Top predators as indicators for ecosystem events in the confluence zone and marginal ice zone of the Weddell and Scotia Seas,Antarctica, November 1988 to January 1989 (EPOS Leg 2)

Summary The pelagic summer distribution of Antarctic seabirds, seals and whales was studied in the marginal ice zone of the northwestern Weddell Sea from November 1988 to January 1989. In order to relate top predators to other components of the ecosystem studied simultaneously, their distribution is mainly described in terms of energy flow. Bird, seal, and probably also whale requirements were highest in ice-covered areas. There was no evidence of higher numbers of top predators along the ice edge: densities generally increased further into the ice. In the pack ice, combined energy requirements of top predators often amounted to about 200.000 kJ/day/km², or about 45 kg fresh food, indicating high abundance and availability of prey under the ice. There was a lack of conformity between top predator abundance on the ice and abundance of other life in the water column below. In open water, bird requirements were generally less than 25.000 kJ/day/km², seals were virtually absent and whales were distributed unevenly. Tubenosed birds concentrated along the outer ice edge in early summer but they moved north to open water during December, leaving the area of maximum phytoplankton biomass associated with the retreating ice edge. This pattern matched northward movements of krill swarms that may be related to changes in quality rather than quantity of phytoplankton stocks.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Early summer distribution of Antarctic krill sexual development in the Scotia-Weddell region: a multivariate approach

Summary The sexual Development of antarctic krill was studied during the EPOS leg 2 cruise (November 1988–January 1989) in the seasonally ice covered Scotia-Weddell Confluence area. Multiple Correspondence Analysis (MCA) was used to elucidate the general trends of variation of biological (body-size, molt stage) and environmental data (geographical position, sea-ice extension, sampling time) associated with female development. In November, female krill from the ice covered area (Weddell Sea) had a juvenile ovary, while pelagic female krill from Scotia Sea were in advanced previtellogenesis, and one third of them had already spawned. The successive samples from the Confluence illustrated a rapid advance of sexual development during the whole period. Both observation of live krill maintained on board and the MCA confirmed the general trend of distribution of sexual development in relation with size structure and environmental factors. Previtellogenesis occurs in relation with the ice-edge, while vitellogenesis is performed in short cycles in the summer pelagic habitat. The degree of sexual development attained by krill samples (measured by the sexual development index, SDI) is then a good indicator of the biological activity of the krill population and of its impact on the pelagic ecosystem.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Organic carbon in the upper layer and its sedimentation during the ice-retreat period in the Scotia-Weddell Sea, 1988

Summary The maximum of POC in the surface layer (200–400 g·l^–1) followed the retreating ice from end November to end December. In the upper 150 m DOC amounted to 10–20 times the POC content. Free floating sediment traps at 150 m showed a daily sedimentation of 0.6–11.6% of the standing stock of POC, 0.1–2.6% of the chlorophyll-a and 5–190% of the gross primary production. Maximum sedimentation occurred during grazing of a krill swarm, indicating the important role of krill swarms in the downward flux. Also at most other stations krill faecal strings formed a large part of the downward flux.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Distribution,size frequencies and maturity stages of krill,Euphausia superba,in relation to sea-ice in the Northern Weddell Sea

Summary The spring (November) and early summer (December) distribution pattern and demography of Euphausia superba in the seasonally ice covered areas of the north-western Weddell Sea are described. Sampling with a Rectangular Midwater Trawl (RMT) and during SCUBA diving were undertaken in the pack-ice during the EPOS expedition 1988/1989 of RV Polarstern. These data were compared with former cruises during late winter (October) and summer (February). The biomass values of krill (1–27 g/m²) in the water column within the ice covered areas were of the same order of magnitude as earlier acoustic estimates in the ice-edge region of the same area. No significant correlation between percentage ice cover and krill abundance in the water column was found. Cluster analyses of size frequency distributions and maturity stage composition revealed the demography of the animals sampled. Samples obtained by SCUBA and RMT are compatible and results show a geographic and demographic separation of the krill. It is proposed that krill in the northern zone are largely emigrants from the Bellingshausen Sea, while the krill in the southern zone are of eastern Weddell Sea origin. Further studies concentrating on the under-ice distribution of krill are recommended.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation. Publication no 62 of the Alfred-Wegener-Institut für Polar- und Meeresforschung     

Krill-feeding behaviour of gentoo penguins as shown by animal-borne camera loggers

