Journal of Plankton Research Volume 6 Number 6 1984
Periodicity of composition, abundance, and vertical distribution ofsummer zooplankton (1977/1978) in Ezcurra Inlet, Admiralty Bay(King George Island, South Shetland)
Juliusz Chojnacki and Teresa Weglenska1
Institute of Fisheries Oceanography and Protection of the Sea, Academy ofAgriculture, Kazimierza Krdlewicza 4, 71—550 Szczecin, and institute ofEcology, Polish Academy of Sciences, 05 — 150 Dziekandw Leiny, Poland
(Received March 1984; accepted July 1984)
Abstract. Seasonal succession and variation in species composition, density, biomass, age distri-bution and frequency of zooplankton (mainly the Copepoda) were analysed during the austral sum-mer of of 1977/1978 in Ezcurra Inlet, a part of the Antarctic coastal ecosystem. Small zooplankters(i.e., cyclopoids of the genera Oncaea and Oithona, and calanoids Drepanopus pectinatus andScolocithricella glacialis) were found to predominate in terms of abundance and percentage contri-bution. The zooplankton biomass was dominated by larger organisms of the Metrididae and Cal-anidae (Calanoida). The maximum abundance and maximum biomass of copepods were recorded inFebruary; two small peaks in copepod biomass being observed in late December and late January, anda lesser biomass peak in late December. The vertical distribution of copepods in terms of their diel andseasonal (December, January, February, March) changes showed a day-time maximum to haveoccurred in the near-bottom layer, the nocturnal distribution being bimodal with peaks within 0 —10 m and 25 — 50 m. The summer zooplankton community in Ezcurra Inlet is controlled by trophic(phytoplankton composition and density) and hydrological (water exchange with Bransfield Strait)conditions prevailing in the area.
Introduction
In spite of numerous studies on the Antarctic zooplankton, the importance ofthese organisms and their role in the Antarctic food chains are not fully under-stood except for the Euphausiacea. One of the main reasons is the fact thatalmost all zooplankton studies, from the earliest Discovery cruises (Hardy andGunther, 1935; Mackintosh, 1934, 1937; Foxton, 1956) to the most recent Sovietand other reports (Voronina, 1977; Voronina et al., 1979, 1980; Vladimirskaya,1978; Zmijewska, 1979) dealt with the biology and ecology of the larger animals.For example, Voronina (1966, 1972, 1975; Voronina et al., 1978) and otherworkers (Murano, 1964; Andrews, 1966; Baker, 1954; Yamanaka, 1976) wereinterested mainly in animals such as Calanus propinquus, Calanoides acutus,Calanus simillimus, and Rhincalanus gigas. On the other hand, the information isscant on the small plankters which, owing to their high abundance (in the neriticzone in particular) and fast turnover, can play a significant part in the Antarcticecosystem. The literature contains only a few references to small copepods.Although Seno et al. (1963) and Ramirez and Dinofrio (1976) mention the pres-ence of Oithona similis and Oncaea spp., their data indicate the contribution ofthose species to the total zooplankton density is <2O°7o. Seno et al. (1963) main-tain that O. similis is abundant in the Antarctic waters at their northernmostboundary rather than in the southernmost region of the Southern Ocean.
Our project was aimed at following the seasonal succession and identifyingvariations in composition, abundance, biomass and age distribution in the zoo-plankton (mainly the Copepoda) over the austral summer in Ezcurra Inlet, a partof the Antarctic coastal ecosystem. The study covers both small (adults < 1 mmlong) and large (adults > 1 mm long) zooplankters. The vertical distribution ofcopepod abundance and biomass as well as frequency of dominants over a 24-hand seasonal (December, January, February, and March) cycle are analysed indetail.
Materials and Methods
The materials were collected at the M/S Antoni GarnuszewskPs anchor stationin the central part of Ezcurra Inlet, Admiralty Bay (62°09 S; 58°28 W), KingGeorge Island, South Shetlands during the austral summer, from 20 December1977 through 10 March 1978.
