antarctic Peninsula in the seasonalpack-ice zone indicated a generally lower speciesdiversity index (mean H'= 0.350to 1.649)than for the Bellingshausencommunity (H'= 1.997).The oceanicand shelf areasof the Peninmacrura.or salps(Piatsula region are known for high dominance of krill, Tbysanoessa kowski 1989;Siegeland Piatkowski i990), which results in a lower diversity index. Ice cover has been shown to have a strong effect on the occurrenceof species(Siegeler al. 1,992).Speciesabundancesdecreasedrapidly when entering the pack-ice of the northern \(eddell Seaand speciesthat were rare in oceanicopen waters disappearedunder the closedpack-ice.Poor food resourcesin the water column under the ice probably prevent most speciesfrom colonizing these waters, becausethey are generally not capable of utilizing the ice algae,the only sufficient food resource. The oceanic closed pack-ice zone supports only poor resources,with ice algae in late winter and spring. \With the exception of krill only very few zooplankton speciesseem to be able to utilize this resource(there may be other species,but we haveno information on the specieslevel about their ability to use or depend on this resource),and thus krill is becoming the dominating element in the community. However, despitethe described relative diversity of this community, abundancesare generally low, becausespeciescannot sustain high numbers and biomass. Results from other investigations on zooplankton biomass are listed in Table 5. Comparisons are difficult, becauseof different size fractions of the studied zooplankton, different gearsizesand different vertical extent of sampling.Someinvestigatorsincluded the mesopelagic zooplankton in their calculations, a community which was found to have a higher diversity than the epipelagic one (Hardy and Gunther 1935; Siegeland Piatkowski 1990),but probably has a lower standing stock. Some studiesexcluded salps and euphausiidsfrom their biomass estimates,becauseof the pronounced net avoidance of larger zooplankton speciesto small plankton nets. Both approachesreduce the biomassesrimatesfor the epipelagicplankton quite substantially,so that theseresults must be considered as minimum zooplankton biomass estimates.
130
Arch.Fish.Mar. Rel 44(1/2),1926
Epipelagiczooplanctoncommunitiesof tbe sowtbernBellingshawsen Sea
Table 5: List of zooolankton biomassdata from Antarctic waters.Resultsin bracketsare Figuresindicatedwith asterisksaremedian transformeddataby Boysen-Ennenet al. (1,991). values,while others are means.
Region
SouthernOcean Indian Ocean Lützow-Holm Bay (Shelf) PrydzBay PrydzBay
Latitude
Depth
D.y weight g/m
55-7A 50 -7A
0 -1000 0 - 100
2.1 (0.9)
69
0 - 660
(1.1)
60 - 68 60 - 68
0 - 200 0 - 200
(2.4)
\(et $üet Dominant weight weight ,species g/m 9/1000m'
Reference
(19.5)
F o x t o n( 1 9 5 6 Voronina and Naumov (1968) Fukuchi et al. (1985)
(19.5) Copepoda 73.6 Copepoda 1.5-25.5 Copepoda 59.7 27
-
Hosie et al. (1988) Hosie and Stolp
(1e8e) 78
0 - 800 0.2 - 2.3
RossSea
78
ScotiaSea
57 - 61 60 - 61,
0-300 0 - 200
Mc Murdo (RossSea)
Scotia Sea
0.9
(1.5-34.4)Copepoda/ E. crystallorophias/ Limacina helicina 2.5 4.6
24 82.8
0-200
Hopkins (1987)
F o s t e r( 1 9 8 2 ,1 9 8 9 )
E. superba Lancraftet al. (1989) Salps/ Sicinskiet al. (1991) E. superba
N. \üüeddellSea (oceanic) N. \iüeddellSea
64 - 67
0 - 1 0 0 0 1 . 1- 1 . 3
65 - 70
o - 200
(0.8)
N. Veddell Sea (ocean.open waters)
58 - 60
0 - 60
1.5
9.0
60
0 - 100
0.45
2.7
9.1+
Salps/ Siegelet al. (1992) Euphausiacea E. superba
N. \Teddell Sea (ocean.marg.ice zone)
8 . 8 -1 0 . 4 30.2
Copepoda Copepoda
149.7't Salps/ Copepoda
Hopkins and Torres (1988) El Sayedand Taguchi(1981) Siegelet al. (tllz)