Animal-borne camera loggers were used to examine the patterns of prey encounter and feeding behaviour of gentoo penguins at King George Island, Antarctica. The still images from the camera loggers showed that the penguins encountered the swarms of krill for 25.5% (range: 8–38%) of their dives (>5 m) on average, during their foraging trips (mean duration of 5.4 h, n = 7 trips). They encountered krill swarms during the dives to 10–70 m depth, in pelagic as well as benthic habitats. In the benthic habitat, the penguins swam just above the sea floor and headed downward over a krill swarm, probably using the sea floor to assist them to feed on mobile swarms. The shallow coastal waters would be the important foraging habitat of gentoo penguins breeding in King George Island.     

Seabird and fur seal responses to vertically migrating winter krill swarms in Antarctica

Summary The foraging behaviour of fur seals and two species of surface feeding seabirds was observed over swarms of vertically migrating krill along the Antarctic Peninsula in July 1987. Fur Seal haul out patterns were correlated with krill in the upper 30 m of the water column. Krill moved to the surface at night; seals subsequently foraged from 1400-0700 hours before returning to floes. Foraging was continuous through the night. Dive duration decreased as krill moved up to the surface; shorter dives may have been more successful than longer ones. It is possible that very deep dives, which occur early in a foraging bout, represent more of an attempt to assess krill depth and distribution rather than being a genuine foraging effort. Seabirds responded to the presence of a surface krill swarm by circling over it and foraging; krill at depths greater than 30 m elicited directional flight and low frequencies of prey capture attempts. Both Snow Petrels and Antarctic Terns preyed on krill, but each species approached the swarms from different habitats. Snow Petrels primarily overflew areas covered by ice; terns preferred open water. This suggested that prey encounters are essentially opportunistic, although the search for prey is limited to rather specific marine habitats. This feature may be important to our understanding of the factors that determine the pelagic distribution of seabirds.     

Population structure and swarm formation of the cyclopoid copepod Dioithona oculata near mangrove cays

The cyclopoid copepod Dioithona oculata forms swarms in water>30 on deep among prop roots of red mangroves (Rhizophoramangle) which fringe protected areas of two lagoonal cays, TwinCays, Belize. During 7 of 8 months surveyed by in situ observation,swarms were present but differed in size from small cylindricalswarms (5–10 cm diameter) to bands extending up to 1200m Swarms were never observed at night Swarms formed at dawnwhen light intensities reached an average value of 13.82 (logioquanta cm^–Abstract. s^–1) and dispersed at dusk atsimilar intensities Swarms observed in June formed earlier anddispersed later in the day than swarms observed in January,their swarming behavior followed seasonal changes in light intensityMean dioithonan density in swarms (10 ml^–1) was much higherthan the mean density (0 15 ml^–1) of non-swarming dioithonansaround mangrove prop roots. In open water 3–5 m away fromthe mangroves, mean dioithonan density was 7 9 x 10^–5ml¹ during the day, and 2 68 x10^–3 ml^–1 at nightSwarms were composed predominantly of adults and copepodid stagesIV and V, although younger copepodid stages could be presentNauplii were never present. The ‘average copepodid stage’for all 95 swarms sampled was 5 3, where 6 0 represents a swarmwith only adults In open water 3–5 m away from the mangroves,the youngest copepodids (stage one) dominated the dioithonanpopulation during the day. At night when swarms dispersed toopen waters, average copepodid stage was higher (3 5) comparedwith the day value (1.2) in open waters. Although densitiesin swarms were higher in June than January, average copepodidstage in June was higher (5 6) than that in January (4.9). Ahigher percentage of adults were females during June than January.Therefore higher densities did not result from increases ofsmaller stages in swarms, but perhaps changes in behavior orpopulation structure.     

The overwintering strategy of Antarctic krill under the pack-ice of the Weddell Sea

Summary Antarctic krill (Euphausia superba Dana) occurs in enormous swarms in Antarctic waters during the ice-free summer months. The winter whereabouts of this stock were hitherto unknown. Evidence collected during the Winter Weddell Sea Project 1986 (WWSP'86, G. Hempel 1988) covering a large area of the eastern and southern Weddell Sea indicates that the seasonal sea ice cover sustains the bulk of the krill population. Results presented here, show that known aspects of krill morphology and behavior are actually adaptations to the ice habitat, suggesting that the dominance of krill in the Antarctic marine ecosystem is a result of its capacity to grow and reproduce in the water column in summer, and find both food and shelter in the ice cover during the rest of the year. This conclusion has far-reaching implications for our understanding of Southern Ocean biology and ecology.     