Ezcuira Inlet is 7 km long and 2 — 3 km wide, it extends over —17 km2 andholds — 1.33 km3 of water. The maximum depths in its eastern and western partsare 270 m and 80 m, respectively. In the central part, east of Dufayel Isle, there isan underwater rocky ridge; the depth over the ridge is 75 m. Our sampling sitewas located above the ridge, in an area with a local upwelling caused by strong,mostly westerly, winds. The hydrological regime in Admiralty Bay and EzcurraInlet is influenced by the exchange of water with Bransfield Strait, tides being theprincipal mechanism underlying the exchange (Rakusa-Suszczewski, 1980a).
Temperature, salinity, and other chemical data collected at the sampling sta-tion during the austral summer of 1977/1978 showed a lack of stratification andan almost total homogeneity of the water column (Bojanowski and Lauer, un-published data). The surface temperature and salinity in the austral summer of1977/1978 ranged within 0 .2- 1.4°C and 33.64-33.98°/oo, respectively (Pahlke,Kowalewski and Lauer, unpublished data).
A standard (0.086 mm mesh size) Nansen net was used to sample the followinglayers: 0-10; 10-25; 25-50; and 50-75 m, while a 0.203 mm mesh Hensen netwas employed to sample the 0 — 25 and 0 — 75 m layers, the nets being hauled upat 0.3 m s"1 winch speed. The samples were taken twice a day; during the day(usually between 12.00 and 15.00 h local time) and at night (between 23.00 and2.00 h local time). The zooplankton collected was fixed in 10% unbuffered for-malin, transferred to a Bogorov trough, and examined under a stereo-microscope.All the individuals and species in a subsample were enumerated, developmentalstages determined, and body length of dominants measured. The zooplanktondensities are given as no. ind m ~3. Copepod biomass was assessed by means ofan indirect method using Chislenko's nomograms (Chislenko, 1968) based onbody length measurements.
Frequencies of dominant copepod species and groups of species in variouslayers were estimated from the formulae proposed by Yamanaka (1976):
mean summer frequency of copepods in a defined layer, F,
Fj = 100 "O- of samples from the layer containing a given speciestotal no. of samples from the layer
Periodicity of summer zooplankton in Ezcurra Inlet
monthly frequency of copepods in a defined layer, F2
no. of samples from the layer taken during a givenp = 100 month and containing a given species
total no. of samples from the layer taken during agiven month
Results
Qualitative composition, abundance, biomass and dominance of the summerzooplankton assemblage
The Ezcurra Inlet summer zooplankton assemblage was found to contain rep-resentatives of 40 holo- and meroplanktonic species and genera belonging to 12higher taxa.
The Copepoda were represented by the highest number of species: Calanoida(18 spp.), Cyclopoida (3 spp.), and Harpacticoida (2 spp.). Crustacean copepodscontributed >95% of all animals in samples. The remaining higher taxa: Hydro-zoa, Nematoda, Polychaeta, Euphausiacea, Mysidacea, Amphipoda, Ostracoda,Gastropoda, Echinodermata, Chaetognatha, and Appendicularia contributed1 — 3 species each.
Almost all the species constituting the summer copepod assemblage wereplanktonic (Vervoort, 1965), only one species, D. pectinatus being typical of thelittoral (Vervoort, 1957; Hardy and Gunther, 1935). The typically epipelagicspecies, Oithona frigida and S. glacialis, were abundant. The remaining speciesinhabit surface and deep waters (Metridia lucens, M. longa, M. gerlachei, O.similis) as do such inter-zonal species as Calanoides acutus, Calanus propinquus,C. simillimus, and R. gigas which occur close to the surface in summer andmigrate to deeper layers in winter (Mackintosh, 1934, 1937; Hardy and Gunther,1935; Brodsky, 1964; Voronina, 1970, 1972). Some night-time samples spor-adically contained some species regarded by Vervoort (1965) as bathypelagic:Euchaeta antarctica, E. rasa, Racovitzanus antarcticus, Scolocithricella dentipes,Haloptilus oxycephalus and Aetideopsis minor.
The assemblage studied contained both typically Antarctic (Calanoides acutus,Calanus propinquus, Metridia gerlachei, R. gigas, Euchaeta antarctica, Pleur-omma robusta f. antarctica, S. glacialis) and sub-Antarctic (e.g., C. simillimus,M. longa, M. lucens) species (Beklemishev, 1958; Ommanney, 1936; Seno et al.,1963).