60 - 63 N. Veddell Sea (ocean.closed pack ice) 64 - 66 N. \WeddellSea E. WeddellSea(oceanic) 66 - 73
0 - 60
0.45
2.2
37.3*
2a
( 1 o o ) E. superba
0 - 300
2.8
24.9
74.9
Euphausiacea/Boysen-Ennen et al (1991) Copepoda
NE Veddell Sea(shelf)
70 - 74
0 - 300
3.4
23.6
78.1
SE Veddell Sea(shelf)
75 - 78
0 - 300
1.2
8.7
28.7
S \Teddell Sea(shelf)
75 -77
o - 200
(1.5)
(2.0)
ee.8
E.oystallorophias/ Copepoda E.crystallorophias/ Copepoda/ Limacina helicina -
Antarctic Peninsula
64 JU- /U
0 - 1000 3.1 0 - 1000 2.7 0 - 200 0.1* 2.0 0 - 200 0.2''
l'acrtrc5ector Bellingshausen Sea (oceanic) (neritic)
67 -73 67 -73
t.J
Arch. Fish.Mar Rel 44(1/2),1996
( 2 8 . 8 ) Copepoda (21.6) Copepoda 0.6't 10.7 1,.2* 6.9
3.1'l 56.8 6.0* 36.8
Siegelet al. (1992) Lancraftet al. (1989)
Boysen-Ennenet al. (1991) Boysen-Ennenet al. (1991)
El Sayedand T a g u c h i( 1 9 8 1 ) Hopkins(1985) Hopkins(1971)
Chaetognatha/present study Copepoda Chaetognatha/presentstudy E. saperba
IJI
V.Siegeland U. Harrn
Comparing our resulrs with those obtained by similar net types and in similar depth ranges(e.g.Hosie et al. 1.988;Hosie and Stolp 1989;Lancraft et al. 1,989;Boysen-Ennen Sicinski et al. 1.991;Siegeler al. tlVZ) we can seethat the BellingshausenSea et al. 199'1,; zooplankton biomass is in the lower range of the estimates.A few lower values were only recorded from the southern shelf of the \Teddell Seaand the PrydzBay, areaswhich are also influenced by extremely prolonged seasonalice cover. Median biomass values demonstrate that zooplankton standing stock in oceanicareasof the BellingshausenSea was evenlower than under the seasonalpack-ice of the northern rVeddellSea.Sincemost of the investigationswere carried out during mid austral summer, one can conclude that the southern'S(eddellSea,Prydz Bay, BellingshausenSeaand probably Ross Seasustain only a low zooplankton biomass with probably low production rates' Most studies carried out in high latitudes recorded copepods as the dominant components of the oceanic zooplankton biomass or Ewpbausiacrystallorophiasfor the neritic areas.\fle can confirm the numerical dominance of copepods for the oceanicas well as the neritic BellingshausenSea. However, it is interesting to note, that chaetognaths, especially Ewkrobnia bamata and Sagitta gazellae substantially contribute to the numerical dominance of species.Mackintosh (1934) noted that the copepod Rbincalanws gigas was the dominant species,with Calanoidesacwtwsranking second.In other years, however, the latter was more abundant, and in some years chaetognathswere very abundant. During our study we observed a situation similar to the alternative described by Mackintosh (1934) where Calanoides Acutus, Calanus propinqaus and Metridia gerlachei outnumbered Rhincalanwsgigas.From the numerical dominance the composition resemblesthe oceaniccommunity describedby Hosie and Cochran (1994)for the Prydz Bay region. Regarding the dominance in biomass (wet weight) we found a slightly different situation. In the oceanicregion chaetognathsdominated the zooplankton, while chaetognaths and krill Ewphausia swperba were the dominant components in shelf waters. Neither copepods nor Ewpbausiacrystalloropbiaswere of major relevanceto the biomass of the neritic zooplankton. According to Mackintosh (193a) chaetognathsare typical warm water speciessometimes found in colder water, while most of the copepods mentioned belong to widespread species.Due to this dominance of warmwater speciesand becauseof the low biomass of high latitude neritic species(r.g. E. crystalloropbiasand Plewragrammaantarcticwm) we suspectthat the interannual differencein the composition of the dominant speciesis causedby variability in ice conditions and that in the