ORIGINAL PAPER

Peter Ward Æ Rachael Shreeve Æ Geraint A. Tarling

The autumn mesozooplankton community at South Georgia: biomass,population structure and vertical distribution

Received: 21 November 2005 / Revised: 20 March 2006 / Accepted: 20 March 2006 / Published online: 20 May 2006� Springer-Verlag 2006

Abstract Mesozooplankton were sampled at shelf andoceanic stations close to South Georgia, South Atlanticduring austral autumn 2004 with a Longhurst HardyPlankton Recorder. Onshelf biomass ranged from 2.18to 5.75 g DM m�2 (0–200 m) and was dominated by thesmall euphausiid Thysanoessa spp. At the oceanicstations (10.57–14.71 g DM m�2, 0–1,000 m) largecalanoids, principally Rhincalanus gigas comprised �47–52% of biomass. Here Calanus simillimus was stillactive and reproducing in surface waters (0–11.2 eggs fem day�1) but R. gigas and Calanoides acutuswere largely resident in the warm deep water andundergoing their seasonal descent. A comparison withspring and summer data indicated increased abundanceand biomass from spring through to summer followedby a decline towards autumn particularly over the shelf.Autumn values in oceanic waters differed little fromsummer. Mesozooplankton biomass in the surface200 m of the oceanic stations as a proportion of thatfound in the top 1,000 m ranged from 63 to 78% of thetotal in spring and 62–73% in summer, but was only 23–29% of the total in this study, following redistributiondown the water column.

Introduction

Lying along the Scotia Ridge in the north-eastern cornerof the Scotia Sea, the island of South Georgia has longbeen recognised as one of the most productive regions ofthe Southern Ocean (Hardy and Gunther 1935; Atkin-son et al. 2001). The island was the centre for thewhaling industry in the early part of the 20th century

and presently supports commercial fisheries for fin-fishand krill as well as abundant higher predators. Pro-duction is often enhanced across a number of trophiclevels and satellite images, in some years, reveal intensephytoplankton blooms which may persist for manyweeks and months, where chlorophyll biomass averages‡ 3 mg m�3 and exceptionally exceeds 20 mg m�3

(Korb and Whitehouse 2004). The underlying causes forsuch elevated production, which broadly consists oflarge and chain forming diatom species such as Thalla-siothrix sp., Thallasiosira spp., Chaetoceros spp. andNitzschia spp., have been linked to the dynamichydrography and to micronutrient supply, in particulariron, particularly downstream of the island (Korb andWhitehouse 2004). Mesozooplankton biomass is alsoelevated and can exceed the Southern Ocean average bya factor of 5–6, particularly in late summer when thenew generation of many species is developing (Wardet al. 1995; Atkinson et al. 1996; Pakhomov et al. 1997).Rapid growth and development of populations of largecalanoid copepods at South Georgia and elsewhere inthe Southern Ocean has been linked to the presence ofphytoplankton blooms containing large diatoms (Shre-eve et al. 2002) with copepod carbon mass being posi-tively and significantly related to the proportion ofmicrophytoplankton > 12 lm present in the water(Ward et al. 2005, 2006). A study by Pakhomov et al.(1997), based on samples taken later in the season inFebruary/March 1994, also indicated high biomass ofmesozooplankton present around the island (4.3–11.5 g DM m�2) although, with the exception of Cal-anus simillimus, feeding rates were low, suggesting thatdevelopmental arrest was occurring prior to winterdescent. However, the vertical distribution of zoo-plankton was not resolved.

Although most biological production takes placeduring austral summer within surface waters, theSouthern Ocean is characterised by the seasonal redis-tribution of plankton biomass within the water column(Hopkins 1971; Atkinson and Peck 1990). The descentof some species in late summer and autumn reduces the

P. Ward (&) Æ R. Shreeve Æ G. A. TarlingBritish Antarctic Survey, Natural Environment Research Council,High Cross, Madingley Road, Cambridge CB3 0ET, UKE-mail: pwar@nerc-bas.ac.ukTel.: +44-1223-221564Fax: +44-1223-221259