Copepods of all developmental stages (nauplii, copepodites, and adults) domi-nated the sampled community in terms of densities, biomass and percentage con-tribution. However, our data may have underestimated the abundance and bio-mass of the Euphausiacea, as a correct assessment of those parameters in euph-ausiids calls for different gear and methods than those employed.
The total density of copepods over the period of study (December — March)was low, mean values for 0 — 75 m ranging from 31 (day samples) to 62 ind m ~3
(night samples) (Table I). The density in the 0-75 m layers varied greatly 1 -480ind m~3 throughout the summer (Figure 1). In the second half of the summer
Table I. Mean density (ind m *) and biomass (mg m " *) of Ezcurra Inlet Copepoda in summer 1977/1978 (December-March) within 0-75 m.
Density(ind m"3)Biomass(mg m"1)
December January February March Seasonal mean
Fig. 1. Seasonal dynamics of copepod density (ind m *) within 0 — 75 m in Ezcurra Inlet in summer1977/1978 (a) day; (b) night.
both the density and biomass of copepods were higher. The mean copepod den-sities (0 — 75 m layer) in February and March were 1.5 — 9 times higher than thoserecorded in December and January (Table I). The seasonal copepod densitydynamics (Figure 1) showed three peaks of abundance: the first, small peak (up to37 and 80 ind m~J during the day and night, respectively) occurred in late Dec-ember; the two subsequent peaks were much more pronounced and were recordedin early (97 and 107 ind m~3 during the day and night, respectively) and late (187and 480 ind m"3) February (Figure 1).
Predominant, in terms of abundance, throughout the period of study weresmall zooplankters: predatory cyclopoids (O. similis, O. frigida and Oncaea spp.)and calanoids, herbivorous filtrators (S. glacialis and D. pectinatus). Their con-tribution to the total copepod abundance within 0 —75 m was found to range, onthe average, from 72% at night to 81% during the day (Table II). On the other
Fig. 2. Seasonal dynamics of copepod biomass (mg m"3) and phytoplankton density (cell ml"1)within 0-75 m in Ezcurra Inlet in summer 1977/1978; phytoplankton data after Kopczynska (1980).(a) day; (b) night.
hand, the contribution of large calanoid species (Calanoides acutus, C. propin-quus, R. gigas, M. lucens, M. longa, M. curticauda and M. gerlachei) was muchlower, ranging from 13% (day) to 28% (night) (Table II).
A marked change in dominance pattern within the copepods was observed dur-ing the summer; the density and percentage contribution of less abundant specieschanged considerably but the principal dominant remained unchanged. Through-out the period of study, the Ezcurra Inlet copepod assemblage abundance wasdominated by O. similis (Figure 3a). At least four dominance periods withcharacteristic dominants and subdominants can be identified in the total copepodabundance: (i) 20 December—4 January: O. similis, Oncaea sp., S. glacialis,Calanoides acutus; (ii) 5-21 January: O. similis, Oncaea sp., S. glacialis; (iii) 22January —23 February: O. similis, D. pectinatus, M. lucens, M. longa, Oncaeasp.; (iv) 24 February—10 March: O. similis, O. frigida, D. pectinatus, Oncaeasp., M. lucens. Mean summer biomass values for copepods in Ezcurra Inlet werealso rather low within 0 — 75 m: 2.7 mg m~3 d"1 and 10.6 mg m~3 night"1
(Table I). The biomass varied greatly over the period of study, particularly atnight, the range being 0.03-165 mg m~3 (Figure 2).
Figures 2 and 3b present a detailed analysis of seasonal dynamics of copepodbiomass within 0-75 m.
Besides several slight biomass increases (late December, late January, earlyMarch), the maximum biomass was recorded in February and accompanied the
Periodicity of summer zooplankton in Ezcurra Inlet
density peak, M. longa, M. lucens, M. curticauda, Calanoides acutus and C. pro-pinquus contributing most biomass. In terms of the total biomass, the EzcurraInlet summer copepod assemblage is dominated (from 89% during the day to95% at night) by larger plankton, particularly by Metridia spp., Calanoidesacutus, and periodically by C. propinquus (Table II).