BellingshausenSeathe summer 1993/94or the preceding winter was warmer than during other years. The tunicate Salpa thompsozi shows a high degree of interannual variation in biomass and abundance.Mean abundanceestimatesfor the Antarctic Peninsulaareaand different years range from 40 specimens/1000m3 for the season1990/91(Nishikawa et al. 1'995) to 5010 specimens/1000m3 in 1.989/90(Park and $üormuth 1993)with maximum densities exceeding25 000 salps/1000m3(Siegel unpubl. data). The season 1993/94 was anorher successfulsalp year: Loeb and Siegel (t99S) recorded a mean density of 9lt salps/1000mi (median 582 salps/l000m3) for the Elephant Island area. For the same period we observed very low salp densities in the BellingshausenSea with 1.3 to 7 mi). m3 (median0.1 to 0.4 specimens/1000 specimens/1000
132
Arch. Fisb.Mar Rel 44(1/2),1996
Sea Epipelagiczooplanctoncomrnunitiesof the soutbernBellingshausen
Regional differenceswere not only observedfor salp abundances,but also for the spatial distribution of salp size groups. A more northern group consistedof larger salpswhich were found under the influence of SüestVind Drift waters, while distinctly smaller salps 'Belgica' drift). occurred in southern waters of the East Vind Drift (the region of the During the sameperiod the large, ubiquitous salp concentrations in the South Shetland Island areawere of similar size to those found in the Bransfield Strait and the northern ranges of the BellingshausenSea.Size classeseven larger than 60 mm were measured around the South Shetlands,which were missing in our samples(Anonymous 1995).A sparial separation of size groups in other years was already reported by HuntJey et al, (1989). These authors concluded that the larger srzegroups (around Elephant Island) were advected from upstream areasalong the Antarctic Peninsula where smaller salps were found. This seemsmeaningful for the Elephant Island/ South Shetland area,with a continuous flow from the southwest. However, for the BellingshausenSeawith the two different current systems,we suspectthat salpson the southern shelf were advected from the oceanic region and trapped under unfavourable conditions. Sufficient food supply leads to the rapid development of large numbers of aggregateforms. Increasein size of aggregateforms and continual releaseof chains by solitary salpsresult in a mixed size distribution and rapid multiplication in early spring (Foxton 1966).This strong salp recruitment obviously occured in the \[est Vind Drift waters during the 1.993/94season. On the other hand long and denseice cover and consequently low phytoplankton concenrrarions in late winter/early spring inhibit rapid salp growth and recruitment (Siegeland Loeb 1995).This situation generally occurs in regions such as the southern Weddell Seaor the BellingshausenSea.As a result salp length is small, reproduction is low and therefore also abundanceis low in the East \find Drift zone, even in years when tVest Vind Drift. salps show a mass development in regions under the influence of the According to Mackintosh (193a) abundance of euphausiids was generally low in the BellingshausenSea.From other parts of the Southern Ocean we know that high latitude shelf areas are dominatedby Eupbawsincrystalloropbiasas one of the main indicator species,e.g.in the southern Veddell Sea(Fevolden 1980;Siegel1982;Boysen Ennen and Piatkowski 1988) or the Prydz Bay area(Hosie and Cochran 1994).A mean density of 54 specimens/lOO0m3was reported for the Veddell Sea by Boysen-Ennen and Piatkowski (19SS),although this value should be treated with caution, becausethe mean was calculatedfrom positive hauls only and zero valueswere deleted,thereby overestimating the