Polar Biol (2006) 29: 950–962DOI 10.1007/s00300-006-0136-3

amount of plankton present in near-surface waters al-though, summed over the top 1,000 m, winter abun-dance has been found to be consistently �80% ofsummer values (Hopkins 1971; Atkinson and Ward1988). Investigating diurnal and seasonal patterns ofplankton distribution is important if we are to under-stand both energy flow within the larger water columnand the population dynamics of key species (see Tarlinget al. 2004). Two previous studies using a LonghurstHardy Plankton Recorder (LHPR) sampler aroundSouth Georgia in late October/early November 1997(Ward and Shreeve 1999) and January 1990 (Ward et al.1995) investigated the biomass, community structureand vertical distribution of mesozooplankton at shelfand oceanic sites. A recent cruise undertaken in lateMarch/early April 2004 provided a further opportunityto use the LHPR to characterise the zooplankton com-munity in the same area later in the year, by assessingcommunity composition, biomass and vertical distribu-tion during the austral autumn.

Materials and methods

Near-surface chlorophyll a concentrations were deter-mined from discrete water samples obtained from theships non-toxic supply which had an inlet located at�7 m depth. Samples were filtered through glass fibrefilters (Whatman GF/F) under low (< 70 mmHg) vac-uum pressure and immediately frozen and stored at�20�C until analysis. The samples on the filters werethen extracted in 10 ml of 90% acetone in the dark, for24 h (Parsons et al. 1984). Fluorescence of the extractwas measured before and after acidification with 1.2 MHCl on a TD-700 Turner fluorometer. The instrument

was calibrated against commercially prepared chl-astandards (Sigma, St Louis, MO, USA).

Longhurst Hardy Plankton Recorder sampling wascarried out at four stations located near the north coastof South Georgia between 21st March and 2nd April2004. Our previous studies (Ward et al. 1995; Ward andShreeve 1999) focussed on just two stations (shelf andoceanic) but this time, because of the amount of icepresent around the island caused by the grounding andbreak-up of iceberg A-38-B, the original shelf stationcould not be sampled. Samples were instead taken asclose as possible to the original positions at 53.60�S,38.15�W and 53.61�S, 38.04�W at local midday and localmidnight, respectively (Fig. 1). As in the previous stud-ies, the LHPR was equipped with a 38-cm dia nose coneand a 200-lm net with the samplers’ gauze advancemechanism set at 60 s. The net was rapidly deployed tothe required sampling depth, allowed to stabilise andthen hauled in an upwardly oblique manner to the sur-face. A ship’s speed of 3.5–4 kts combined with a towingspeed of between 1.75 and 2.0 m s�1, ensured a depthresolution of �10 m per patch at the shelf station. Dueto the onset of bad weather, only a single 0–1,000 mprofile was possible at the original oceanic station(52.9�S, 39.48�W) and we have therefore used an addi-tional 0–1,000 m haul taken in the approximate locality(53.25�S, 38.14�W) 1 week earlier albeit at a similar timeof day (night through the early hours of the morning).At the oceanic stations the sampler was deployed in thesame manner as over the shelf but the gauze advancewas extended to 90 s which resulted in a depth resolu-tion of �20 m per patch. Upon recovery, the gauzeswere inspected, cut to correspond with each samplinginterval and frozen at �20�C. In the UK, the sampleswere gently thawed and plankton washed off the gauzes

Fig. 1 The study area duringcruise JR100. The fourLonghurst Hardy PlanktonRecorder stations are annotatedwith the station number. Bongonet stations are indicated by thefilled circles

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and preserved in formalin (10%, v:v). Samples were sub-sampled with a Folsom plankton splitter when necessaryand examined under a stereo-microscope. Plankton wereidentified where possible to species and developmentalstage and counted (usually 500–800 species stages persample). Raw counts were standardised using flow data.Temperature data were obtained via a sensor incorpo-rated as part of the net monitor control system and anaverage determined within the depth range of eachsample patch.