A Southern Ocean archipelago enhances feeding opportunities for a krill predator

Productivity in the oceans is heightened around oceanographic and bathymetric features such as fronts and islands. This can have a flow-on effect, providing increased food availability for higher trophic level species. Using data from a 5-day combined visual and acoustic survey, we examined the hypothesis that higher Antarctic krill (Euphausia superba) density provides a lucrative resource for humpback whales (Megaptera novaeangliae) at a remote Antarctic feeding area, the Balleny Islands (67^oS, 164°E). We assessed whale presence at the feeding area in relation to prey (krill), productivity and environmental variables using density surface modeling. We found stark differences between krill swarms near the islands and those in adjacent open water. Swarms were twice as dense and three times more numerous near the Balleny Islands compared to an open water pelagic environment, suggesting that the islands offered a profitable feeding opportunity. At the feeding area, whales were found in deeper and more productive waters with medium krill densities. These relationships, along with the high krill availability around the islands, may be the result of the Island Mass Effect.     

The influence of copepod and krill grazing on the species composition of phytoplankton communities from the Scotia Weddell sea

The influence of copepods (mainly Oithona sim-ilis) and krill (Euphausia superba) grazing on the species composition of plankton communities in ship board con tainers was investigated during the spring and post spring period in the Scotia Weddell Sea in the Antarctic ocean. Numbers of grazers were experimentally manipulated in containers with natural phytoplankton assemblages. With ratural levels of copepods but no krill a high (700–950 g C·l¹, ca 30 g chl a·.l¹) phytoplankton biomass developed. In these cultures large diatoms, e.g. Corethron criophilum and chains of Thalassiosira sp., made up 80% of total phytoplankton cell carbon at the end of the experiment. In cultures with elevated numbers of copepods (5X or 10X the natural level) phytoplankton biomass was somewhat reduced (ca 23 g chl a · l¹) compared to cultures with natural copepod abundance, but still high. Phytoplankton species composition was on the other hand greatly influenced. Instead of large diatoms these cultures were dominated by Phaeocystis pouchetii (70%) together with small Nitszchia sp. and Chaetoceros neogracile (20%). In containers with krill (both juveniles and adults), but without elevated numbers of copepods, phytoplankton biomass rapidly approached zero. With 10X the in situ level of copepods, krill first preyed on these before Corethron criophilum and Thalassiosira sp. were grazed. When krill were removed a plankton community dominated by flagellates (60–90%), e.g. Pyramimonas sp. and a Cryptophycean species, grazed by an unidentified droplet-shaped heterothrophic flagellate, developed. These flagellates were the same as those which dominated the plankton community in the Weddell Sea after the spring bloom. A similar succession was observed in situ when a krill swarm grazed down a phytoplankton bloom in a few hours. Our experiments show that copepods cannot control phytoplankton biomass in shipboard cultures even at artificially elevated numbers. Krill at concentrations similar to those in natural swarms have a great impact on both phytoplankton biomass and species composition in shipboard cultures. Both copepods and krill may have an impact on phytoplankton species composition and biomass in situ since the rates of phytoplankton cell division were probably artificially increased in shipboard cultures compared to natural conditions, where lower growth rates make phytoplankton more vulnerable to grazing. A similarity between phytoplankton successions in containers and in situ, especially with respect to krill grazing, supports the conclusion that grazing may structure phytoplankton communities in the Scotia-Weddell Sea.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Food and feeding ecology of emperor penguins in the eastern Weddell Sea

Summary The diet of the emperor penguin Aptenodytes forsteri in the eastern Weddell Sea, Antarctica was studied during October and November 1986 by stomach content analysis. Emperor penguins fed mainly on Antarctic krill Euphausia superba, Antarctic silverfish Pleuragramma antarcticum and squid Psychroteuthis glacialis. Benthic prey was not found. The prey composition suggests two different feeding strategies, shallow dives exploring the rugged underside of sea ice where krill is taken, and deep dives when mesopelagic fish and squid are consumed. Chicks were fed on average every 1.44 days.     

The association of Antarctic krill Euphausia superba with the under-ice habitat

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0-2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m(-2) in summer and autumn, and 2.7 individuals m(-2) in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0-2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0-2 m layer were higher than corresponding values from the 0-200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0-200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.     