abundance to an unknown degree. Our results from the BellingshausenSea are probably in the same range (mean 39.8 individuals/1000m3 and median 2.1 individuals/1000m3). During our study we did not encounter larval stagesof Ewpbausia cvystallorophias and adult stageswere still in the processof spawning. Investigationscarried out in the Indian Ocean secror over many years indicate the peak spawning seasonfor this speciesduring late November/December with few exceptionsof late spawning inJanuary (Pakhomov and Perissinotto 1994).Fevolden (1980) found furcilia stagesin the \WeddellSeafrom mid February onwards. In February/March Boysen-Ennen and Piatkowski (1988)samwith pled rather high densities of larvae in the Veddell Sea(1927 specimens/lOOOm3)
Arch.Fish.Mar. Res.44(1/2),1996
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V Siegeland U. Harrn
highest concentrations in the upper 50 m water column. Since these authors used the same net equipment as in the present study, our results can hardly be explained by methodological biases.On the other hand results obtained for the recruitment index for age group 1+, year-class 1992/93,show a very successfulspawning for this euphausiid stock in the precedingyear. Pakhomov and Perissinotto (1994) suggestedthat spawning successof Eupbawsia crystallorophias r.rraystrongly depend on the timing of ice breakout and formation of polynias above the continental shelf. This hypothesis should be consideredfor this speciesin more detail by future studies,becauseit has great relevancefor the krill Ewpbausiasuperba (seebelow). Krill is almost completely absent on the shelf of the southern Veddell Sea(Fevolden 1980;Siegel1982;Boysen-Ennen and Piatkowski 1988),and in the neritic waters of the Ross Sea(Marr 1,962),but seemsto occur regularly on the shelf of the ice covered BellingshausenSea.Mackintosh (1973) defined various krill stocks around the Antarctic on the basisof regional differencesin krill densities.He also defined a Bellingshausenstock for the area berween 72o and 97" \X/,but pointed out that this does not imply that krill within the different stocks are isolated from the rest. Marr (1,962)and Lubimova et al. (1982) concluded from their studies that areasof high krill density occur around 90" \f and south of 65o/66" S latitude. A recent UK hydroacoustic survey indicated that the northern distribution limit of krill in this areais probably around 66" to 67o S (Murray et al. in press).These authors estimaredmean biomass of 19.6 and 42 g/m2 for two con'1,992. Hewitt and Demer (1994) sumsecutive surveys in November and December marizedbiomassestimatesfrom hydroacoustic surveysin the Elephant Island areawhich is thought to be an areaof high krill concentrations.They listed resultswhich rangefrom 8.4 to 134.5g/m2, most valuesbetween24 and78 g/ m2.Biomassestimatesfrom RMT net samplesin the Antarctic Peninsularegion rangefrom 4 .2 to 13.9g/m2 (Siegel1,986,L992). From the present study we calculated a mean biomass of 2.0 g/m2 for the entire area. From the comparison it can be concluded that krill biomassin the BellingshausenSeais in the lower rangeof the values,generallyfound in the Antarctic Peninsulaand Elephant Island region. Krill size classeswere spatially separatedwith larger size groups preferably in offshore water. This confirms the description of Makar ov (1979)and Lubimova et al, (1982),who found the large mature specimenspredominately between 66" and 68' S in the northern periphery of the krill distribution range, gradually extending to the north in the eastern part in the vicinity of the Antarctic Peninsula.This spatial successionof krill size/age groups was also described for other areas,e.g. the Peninsula