Additionally, a series of 19 motion-compensatedpaired Bongo net (62 cm dia, 200 lm mesh net, 5l solidcod-end) hauls were made in the surface 200 m at aseries of stations near to and west of the LHPR samplingsites towards Shag Rocks (Fig. 1). The contents of oneof the paired net cod-ends were preserved in formalin(see above) for analysis in the UK. From the other cod-end, groups of ten female Rhincalanus gigas and C. si-millimus were placed in perspex inserts (750 ml volclosed off with 800 lm mesh) and suspended in 1.5lcontainers of filtered sea water (FSW) maintained atambient water temperature (�2�C) in low light. After24 h females were removed and eggs were filtered fromthe FSW, backwashed into a Petri dish and counted.Egg production rate (EPR) was expressed aseggs fem day�1. Females from EPR experiments andindividuals of Calanoides acutus CV were rinsed inammonium formate and placed in pre-weighed tin foilcapsules and dried and stored as above to await drymass determination in the UK. Dry mass values for theother epipelagic and mesopelagic species were derivedfrom length vs dry mass regressions based on valuesdetermined by Ward et al. (1995).

Results

Oceanography

Temperature profiles obtained during the LHPRdeployments are presented in Fig. 2. At the shallowLHPR stations, the near-surface temperature maximum(T� max) was just over 3�C with the start of a temper-ature minimum (T� min) located between 130 and140 m. The T� max indicated warming during the pre-ceding summer months with the upper 100 m still beingreasonably well stratified with little evidence of storminduced mixing. At the deep stations, the T� max wasagain a little over 3�C with the T� min indicating theremnants of the previous winter water occurring around130 m for E213 and 210 m for E142. The underlyingcircumpolar deep water (�2�C, Orsi et al. 1995) canclearly be seen extending to depth from about 400 m.

Chlorophyll data

Hourly underway measurements indicated that themedian surface chl a value within the study area (see

Fig. 1) was 0.445 mg m�3 (quartile ranges; Q1 = 0.293,Q3 = 0.99, n = 131). Values recorded over the innershelf were uniformly < 1 mg m�3 with the minimumvalue (0.07 mg m�3) measured over the shelf close to theNW corner of the island. Offshelf, values were also gen-erally low, although a bloom (max value 14.9 mg m�3)was present at the western outer shelf break (500 m) andwestwards towards Shag Rocks (Fig. 3).

LHPR abundance and biomass

Shelf stations (E129 and E131)

The distribution of total mesozooplankton in the sam-pled water column is illustrated in Fig. 4a. Integratedbiomass was equivalent to a standing stock of 2.18 and5.75 g DM m�2 for the midday and midnight hauls,respectively. The difference was predominantly due tothe increased abundance of Thysanoessa spp. at night(3.36 g DM m�2 representing 58% of total biomass;Table 1) but also to the decreased numbers of large

Fig. 2 Temperature profiles obtained during the Longhurst HardyPlankton Recorder deployments. a Shelf stations—filled circleE129, filled upward triangle E131; b oceanic stations—filleddownward triangle E142, filled square E213

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calanoids during the daytime haul. Differences in overallabundance were not as pronounced as those in biomass.The rank order of abundance and biomass was largelyconsistent between hauls with Oithona spp., Metridia

spp. and Ctenocalanus spp. accounting for 82–87% ofabundance and Thysanoessa spp., R. gigas, C. simillimus,C. acutus and Metridia spp. accounting for 85–88% ofbiomass (Table 1).

Vertical distribution

Diurnal migratory behaviour was strongly apparentamong many of the dominant taxa with pronouncedvertical migrations apparent in Calanus propinquus,C. simillimus, Metridia spp. and Ctenocalanus spp.,although the evidence for such behaviour in R. gigas andC. acutus is ambiguous at best. The vertical distributionof taxa contributing ‡ 2% to either abundance or bio-mass are illustrated in Fig. 5.

Cumulative frequency plots indicated that, during themidday haul, 50% of total zooplankton abundance laybetween the surface and 126 m whilst, at midnight, 50%lay within the surface 80 m. In contrast, the diurnaldepth differences in biomass distribution were less pro-

Fig. 3 Surface chlorophyll values (mg m�3) within the study areaduring cruise JR100

Fig. 4 Abundance(ind. · 103 10 m�3) andbiomass (g DM 10 m�3)profiles at the shelf and oceanicLonghurst Hardy PlanktonRecorder stations

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nounced, with 50% of biomass occupying the top 65 mcompared to the top 50 m at midnight. This was largelydue to the lack of migratory behaviour in the biomassdominant euphausiid Thysanoessa spp.