Age structure of the summer copepod assemblage
The summer copepod assemblage in Ezcurra Inlet included both older develop-mental stages and adults of the overwintering generation and younger stages of anew, spring generation (Figure 4).
The population of Calanoides acutus comprised two generations: those of thewinter and spring. At the onset of our project the population was at an advancedstage of vernal development and comprised copepodite stages IV and V andadults of the winter generation as well as copepodite stages I, II and III of the newspring generation; however, the overwintering generation predominated. Theabundance of winter generation declined gradually throughout January, and inFebruary the C. acutus population was dominated by copepodite stages II and IIIof the new spring generation. In March, the previous year's generation virtuallyceased to exist and the population was almost totally made up of individuals ofthe new generation, copepodite stage IV predominating and stage I absent (Figure4).
Spring generations of C. propinquus and R. gigas delayed their developmentrelative to Calanoides acutus. In the last decade of December, the C. propinquusand R. gigas populations comprised exclusively copepodite stages IV and V andadults of the overwintering generation (Figure 4). Copepodite stage I of the newspring generations appeared in early and late January in the C. propinquus andR. gigas populations, respectively. In March the C. propinquus population in-cluded a low percentage of copepodite stage V and adults of the previous year'sgeneration; stage III of the new generation predominated. At the same time theR. gigas population consisted of copepodite stage I and adults of the winter gen-eration along with the first two copepodite stages of the new one.
The Metridia species in December were represented by copepodite stages IVand V and adults of the winter generation. Representatives of new spring gener-ations appeared in January, first in the M. curticauda and M. lucens and later inM. longa populations. In January, February and March the populations of thethree species showed the presence of younger copepodites of new generations aswell as stage V and adults of the old ones. M. gerlachei was represented presum-ably by older copepodites and adults of the previous year's generation.
Populations of the small calanoids D. pectinatus and S. glacialis consisted ofall copepodite stages and adults over the entire period of study, with oldercopepodites and adults being most numerous (Figure 4). More than one gener-ation of each of the two species is likely to appear during the summer.
No males were ever found among the adults of the calanoid species encountered.The Cyclopoida showed the most complex populations structure. Throughout
the summer, populations of O. similis, O. frigida, and Oncaea sp. contained all
Fig. 4. Population structure of selected copepod species within 0 — 75 m in Ezcurra Inlet in summer1977/1978.
copepodite stages, adult females (including numerous ovigerous ones) and males(Figure 4). The continuous presence of males and ovigerous females indicatedcontinuous breeding. The samples were, as a rule, dominated by older copepo-dites and adults; this domination might have been enhanced by gear selectivity forlarger individuals.
Vertical distribution of the summer copepod assemblage
Pronounced did and seasonal variations were observed in the vertical distri-bution of copepods in Ezcurra Inlet. Figure 5 illustrates changes in mean summer
Fig. 5. Vertical distribution of copepod density (A) and biomass (B) during the day and at night inEzcurra Inlet in summer 1977/1978.
density and biomass of the Copepoda occurring in the four water layers sampled.Both the densities and biomass in almost the whole water column were higher atnight than during the day, except for the bottom layer (50 — 75 m) which showednumerically large concentrations of copepods during the day. Generally, the dielvertical density variations were less extensive (the largest change being 2-fold)than those of biomass; the latter changed by a factor of five at the most.
The vertical distribution of copepods at night was bimodal with density peakswithin 0 — 10 and 25 — 50 m. During the day the densest concentrations wererecorded within the 50 — 75 m layer. The uppermost layer (0 — 10 m) showed themost extensive diel variations in the copepod density and biomass (Figure 5).
The frequency distribution of dominant copepods in various water layers forthe whole summer (Figure 6, Table III) shows that during the day most speciesprefer deeper layers and avoid those close to the surface. The highest day-timefrequencies of O. similis, O. frigida, Oncaea spp., D. pectinatus, S. glacialis, M.longa, M. lucens, M. gerlachei and M. curticauda were observed within 50 —75 m; Calanoides acutus, C. propinquus and R. gigas occurred with the highestfrequency within 25-50 m. At night most species either migrate upward, likeOncaea sp. (to 0 -10 m), O. similis (to 10-25 m), O. frigida, D. pectinatus andMetridia spp. (to 25—50 m) or are evenly dispersed throughout the water col-umn, like S. glacialis. It is only the larger calanoids Calanoides acutus, C. propin-quus and R. gigas that remain with the highest frequency within 25 — 50 m.