region and the southern Scotia Sea/norrhern Veddell Sea (Siegel 1988; Siegel et al. 1990). The overall length frequency distribution showed a dominance of length classes> 40 mm indicating a poor availability of age groups 1+ and 2+.The same situation was found around Elephant Island (Anonymous 1995), with larger krill beyond the continental shelf break, and overall dominance of krill > 40 mm. Larger sized length classeswere slightly underestimated, becausethe study did not cover the areascloseto the northern distribution range adaeqately,where larger specimensdominate. The recruitment index for age group 1+ in the BellingshausenSeawas calculatedas R1 = 0.076,the mean index from two surveys
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off Elephant in the same summer seasonwas R1 = 0.064 (Siegeland Loeb 1995).This is in very close conformity and shows that not only the actual krill stock composition but also the recruitment of year class1.992/93was very similar over a wide spatial scalein the Southern Ocean. It was shown by Siegeland Loeb (1995) that krill recruitment strongly dependson the extent, duration and concentration of ice coverageduring winter. Long and wide ice cover favours krill spawning successand larval survival, while weak ice conditions negatively affect krill recruitment. In this respectit is interesting to note, that seaice conditions between the BellingshausenSeaand the Antarctic Peninsula region are closely positively linked over the years, while at the same time other regions show an inverse trend in ice conditions to the area under consideration (Stammer;'ohnand Smith in press).This would support the similarities within the krill stock composition between the two regions. Earlier studies reported that krill spawning in the BellingshausenSeaoccurs mostly in January and February and that during this period 53 to 98 %" of the females were of gravid or spent maturity stages(data of four yearspublished by Spiridonov 1995).In the presenrstudy the proportion of gravid stagesreachedonly 7.6"/o and spent femaleswere not observed. Furthermore no krill larvae were caught in the surfacewater during the present investigation. From this it can be concluded that spawning was rather late in the BellingshausenSeain 1.994,even later than off ElephantIsland (seealso Fig.6d). However,the krill maturation processaround Elephant Island was alreadyinterpreted aslittle advancedand as too late in the seasonto support a successfulspawning and later recruitment of the year class 1993/94 (Siegel and Loeb 1995). The same conclusion can be drawn for the BellingshausenSeastock. The recruitment indices for krill and for the other euphausiid speciesshowed opposite trends. Ewpbawsiaswperbahad a low recruitment value in the BellingshausenSeain1,994, while Eupbawsia crystalloropbias and the omnivorous Tbysanoessamacrura had rather high values. It is not known, which environmental or biological parameters control spawning and recruitment of these two species.Flowever, if ice conditions regulate reproduction timing and offspring success,then ice conditions must act in quite a different way than they do for Ewpbawsiaswperba.
References Anonymous, 1995:Report of the CCAMLR \florkshop: Temporal changesin marineenvironmenrsin the Antarctic Peninsulaareaduring the 1994/95australsummer.(Hamburg 17-2rJuly 1995).CCAMLR \üG-EMM-95/59: 1-47. Baker, A.de C.; Clarke, M.R.; Harris,MJ., 1973:The N.I'O. combination net (RMT 1+8) and further developmentsof rectangularmidwatertrawls.J. Mar. Biol. Assoc.U.K. 53: 167-r84. Boysen-Ennen,E.; Hagen,\fl.; Hubold, G.; Piatkowski,U., l99l:Zooplankton biomassin the ice-covered\fleddell Sea.Antarctica.Mar. Biol. 111:227-235.