Oceanic stations (E142 and E213)

Although the profiles look superficially similar (Fig. 4b),E142, taken closer to the shelf break, hadmarkedly higherzooplankton abundance (3.8 · 105 vs 1.98 · 105 in-d. m�2), although both were higher in this respect thanover the shelf (Fig. 4a). The difference in biomass was notso marked, �11 and 15 g DM m�2, between 2 and 7times as high as onshelf. With the exception of Drepan-opus forcipatus, which effectively only occurred in haulE213, the rank order of abundance of taxa contribut-ing > 2% was the same in both hauls (see Table 2).

The large interzonal calanoids collectively contrib-uted 47–52% of biomass with R. gigas accounting forthe greatest proportion.

Vertical distribution

Given that the deep profiles were obtained from differentstations a week apart, it is not possible to infer anythingabout diurnal migratory behaviour although there weresome clear differences illustrated in E142 where therewere two regions of increased biomass, one in the top100 m comprising mainly Thysanoessa spp., Themistogaudichaudii and Metridia spp. and another at �500 mcomprising mostly R. gigas and C. acutus.

In E213, a greater proportion of total biomass(0–1,000 m) was present in the surface 200 m (40%compared to 19% in E142) due largely to the near-surface presence of D. forcipatus in addition to Thy-sanoessa spp. The former, more usually neritic, specieswas the most abundant species in this profile (Table 2).Distribution of these and other taxa comprising > 2%to either abundance or biomass are shown in Fig. 6.

Stage structure and vertical distributionof the large calanoids

Rhincalanus gigas

This species was the most abundant large calanoid off-shelf and the second most abundant over the shelf. Itaccounted for 37–44% of total mesozooplankton bio-mass offshelf and 9–18% over the shelf (Tables 1, 2). Allcopepodite stages of this species were present at bothsites with the exception of males which were absent overthe shelf. In all hauls at all four sites CIV was thedominant stage comprising an invariant 43–50% of thisspecies abundance. Onshelf, the next dominant stagewas CIII (�30%) and there was also a greater propor-tion of earlier stages whereas offshelf, the next dominantstage was CV (�27%) with few CIs and CIIs present.Median depths determined for the dominant stages atthe shelf stations indicated that they generally lay deeperat night, whereas, at the two oceanic stations, the pop-ulation was deeper in the day by up to 100–200 m perrespective stage (Table 3).

Calanoides acutus

This species contributed around 1% of total abundanceand between 4 and 8% of total biomass across all fourstations (Tables 1, 2). Populations everywhere weredominated by CVs (76–91%) with CIV effectively con-tributing the remainder, although a few CIIIs (< 1%)were found at two of the sites and females (�2%) at oneof the deep stations (E213).

Over the shelf, median depths were deeper at night,although consistently shallower than for R. gigas.Examination of Fig. 5 does however reveal a virtualabsence from the near-surface layers during daylight.

At the oceanic station, median depths were consid-erably deeper in the day with highest abundances beingfound between 900 and 1,000 m.

Calanus simillimus

This species was the most abundant large calanoid overthe shelf (2–3% of total abundance and 10–11% of totalbiomass, Table 1). Copepodite stages CV and CVI fe-male dominated at all sites (�75–90%) and it was theonly species where males formed an average of ‡ 5% of

Table 1 The percentage contributions of the principal zooplanktontaxa and species in the shelf Longhurst Hardy Plankton Recorderhauls with respect to abundance (ind. m�2) and biomass(g DM m�2)

Taxa Midday Midnight

Abundance Biomass Abundance Biomass

Oithona spp.a 47.0 2.0 40.0 0.5Metridia spp.b 20.0 7.0 18.0 5.0Ctenocalanus spp. 20.0 3.0 25.0 2.0Copepod nauplii 3.0 < 0.1 0.3 < 0.05Calanus simillimus 3.0 11.0 5.0 10.0Rhincalanus gigas 2.0 18.0 1.5 9.0Drepanopus forcipatus 1.5 0.2 4.0 1.5Calanoides acutus 1.0 8.0 1.0 5.5Thysanoessa spp.c 1.0 40.0 3.0 58.0Scolecithricella minor 0.5 0.5 0.5 0.3Calanus propinquus 0.3 4.0 0.2 1.5Chaetognathad 0.2 < 0.1 0.2 <0.5Clausocalanus laticeps 0.2 < 0.1 < 0.1 < 0.5Euchaeta antarctica < 0.1 0.5 < 0.1 0.5Euphausia frigida < 0.1 2.5 < 0.1 3.0Themisto gaudichaudii < 0.05 3.0 < 0.05 2.0Others < 0.1 < 0.1 0.8 0.6