The vertical frequency distribution of dominant copepod species in varioussummer months showed once again that the small cyclopoids and calanoids werea constant element in the summer zooplankton assemblage of the Antarctic
coastal ecosystem (Table IV).A clearly marked seasonal succession and asynchrony was found in the occur-
rence of the large species Calanoides acutus, C. propinquus and R. gigas. At theonset of the summer, in December, the highest frequency (up to 100%) within theentire water column was shown by Calanoides acutus, the remaining two speciesappearing with negligible frequencies. In January, the C. acutus frequency wasstill high, particularly at night, and that of C. propinquus began to increase. InFebruary, the frequencies of Calanoides acutus and C. propinquus declined asopposed to that of R. gigas which increased during that month. However, it isonly at the end of the summer that the latter species' frequency in night samplesfrom the bottom layers equalled the frequencies of Calanoides acutus and C. pro-pinquus (Table IV).
Discussion
Summer concentrations of zooplankton in Ezcurra Inlet do not seem to behigh; the mean copepod density and biomass within 0 — 75 m ranged from 31 to62 ind m ~3 and from 3 — 11 mg m ~3, respectively, depending on the time of theday (Table I). This is in contrast to opinions expressed by numerous authors whostate that the Antarctic waters harbour large amounts of zooplankton; Vino-gradov and Naumov (cited by Kanajeva et al., 1969) determined the mean zoo-plankton biomass in the whole Antarctic as 300 mg m ~3, vertically they found1-20 rngm"3 within 0-50 m and 100-150 mgm~3within 100-200 m. Hold-gate (cited by Knox, 1970) found a biomass value of 55 mg m~3, while accordingto Voronina (1966) the mean biomass of Antarctic zooplankton within 100-200 m ranged from 10 to 50 mg m"3 and tended to increase northwards.
On the other hand our results showing the numerical domination of copepodsin the Antarctic zooplankton are consistent with those of Voronina and Naumov(1968) who estimated the copepod contribution to the total abundance of zoo-plankton collected with a Juday net as 94.5%. The Discovery expedition foundthat 74.5% of the zooplankton was made up of copepods. The percentages ofvarious components of the Antarctic zooplankton assemblages often differ as aresult of the collecting gear used. Had an actual abundance of Euphausiidae beenincluded, the copepod contribution to the total density found in our studieswould have been lower by several per cent. Assuming that the euphausiid biomassin Admiralty Bay and Ezcurra Inlet ranges from 0.065 to 0.329 g m~2 (Rakusa-Suszczewski, 1980a) with the copepod biomass (our studies) amounting to—0.5 g m ~2 (the value was arrived at by multiplying the mean summer biomassof copepods by 75 m), and assuming a negligible contribution of the remainingzooplankters, one can approximate the percentages of the two groups in the totalsummer zooplankton biomass. Considering the biomass fluctuation range ofeuphausiids, they would make up 13 — 40% of the total zooplankton biomass inEzcurra Inlet; similarly, copepods would contribute 60 — 87%. These resultsdiffer from the main premise entertained by Knox (1970) and Hempel (1970) thatthe euphausiid biomass in the Antarctic equals the biomass of the remaining zoo-plankton. On the other hand, our values are well within the euphausiid biomasscontribution to the total zooplankton biomass (10—50%) as given by Everson
Periodicity of summer zooplanklon in Ezcurra Inlet
(1977). The Ezcurra Inlet summer assemblage of copepods with its 23 species onlyhad a low diversity when compared with the zoopiankton composition in openwaters. According to Vervoort (1965), the Antarctic waters are inhabited by 126copepod species; 28 species typical of the epipelagic water and the rest occurringin the deep layers. Hardy and Gunther (1935) recorded 100 copepod species offSouth Georgia, 31 of which were typical of the upper 250 m. On the other hand,Zmijewska (1979) observed a low diversity in the neritic zoopiankton in thecoastal zone of the Davis Sea and Olaf Prydz Bay, as compared with that in theopen ocean, an observation similar to ours.