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Boysen-Ennen,E.; Piatkowski,IJ., 1988:Meso- and macrozooplanktoncommunitiesin the \feddell Sea.,Antarctica.Polar Biol.9: 17-35. der Bericht über die Entdeckungsreise Cook, F.A., 1903:Die ersteSüdpolarnacht1898-1899. 'Belgica'in der Südpolarregion. (German 415 Pub. transl.): pp. Kösel Jos. de la Mare, \ü.K., 1994:Modelling krill recruitment.CCAMLR Science7:49-54. Taguchi, S., 1981:Primary production and standingcrop of phytoplankton El Sayed,S.Z.; along the ice-edgein the \Teddell Sea.Deep-SeaRes.28:1017-1032. Fevolden,S.E.,1980:Krill off Bouvetöyaand in the southern\üeddell Seawith a description of larval stagesof Euphausiacrystallorophias.Sarsia65: 149-162. Foster,8.A., 1987:Composition and abundanceof zooplankton under the spring sea-iceof McMurdo Sound,Antarctica.Polar Biol. 8: 41-48. Foster,8.A., 1989:Time and depth comparisonof sub-icezooplanktonin McMurdo Sound, Antarctica.Polar Biol. 9: 431-435. Foxton, P., 1,956:The distribution of the standing crop of zooplankton of the Southern Ocean.Discovery Rep. 28: 191-236. Foxton, P.,7966:The distribution and life history of SalpatbornpsoniFoxton with observations on a relatedspeciesSalpagerlachelFoxton. Discovery Rep. 34: 1-116. Fukuchi, M.; Tanimura, A.; Ohtsuka, H., 1985:Zooplankton community conditions under the seaice near Syowa Station,Antarctica.Bull. Mar. Sci.37: 518-528. Hansen,H.J., 1908:Schizopodaand Cumacea.Resultatsdu Voyagedu S.Y.Belgicaen 1897Rap.Scient.Zool. Anvers.1-20. 1898-1899. E.R., 1935:The plankton of the South Georgiawhaling groundsand Gunther, Hardy, A.C.; adjacentwaters 1926- 1927.Discovery Rep. ll: l-456. Hewitt, R.P.; Demer, D.A., 1994:Acoustic estimatesof krill biomassin the ElephantIsland CCAMLR Sci.1: 1-5. area:7981-1993. Hopkins,T.L., l97l: Zooplankton standingcrop in the Pacific sector of the Antarctic. In: Llano, G.\(/.; Wallen,I.E.(eds.):Biology of the Antarctic SeasIV. Antarct. Res.Ser.17: 347-362. Antarctic Peninsula. Hopkins, T.L., 1985:The zooplankton community of Croker Passage, P o l a rB i o l .4 : 1 6 l - 1 7 0 . Hopkins, T.L., 1987:Midwater food web in McMurdo Sound, Ross Sea,Antarctica.Mar. Biol. 96: 93-106. Hopkins, T.L.; Torres,JJ., 1988:The zooplankton community in the vicinity of the ice edge, western\üeddell Sea,March 1986.Polar BioL 9: 79-87. Hosie, G.\7., 1994:Multivariate analysesof the macrozooplanktoncommunity and euphausiid larval ecologyin the Prydz Bay region,Antarctica.ANARE F.ep.1'37:l-209. Hosie, G.\ü.; Ikeda, T.; Stolp, M., 1988:Distribution, abundanceand population structure of the Antarctic krill (EupbausiasuperbaDana) in the Prydz Bay region, Antarctica. P o l a rB i o l . 8 : 2 1 3 - 2 2 4 . Hosie; G.\ü.; Stolp, M, 1989:Krill and zooplankton in the westernPrydz Bay. Symp. Polar Biol.2:34-45. Hosie, G.\ü.; Cochran, T.G., 1994:Mesoscaledistribution patternsof macrozooplankton communities rnPrydz Bay, Antarctica -January to February 1997.Mar. Ecol. Prog. Ser.106:21-39.