aMainly Oithona similisbMainly Metridia lucenscThysanoessa vicina and Thysanoessa macruradMainly Eukrohnia hamata

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the population, although younger stages were essentiallyabsent.

Median depths were, in contrast to the two speciesabove, shallower at night, when median depths for allspecies stages were within approximately 10 m of thesurface (Table 3).

At the two deeper sites the population was locatedwithin the near-surface layer with median depths around50–70 m for all stages except CV. Although 40–50% ofCVs were found in the top 100 m, median depths were

much deeper as secondary peaks were observed basedaround 500 m or deeper (Table 2).

A seasonal comparison of abundance and biomass atthe LHPR stations from this and previous studies isshown in Table 4, along with average abundance esti-mates derived from Bongo nets. The presence of theneritic copepod D. forcipatus can have a marked impacton both zooplankton estimators in all seasons, particu-larly over the shelf. The trends in abundance and bio-mass, once this patchily distributed species has been

Fig. 5 Abundance vs depth ofthe taxa that contributed ‡ 2%to either abundance or biomassat the shelf Longhurst HardyPlankton Recorder stations.Large calanoids figured first.For details of relativeabundance see Table 1

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subtracted from the totals, are consistent, in so far asthey indicate a rising trend in abundance and biomassfrom spring through to summer and then a decline to-wards autumn over the shelf. Oceanic abundance andbiomass follow a similar albeit less dramatic trend with adoubling of both from spring to summer and althoughautumn values differ little from summer, a dramaticredistribution of zooplankton down the water columnhas occurred. Mesozooplankton biomass in the surface200 m of the oceanic stations as a proportion of that

found in the top 1,000 m ranged from 23 to 29% of thetotal in this study compared with 63–78% of the total inspring and 62–73% in summer.

Egg production

Of the two species selected for egg production experi-ments, only C. simillimus females were located within thenear-surface layer at the majority of stations. AlthoughR. gigas was present in near-surface layers at stations

Fig. 5 (Contd.)

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over the shelf, females comprised only a small part of thepopulation and at only 2 of 29 stations were sufficientencountered to set up egg production experiments. Off-shelf, the LHPR profiles indicated female distributionsof ‡ 500 m and none were encountered in the 0–200-mBongo net hauls.

In the two experiments setup for R. gigas, egg pro-duction exceeded 10 eggs fem day�1 (Table 5). Femalesin both were taken from stations where chl a valuesexceeded 1 lg l�1. Egg production in C. simillimus wasextremely variable (0–17.8 eggs fem day�1, Table 5).

Discussion

Zooplankton occurring around South Georgia has beenwell described, particularly in recent years, althoughstudies have, in the main, taken place during the summerperiod (Ward et al. 1995; Atkinson et al. 1996; Shreeveet al. 2002). In trying to understand how this productiveregion functions and compares to other parts of theSouthern Ocean, it is necessary to extend temporal

coverage to include other seasons (Atkinson et al. 2001;Tarling et al. 2004). Two previous studies have investi-gated the spring and summer mesozooplankton popu-lations using the LHPR (Ward and Shreeve 1999; Wardet al. 1995) and have provided finely resolved informa-tion on depth distribution, abundance and biomass ofthe mesozooplankton community. Although the firstdetailed plankton surveys of the South Georgia regionwere carried out as long ago as 1926–1927, conclusionsregarding seasonal changes were largely limited to not-ing the decline of some species in near-surface waters inthe autumn/early winter periods (Hardy and Gunther1935, p. 265 et seq). The only other recent study duringthe autumn period was by Pakhomov et al. (1997) al-though samples were only obtained in the surface 200 m.Detailed depth resolved data for the autumn period havebeen lacking until now. In this study we have sampled asclose as possible to the stations sampled during spring1997 and summer 1990, although the presence of frag-menting ice bergs and severe weather occasioned someminor relocation to enable safe operation.