Effectiveness of the RMT8 system for sampling krill (Euphausia superba) swarms

Summary The effectiveness of the Rectangular Midwater Trawl (RMT) for sampling krill swarms was studied by observing the behaviour of swarms in the vicinity of the net using echosounders. Large scale avoidance was observed by day but not at night. By day krill layers dispersed to such an extent that few krill were present less than 10 m below the net. Limited avoidance by night indicates that the krill are acting on visual clues of the net's presence.     

Krill diet affects faecal string settling

Summary Free-floating sediment traps used on a transect from Scotia Sea to Weddell Sea collected larger, more degraded, krill faecal strings in the deeper (150 m) than in the 50 or 75 m traps. The smallest faecal strings were only present in the shallower traps. Sinking velocity of smaller faecal strings was — as expected — much lower than for larger ones, with a total range of 50 to 800 m · day ^–1 for faecal string volumes of 0.007 to 0.53 mm³. Krill feeding largely on diatoms produced faeces with higher settling velocity than those feeding on non-diatom phytoplankton. Smaller krill faecal strings do not leave the upper mixed layer. Potential settling velocities as measured in settling tubes (without turbulence), may in this respect be misleading. Small oval faecal pellets of unknown origin showed relatively high settling velocities (80 to 250 m·day^–1 for 0.002 to 0.013 mm³) due to higher compaction and lower form resistance to sinking.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Variability of Euphausia superba populations near Elephant Island and the South Shetlands: 1981 vs. 1984

Summary March 1981 and 1984 Euphausia superba populations were compared using acoustics and net catches near Elephant Island, the South Shetlands, and in the Bransfield Strait. In 1981, krill tended to form large, thick swarms and in 1984, smaller, more dispersed, shallower patches. March body lengths of juvenile-adult krill were 22–59 mm in 1981 and 13–59 mm in 1984. Near Elephant I. in 1981, krill >45 mm were most numerous; in 1984 sizes <45 mm were dominant. In March 1984, the larger (>45 mm) body-size group was prereproductive and occurred from just west of Elephant I. westward into waters north of the South Shetlands; in 1981 the larger krill were postreproductive and more widely distributed in the Elephant I. area. Overall, netted postlarval krill, 1981 vs. 1984, averaged 73 vs. 48 individuals/m², or 54 vs. 16 g/m²; acoustic biomass estimates were 229 vs. 134–201 g/m². Larvae near Elephant I. averaged >2000/m² in 1981 vs. <1/m² in 1984—compatible with respective March reproductive states. Net-type comparisons revealed short-term (15 min to 6h) variability of a similar scale in both MOCNESS and bongo net catches, but bongo abundances averaged greater. Variation in maturity composition across 1981 swarms, patches, and random transects was like variation among the random 1984 tows; spatial distributions were more heterogeneous in 1984. The March 1984 krill of 20–44 mm (Year-2, mode 34 mm) relate to November 1983 krill of 9–30 mm (mode 21 mm), indicating growth averaging 12 mm during the season. Body-lengths and size-frequency modes of Year-2 and combined Years-3,3+ krill from comparable Feb-Mar data collected since 1968 suggest trends between times when (1) Year-2 krill average small and peak reproduction seems to be late in the season and/or weak (1979, 1982–1984), and (2) Year-2 krill are larger, and reproduction is possibly earlier and more successful (1976, 1980, 1981).     

The diet of Antarctic fur seals, Arctocephalus gazella, at King George Island, during the summer–autumn period

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at Stranger Point, King George Island, by scat analysis from February to April 1996. Overall, krill and fish were the most frequent prey, occurring in an average of 97% and 69% of samples (n=128), followed by cephalopods (12%). Myctophids constituted almost 90% of the fish predated, with Electrona antarctica and Gymnoscopelus nicholsi being the most abundant and frequent species consumed. All fish taxa identified were krill-feeding species suggesting that seals foraged primarily on krill and opportunistically on fish species associated with krill swarms. A seasonal change observed in the relative proportions of the different fish prey taxa indicates that fur seals spent more time foraging over the shelf in summer and off the shelf in autumn. During the study period, commercial fishing in the area was not based upon any of the fish identified in this study.     