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Huntley, M.E.; Sykes, P.F.; Marin, V., 1989: Biometry and trophodynamics of Salpa thompsoniFoxton (Tunicata:Thaliacea)near the Antarctic Peninsulain australsumPolar.Biol. 10:59-70. mer, 1983-1984. Kalinowski, J.; \7itek, 2., 1,980:Diurnal vertical distribution of krill aggregationsin the westernAntarctic. Pol. Polar Res. 1: 727-146. Lancraft,T.M.; Torres,JJ.; Hopkins, T.L., 1989:Micronekton and macrozooplanktonin the open warersnear Antarctic ice edgezones(AMERIEZ 1983and 1986).Polar Biol. 9: 225-233. Latogursky, V.I.; Makarov, R.R.; Spiridonov,V.A.; Fedotov, A.S., 1990:Distribution and biology of Eupbawsiasuperbain the areaof the Antarctic Peninsulaand the adjacent waters.Trudy AtlantNIRO 1990:20-40. Loeb, V.; Siegel,V., 1995:AMLR program: Krill and macrozooplanktonin the Elephant Island area,Januaryto March 1994.Antarct.J. U.S. 29: 185-188. stocksandroleof E. superbarnthe Lubimova,T.G.;etal.,l982The ecologicalpeculiarities, trophic structureof the Antarctic ecosystem.SelectedPapersPresentedto the Scientific Committee of CCAMLR 1982-1984,p.391-505. Ludwig,J.A.; Reynolds,J.F.,1988:Statisticalecology.New York: John \üiley and SonsPubl. 3 3 7P P . Macdonald,P.D.M.; Pitcher,T.I. 1979:Age groupsfrom sizefrequencydata:A versatileand efficientmethod of analysingdistribution mixtures.J. Fish.Res.Bd. Can. 36:987-1001. Mackintosh,N.A., 1934:Distribution of the macrozooplanktonin the Atlantic sectorof the Antarctic. Discovery Rep. 9: 65-160. Mackintosh,N.A., 1937:A seasonalcirculation of the Antarctic macroplankton.Discovery Rep. 16:365-412. Mackintosh, N.A., 1973:Distribution of post-larvalkrill in the Antarctic. Discovery Rep. 36:95-156. London: Croom Helm Ltd. Magurran,A.E., 1988:Ecologicaldiversity and its measurement. Publ. 179pp. Makarov; R.R., 1979:Size composition and conditions of existenceof Ewpbawsiaswperba in the easternpart of the Pacificsectorof the Southern Dana (Crustacea:Euphausiacea) Ocean.Oceanology19:582-585. Makarov, R.R.; Denys, CJ., 1981:Stagesof sexualmaturrty of Eupbausiaswperba.BIOM A S S H a n d b o o kS e r .1 1 :1 - 1 1 . Makarov, R.R.;Maslennikov,V.V.;Movchan,O.A.; Solyankin,E.V., 1982:Oceanographical in plankton off the Antconditions and regionalpeculiaritiesof seasonalsuccessions arcric Peninsula(in Russian).In: The Antarctic. Soviet Committee of Antarctic Research,CommitteeRep.21: l0l-117. Marr, J.\(.S., 1962: The natural history and geography of the Antarctic krlll (Ewphawsia Dana). Discovery Rep. 32: 33-464. suPerba. Miller, H.; Grobe, H., 1996: The expedition ANTARKTIS-XI/3 of RV Polarstern in 188.155pp. 1993/94.Ber.Polarforsch. rWatkins, Murray, A.V.A.; J.L.; Bone, D.G., 1995:A biologicalacousticsurvey in the marSea.Deep-SeaRes.II, 42:1159-L175. ginal ice edgezone of the Bellingshausen
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Epipelagiczooplanctoncommunitiesof the southernBellingshawsen Sea \witek, Z.; Koronkiewicz, A.; Soszka,G.J., l98o: certain aspectsof the early life history of krllI EuphausiaswperbaDana (Crustacea).Pol. Polar Res.1:97-115. \flitek, Z.;Pasttszak, M.; Grelowski, A., 1982:Net-phytoplankton abundancein wesrern Antarctic and its relation to environmentalconditions.Meeresforsch.2g:166-180.
Acknowledgements The authors like to express their gratitude to the crew of RV Polarstern for their friendly help during the course of sampling and to the chief scientist Dr. H. Miller (A\rI) for his support of the zooplankton programme. \fle greatly appreciatethe expertise of Dr.V Loeb (Moss Landing) in verifying the determination of larval fish species.Thanks are due to the helpful comments of the two refereesand their effort to improve the manuscript. Authors' address: Dr. Volker Siegel and Urte Harm, Bundesforschungsanstaltfür Fischerei, Institut für Seefischerei,Palmaille 9, D-22767 Hamburg, FRG. Fax: +4038905-263, e-mail:
[email protected]
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