With the exception of some of the Bongo hauls takenduring autumn (this study), when sampling extended asfar west as Shag Rocks, all data used in this study wereotherwise obtained from the region immediately to thenorth-west of the island, where, after passing up thecoast and round the Northwest Georgia Rise, waters areentrained in the circulation around the Georgia Basin(Meredith et al. 2003; Ward et al. 2005). This area ishistorically very productive (Korb et al. 2003; Korb andWhitehouse 2004) with blooms present for up to6 months of the year and elevated chl a levels (up to 15–20 lg l�1) occasionally recorded (Atkinson et al. 2001;Korb et al. 2003). Secondary producers can also benefitfrom this enhanced production through increased car-bon mass (Shreeve et al. 2002) and EPR (Ward et al.2002), although the generation times of copepods andkrill in relation to residence times around the islandimply that advection may be equally important indetermining biomass (Atkinson et al. 2001).

Over the shelf, the small euphausiid Thysanoessa spp.comprised �50% of zooplankton biomass. This species,along with Euphausia frigida, has been found to formextensive horizontal scattering layers which were par-ticularly persistent during daylight, in an acoustic studycarried out in January/February 1996 (Brierley et al.1998). Small euphausiids were also found to be a dom-inant component (15–63% biomass) by Pakhomov et al.(1997).

At the oceanic stations, the interzonal copepod mi-grants R. gigas and C. acutus that populate surfacewaters in spring and summer were largely absent duringautumn having descended into the mesopelagic zone.C. simillimus, in contrast, had a distribution similar tothat observed in spring, in that whilst the bulk of thepopulation was resident in the surface 200 m, themigratory stage CV was present throughout the top1,000 m. Other seasonal migrants included those thathad either departed the surface 200 m (Oncaea spp.) or

Table 2 The percentage contributions of the principal zooplanktonspecies and taxa at the oceanic stations to abundance (ind. m�2)and biomass (g DM m�2)

Taxa E142 E213

Abundance Biomass Abundance Biomass

Oithona spp.a 35.0 2.0 22.0 0.5Metridia spp. 28.5 20.0 17.0 9.0Ctenocalanus spp. 11.5 2.0 13.0 1.0Oncaea spp. 11.0 0.6 7.0 0.3Microcalanus pygmaeus 4.0 0.4 3.5 0.3Rhincalanus gigas 3.0 44.0 3.0 37.0Scolecithricella minor 1.0 1.0 0.4 0.2Copepod nauplii 0.9 < 0.05 0.5 < 0.05Calanus simillimus 0.7 2.0 1.0 3.0Calanoides acutus 0.6 4.0 1.0 5.0Chaetognatha 0.5 0.1 0.7 0.1Heterorhabdus spp.b 0.5 1.0 0.5 1.0Ostracodac 0.5 1.0 0.6 1.0Pteropoda spp.d 0.5 < 0.05 – –Spinocalanus spp. 0.4 0.2 0.7 0.3Thysanoessa spp.e 0.4 15.0 0.7 18.0Pleuromamma robusta 0.2 0.2 0.3 0.2Calanus propinquus 0.1 1.5 0.2 2.0Euchaeta spp.f 0.1 3.0 0.1 2.0Scaphocalanus spp. < 0.1 < 0.05 < 0.1 < 0.05Metridia curticauda < 0.1 < 0.1 0.15 < 0.1Gaidius spp.g < 0.1 < 0.1 < 0.1 0.2Drepanopus forcipatus – – 26.0 17.0Themisto gaudichaudii < 0.1 1.0 < 0.01 –Euphausia frigida < 0.05 0.2 < 0.05 1.0Others 0.3 0.5 1.5 0.7

aMainly Oithona similisbIncludes Heterorhabdus farrani, Heterorhabdus austrinuscConchoecia spp.dMainly thecate PteropodaeThysanoessa vicina and Thysanoessa macrurafMainly Euchaeta antarctica but includes Euchaeta rasa andEuchaeta parvulagMainly Gaidius pungens but includes Gaidius intermedius

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were found there in greatly reduced numbers comparedto other seasons (e.g. Oithona spp. and Ctenocalanusspp.). Other taxa such as Metridia spp., Euchaeta spp.,Scaphocalaniids and Scolecithriciids occupied the samebroad mesopelagic distributions year round.