Almost all species in the summer copepod assemblage in the layers sampled inEzcurra Inlet are pelagic. Only one species typical of inshore waters D. pectinatuswas encountered, the species occurring in large concentrations off South Georgia(Hardy and Gunther, 1935) and Kerguelen Archipelago (Vervoort, 1957).
As shown previously (Table II) the total density of summer copepod assem-blage in Ezcurra Inlet was dominated by small organisms of two trophic groups:predatory cyclopoids (O. similis, O. frigida, Oncaea sp.) and herbivorous cala-noids (D. pectinatus and S. glacialis). In spite of their relatively small contri-bution to the total zoopiankton biomass, all those species (by virtue of their highdensities, 100% frequency over the summer, assumed fast turnover, and highreproductive and developmental rates) play a very important part in the organicmatter destruction and nutrient regeneration in the coastal Antarctic ecosystem.Large calanoids (Calanoides acutus, C. propinquus, R. gigas, Metridia spp.)occurred with low densities, but owing their their large size, dominated the zoo-plankton biomass (Table II).
In the opinion of many workers (Brodsky, 1964; Voronina 1970; Seno et al.,1963; Ramirez and Dinofrio, 1976), the species composition and dominance pat-tern in zoopiankton of the Antarctic open waters differ significantly from thosein the neritic region. Numerically, the leading role in the open waters is played byinterzonal copepods such as Calanoides acutus, C. propinquus, and R. gigas.
The abundance, composition and structure of the summer zoopiankton assem-blage in Ezcurra Inlet are affected by specific conditions of productivity andhydrography of the area. An ample inflow of fresh water from the shore andglaciers as well as a considerable supply of terrigenous materials (~200 t d"1;Pecherzewski 1980), resulting in decreased salinity and pH (Szafranski and Lip-ski, 1982) coupled with high turbidity and poor light conditions due to a highcontent of suspended matter (2.8-183 mg I"1; Pecherzewski, 1980) limit theprimary productivity. During the Antarctic summer of 1977/1978, the primaryproductivity in Ezcurra Inlet ranged from 0.4 to 0.9 g C m~ l d"1 (Dera et al.,unpublished data). The summer phytoplankton in Ezcurra Inlet is dominated bynanoplanktonic dinoflagellates (13-15/un diameter) and monads (4 -6 /tmdiameter) making up 75 -90% of the total phytoplankton abundance (Kopczyn-ska, 1980, 1981). Moreover the density of total phytoplankton (2.5-217 cellsml-1) and of diatoms (0.5-199 cells ml"1) were low (Kopczynska, 1981).
It seems probable that the Ezcurra Inlet phytoplankton density and compo-sition significantly affect the abundance and composition of zoopiankton in thearea. Nanoplanktonic forms, monads, and small diatoms (<20 /tin), mainly of
the genus Thalassiosira, constitute an excellent food supply for small calanoidsand euphausiids.
Wide month-to-month fluctuations of the zooplankton abundance and bio-mass were observed over the summer (Figures 1 and 2). As a rule, periods of themaximum density and biomass of the zooplankton coincide with the maximalphytoplankton development (Figures 1 and 2). The peak phytoplankton densityin late February occurred at the same time as the highest and longest-lasting peakof copepod abundance and biomass. Moreover, two lesser phytoplankton bloomsin mid-January and in late January/early February overlapped or slightly pre-ceded increases in the copepod abundance and biomass.
The vertical distribution of copepods showed the highest nocturnal concen-trations to have occurred within the uppermost layer of 0—10 m and within 25 —50 m (Figure 5). The vertical distribution of copepods is well correlated with thatof phytoplankton. According to Kopczynska (1980, 1981) a characteristic featureof the vertical phytoplankton distribution was two peaks of density (> 100 cellsml"1), one occurring within the uppermost layer (mainly at 5 — 6 m) and theother close to the lower limit of the euphotic zone (15 —30 m).
The abundant small calanoids and younger stages of large copepods presum-ably constitute the principal food source for predatory cyclopoids in EzcurraInlet. Large populations of Oithona spp. and Oncaea sp. as well as their intensivedevelopment and reproduction may be taken as evidence that their food re-quirements are met in the coastal zone.