Biomass and composition of size fractionated phytoplankton in the Weddell-Scotia Confluence area

Summary The measurement of Chl a, Chl b and Chl c contents in four size fractions (Nuclepore filters of 10 m, 3m, 1 m and 0.2 m pore-size) together with microscopic examination illustrate the structure and the relative importance of the micro-, nano and pico-phytoplankton in the production system in the Weddell/Scotia Confluence area. In the Scotia Sea, large diatoms were prevalent and their biomass increased during the six week cruise period, exceeding 1 mg Chl a m^–3 at the beginning of January. In contrast, in the Marginal Ice Zone of the Weddell Sea, the biomass remained low, up to 0.3 mg Chl a m^–3. A diversified nanoplankton community accounted for more than 90% of this biomass: small diatoms, naked dinoflagellates, cryptophyceans, prymnesiophytes and green flagellates which increased the Chl b/Chl a ratio to values >0.20. An important trend affected the Confluence area, where a high biomass net-plankton community (4 mg Chl a m^–3) rapidly changed towards a uniform nanoplankton system of the same kind as in the Weddell Sea. At times, autotrophic cryptophyceans were almost dominating (>4.10⁶ cells/l), with a biomass up to 2 mg Chl a m^–3 and a low phaeopytin ratio (<10%). This situation probably arises because of a grazing pressure by krill. However, due to the geographic and oceanographic peculiarities of this area, it is not possible to extrapolate these observations concerning the size structure of the primary producers to the Southern Ocean in general.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Vertical distribution and abundance of juvenile chaetognaths in the Weddell Sea (Antarctica)

The composition, abundance and vertical distribution of chaetognaths were analysed along a transect in the Weddell Sea during late spring. Three species were identified: Eukrohnia hamata (90.8%), Sagitta marri (6.4%) and S. gazellae (2.8%). Only juvenile stages were collected in the samples, a result related both to the type of sampling gear employed (mesh size: 100 μm) and the species' life-cycles. The vertical distributions showed that the juvenile stages of these species tended to aggregate at considerable depth (1000–500 m). It is postulated that this pattern may be related to the life-cycles of these species in association with seasonal Antarctic conditions, similar to the pattern postulated for krill and other polar crustaceans. Accepted: 10 July 2000     

Long-term changes in Krill biomass and distribution in the Barents Sea: are the changes mainly related to capelin stock size and temperature conditions?

Krill plays a significant role in the Barents Sea ecosystem, providing energy transport between different trophic levels. The current paper presents the results of a long-term study (1980–2009) based on pelagic trawl catches from August to September. Our investigations show that the krill species were distributed widely in the Barents Sea and that the largest krill concentrations were restricted to the west-central and eastern parts of the Barents Sea. The current paper presents the relative biomass indices, and the estimates must be interpreted as minimum biomass. The mean annual krill biomass was estimated to be 22 million tonnes in wet weight, with the highest values being as much as 48 million tonnes. Capelin is the largest pelagic stock, and in some years, their biomass can amount to 4–7 million tonnes, which can impose high predation pressure on krill. When their biomass is high, capelin may consume close to 26 million tonnes annually. The predation from pelagic (herring and blue whiting) and bottom (cod and haddock) fish species was much lower, being 9 and 1 million tonnes, respectively. A negative relationship between krill biomass and capelin stock size above 74°N was observed during the study period. However, during the last decade, the krill biomass has increased despite heavy predation from capelin in some years. A positive significant linear relationship between the mean annual Kola temperature and the krill biomass seems to indicate that the recent warming conditions have favourable impacts on the krill populations in the Barents Sea.     

SIZE-ASSORTATIVE MATING IN A NATURAL POPULATION OF VIVIPARUS ATER (GASTROPODA: PROSOBRANCHIA) IN LAKE ZURICH, SWITZERLAND

The number of mating pairs, the size of the mating partners,and the distribution of individuals of Vivi-parus ater on agrid in Lake Zürich were recorded during one breeding seasonin 1990. There was positive assortative mating with respectto shell size. The proportion of copulating individuals rangedfrom 1% to 6% (average 3%) of the active population at any onetime. Individual snails copulated 60 times on average from Apriluntil November. Snails were abundant and copulated in shallowwater close to the shore in Spring. They moved towards deeperareas in Autumn. V. ater copulated on all substrates at anydepth (1–9 m)of the grid. The spatial distribution ofcopulations throughout the summer reflected the pattern of snailabundance. (Received 29 January 1993; accepted 14 November 1994)