The snapshots of species abundance and verticaldistributions collectively presented in this and twoadditional papers (Ward et al. 1995; Ward and Shreeve1999) reflect stages in the development of zooplanktoncommunities and the diurnal and seasonal movement oftheir component parts over the productive part of theyear. Over annual time-scales, it is apparent that the

‘seasonal’ alterations in the depth distribution of manyspecies is a broadly continuous process. This is partic-ularly true of the biomass dominants (see Atkinson et al.1997; Ward et al. 1997) where a relatively small pro-portion of each year is spent in the near-surface layers.By deploying the LHPR as deep as 1,000 m a very largeproportion of the interzonal copepod populations aresampled and differences in terms of stage structure,abundance and depth distribution have been readilyhighlighted. Differences between species in the chronol-ogy of recruitment have been known for some time (seeVoronina 1970, 1978; Atkinson 1991) and this is clearly

Fig. 6 Abundance vs depth ofthe taxa that contributed ‡ 2%to either abundance or biomassat the oceanic Longhurst HardyPlankton Recorder stations.Large calanoids figured first.For details of relativeabundance see Table 2

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reflected in their stage structure and depth distributionin this present study. Thus, the early spawning C. acutushas reached an overwintering stage by late January andhas started its seasonal descent (see Atkinson et al. 1997;Tarling et al. 2004). By March and early April, at theoceanic station, C. acutus stage CIV is distributed spo-radically down the water column in both hauls whereasCV has a modal peak at a depth of �500 m in haul E213but is skewed to deeper horizons in haul E142. The mainoverwintering stages of R. gigas are CII–CIV (seeAtkinson 1991) and the preponderance of CIVs goinginto winter is clearly seen in our data. Although themajority of R. gigas females were resident in deep water;

at two of the stations to the west of the LHPR sites, theBongo nets revealed their presence in surface watersproducing appreciable numbers of eggs (Table 5), com-parable to maximum EPRs recorded in previous studiesduring early summer at South Georgia (Ward et al.2005). R. gigas is known to have a protracted period ofrecruitment through the summer into autumn and nau-plii have been found as a dominant stage in populationsnear the Antarctic Peninsula as late as May (Marin andSchnack-Schiel 1993). It is unclear how unusual thelatter observation is, due to a lack of samples from thattime of year. In this study chlorophyll levels were gen-erally low (< 1 lg l�1) although the species does appear

Fig. 6 (Contd.)

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able to respond to increased food availability byincreasing EPR (Ward and Shreeve 1999).

Table 3 indicates that the median depths of several ofthe older copepodite stages of the biomass dominantspecies, R. gigas and C. acutus, are located deeper duringthe night than the day by up to 50 m at shelf stationsand over 150 m at oceanic stations. This suggests thatthese stages are performing a reverse migration. Suchmigrations have been considered by some to be evidenceof high levels of invertebrate predation, which use tactilestimuli rather than vision to locate prey (Hays 2003).The voracious copepod predator, the chaeognathSagitta gazellae, reaches some of its highest concentra-tions around South Georgia (David 1955). However, thefraction of the population that has already started tomake its seasonal descent to the deeper layers may

contribute to the deepening of median depth at night. InFig. 5, it can be seen that there is an increased concen-tration of R. gigas and C. acutus in the deepest layers. Itis possible that these individuals have migrated up frombelow 200 m for the duration of the nighttime period.Whether individuals entering seasonal diapause arecapable of such diel vertical migration requires furtherinvestigation.