The composition and density of summer phytoplankton in Ezcurra Inlet is notoverly favourable for large filtrators such as Calanoides acutus, C. propinquusand R. gigas which prefer larger algal cells (Voronina and Suchanova, 1976).Unfavourable trophic conditions in terms of both the phytoplankton compo-sition and density may be the reason for the limited abundance of those species inthe neritic zone. The presence of those calanoids in Ezcurra Inlet, and even theirdominance in the summer zooplankton biomass, probably resulted from thespecific hydrology of the area. Rakusa-Suszczewski (1980a) maintains that a con-stant supply of organic matter from Bransfield Strait to Admiralty Bay and Ez-curra Inlet is indispensable for functioning of that compartment of the coastalecosystem. Studies on the Admiralty Bay current system (Pruszak, 1980) revealeda constant flow of cold water from Bransfield Strait along the bottom. The pre-vailing winds, mainly the westerlies, cause a local upwelling whereby the in-coming water is transported up toward the surface around the central underwaterridge in Ezcurra Inlet. The constant exchange of water with Bransfield Straitresults in a continuous arrival of calanoid species typical of the open ocean(Metridia spp., Calanoides acutus, C. propinquus and R. gigas) and controlstheir density levels in Ezcurra Inlet. As a rule, the abundance and biomass ofthose calanoids increased at night (Table II). The phenomenon can be accountedfor by the fact that at night, owing to their extensive vertical migrations, theanimals can be carried away from Bransfield Strait deep water into Ezcurra Inlet.This line of reasoning is further substantiated by the fact that such typicallybathypelagic species as S. dentipes, Aetidopsis minor, Haloptilus oxycephalusand Racovitzanus antarcticus were encountered in the coastal waters of Ezcurra
Periodicity of summer zooplankton in Ezcurra Inlet
Inlet in night samples only. Additionally, tides seem to be of importance for thewater exchange with Bransfield Strait.
The seasonal dynamics of abundance, biomass, population structure, and fre-quency of the Copepoda (Figures 1, 2, 3a and 3b; Table III) showed a clearperiodicity and asynchrony in the occurrence and development of Calanoidesacutus, C. propinquus and R. gigas. Our results are in agreement with Voronina's(1966, 1970, 1975) observations on the succession of life cycles of the three speciesin the Antarctic open waters. In her opinion the order of spring migration andbreeding, and also the maximum densities of the new generations at various timesover the spring season are controlled by differences in depth of occurrence anddevelopmental stages of the overwintering generations of the species. Voronina'sdata indicate Calanoides acutus to be the first to migrate upward and breed,followed by C. propinquus and R. gigas.
The summer population structures of the three species in Ezcurra Inlet showedthe spring generation of Calanoides acutus to develop earliest. The species is alsothe first dominant in the zooplankton abundance and biomass. The spring gener-ations of C. propinquus and R. gigas in Ezcurra Inlet were observed to developalmost simultaneously, the first clearly predominating in terms of abundance.
The vertical distribution of copepods in Ezcurra Inlet in summer showed somediel and seasonal variations with respect to abundance and biomass (Figure 5).During the day, the highest copepod density was recorded near the bottom,within 50 — 75 m, which is a pattern typical of plankton communities (Mann,1982).
On the other hand, at night (i.e., at the time of presumed active feeding) thevertical distribution was more diverse. That was particularly the case in thosespecies having almost 100% frequency in summer (Table III), simultaneous oc-currence of all developmental stages, and similar feeding habits, i.e., the speciesthat would be expected to be strong competitors for food. The frequency ofpredatory cyclopoids showed Oncaea sp., O. similis and O. frigida to be most fre-quent within 0 —10 m, 10 — 25 m and 25 — 50 m, respectively. On the other hand,S. glacialis was uniformly distributed throughout the water column.
The calanoid species showing a clear periodicity and asynchrony of occurrenceand development were more evenly distributed vertically at night. Calanoidesacutus, C. propinquus, R. gigas and M. longa were practically limited in theirnocturnal occurrence to the 25 — 50 m layer (Table IV).
Acknowledgements
The work was supported by grants received by the Arctowski Station from thePolish Academy of Sciences Second Antarctic Expedition (Project MR-II-16).
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