Calanus simillimus was the only large calanoid specieslocated within the near-surface layers and with spawningfemales, albeit producing low numbers of eggs. Thisspecies appears to be less dependent on chlorophyllper se and studies by Atkinson (1996) has demonstratedits diet can comprise significant amounts of heterotro-phic carbon. Stage CV at the deep station was distrib-uted all the way down the water column in line with thefindings of Atkinson (1991) who indicated that this wasthe only strongly migratory stage. The populationdynamics and recruitment cycles of individual speciestherefore have a marked influence on where they arelikely to be found during much of the year. One of theconsistent differences seen in summer studies of plank-ton at South Georgia has been the different stage com-position of C. acutus and R. gigas over the shelf relativeto surrounding oceanic waters (Atkinson 1989). Gener-ally population development was found to be retardedover the shelf and has been attributed to the fact thatboth species spawn at depth and that the shelf in someway inhibits this behaviour and influences populationdevelopment. In spring, prior to the recruitment of anew generation of copepodites, over-wintered popula-tions of both species were present at both shelf andoceanic stations and differences were negligible (Wardand Shreeve 1999). In this study, such a difference wasonly observable for R. gigas, which spawns later thanC. acutus. Population recruitment had been completedin the latter species and stage CV dominated populationsat all stations. In contrast, C. simillimus, a near-surfacespawner, was found to have a similar stage compositionin all seasons.

Recent research using the LHPR in the Scotia Sea hasdocumented pronounced spatial changes in speciesdepth distribution and stage composition across itsbreadth within a temporal period of some 12 days(Ward et al. 2006). Sampling took place in late January/early February 2003 when recently over-wintered pop-ulations of C. acutus and R. gigas were generally lyingdeeper down the water column (�350 m median depth)in the southern Scotia Sea whereas, further north, thespring generation was present in surface waters. AtSouth Georgia, sampled 14 days after the last Scotia Seastation on 18th February, C. acutus was already in theprocess of undergoing its autumn descent and had amedian population depth of 177 m whereas R. gigas wasstill active in surface waters (median population depth38 m). Data collected in the present study indicated that,by late March and early April, median depths had in-creased to between 550 and 750 m for C. acutus and tosimilar depths (418–739 m) for later copepodite stages of

Table 3 Median depths of large calanoid species stages in the shelfand oceanic Longhurst Hardy Plankton Recorder hauls Shelf haulswere taken at local midday and midnight whereas oceanic hauls,although taken a week apart, were only separated diurnally by afew hours

Shelf Haul 129(day)

Haul 131(night)

Rhincalanus gigasMale – –Female – –CV 101 151CIV 110 122CIII 94 111CII 94 98CI – –Calanoides acutusCV 56 84CIV 54 101Calanus simillimusMale 97 7Female 90 6V 78 7IV 58 13III 54 –

Oceanic Haul 142 Haul 213

R. gigasMale 739 773Female 651 540CV 522 385CIV 418 292CIII 385 192CII 299 128CI – 125C. acutusCV 748 569CIV 557 544C. simillimusMale 67 68Female 65 67V 522

(41% in top 100 m)363(48% in top 100 m)

IV 59 50

Shelf hauls were taken at local midday and midnight whereasoceanic hauls, although taken a week apart, were only separateddiurnally by a few hours

960

R. gigas. Although the general pattern of biologicalproduction commencing earlier in the northern parts ofthe Southern Ocean is a long established idea (Hart1942; Voronina 1970; Atkinson 1991) it should be em-phasised that populations do not slavishly follow a lat-itudinal pattern and there are regional variations. Forexample, sampling at the furthest south (�62�S) duringlate January 2003 disclosed populations of C. acutusthat, although comprised mainly of over-winteredstages, were present in near-surface waters, apparentlyresponding to increased levels of phytoplankton. Thedifferences in depth distribution of zooplankton betweenthe two oceanic stations sampled in this study may also

reflect spatial differences in development albeit overmuch smaller scale.

Acknowledgments We thank Martin Collins, cruise leader of cruiseJR100, for enabling this field work to take place within an alreadybusy cruise schedule and Captain Jerry Burgan and his crew aboardRRS James Clark Ross for facilitating the scientific work under thesometimes difficult conditions. Beki Korb kindly provided thechlorophyll data and Mick Whitehouse constructed Fig. 3. Thispaper is a contribution from the FLEXICON project to the BASDISCOVERY 2010 programme.

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Min 10.60Max 22.2

C. simillimus 5.62 ± 5.30One hundred andseventy three femalesincubated in 19experiments fromseven stations

Min 0.0Max 17.80

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