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Iron is an essential nutrient for phytoplankton, but low concentrations limit primary production and associated atmospheric carbon drawdown in large parts of the world’s oceans [ 1, 2 ]. Lithogenic particles deriving from aeolian dust deposition, glacial runoff, or river discharges can form an important source if the attached iron becomes dissolved and therefore bioavailable [ 3–5 ]. Acidic digestion by zooplankton is a potential mechanism for iron mobilization [ 6 ], but evidence is lacking. Here we show that Antarctic krill sampled near glacial outlets at the island of South Georgia (Southern Ocean) ingest large amounts of lithogenic particles and contain 3-fold higher iron concentrations in their muscle than specimens from offshore, which confirms mineral dissolution in their guts. About 90% of the lithogenic and biogenic iron ingested by krill is passed into their fecal pellets, which contain ∼5-fold higher proportions of labile (reactive) iron than intact diatoms. The mobilized iron can be released in dissolved form directly from krill or via multiple pathways involving microbes, other zooplankton, and krill predators. This can deliver substantial amounts of bioavailable iron and contribute to the fertilization of coastal waters and the ocean beyond. In line with our findings, phytoplankton blooms downstream of South Georgia are more intensive and longer lasting during years with high krill abundance on-shelf. Thus, krill crop phytoplankton but boost new production via their nutrient supply. Understanding and quantifying iron mobilization by zooplankton is essential to predict ocean productivity in a warming climate where lithogenic iron inputs from deserts, glaciers, and rivers are increasing [ 7–10 ].

High densities of ice krill Euphausia crystallorophias were observed along six acoustic transects within the Amundsen Sea Coastal Polynya, Antarctica. Two-frequency acoustic backscatter data was examined in the austral summers of January 2011 and February 2012. A dB identification window (Sv120−38) identified ice krill dominating the acoustic backscatter. The density of ice krill, calculated with the stochastic distorted-wave born approximation model, ranged between 4.5 and 30 g wet mass m−2 for each transect (a mean of 16 g wet mass m−2 for all transects), these high values are an order of magnitude higher than recorded previously in the Ross Sea Polynya. High densities were detected along the ice shelf and near the boundary between pack ice and coastal polynya, and we postulate that these could be important habitats for ice krill. The high densities observed along the transects make ice krill a potentially important, but poorly known contributor to these high-latitude shelf food webs.

‘Wasp-waist’ systems are dominated by a mid trophic-level species that is thought to exert top-down control on its food and bottom-up control on its predators. Sardines, anchovy, and Antarctic krill are suggested examples, and here we use locusts to explore whether the wasp-waist concept also applies on land. These examples also display the traits of mobile aggregations and dietary diversity, which help to reduce the foraging footprint from their large, localised biomasses. This suggests that top-down control on their food operates at local aggregation scales and not at wider scales suggested by the original definition of wasp-waist. With this modification, the wasp-waist framework can cross-fertilise marine and terrestrial approaches, revealing how seemingly disparate but economically important systems operate.

The pelagic food web of the Scotia Sea was studied by analysing natural abundances of nitrogen and carbon stable isotopes of primary producers and pelagic consumers, sampled from the seasonal ice edge in the south to the Antarctic Polar Front in the north. The analysis covered, within a single mid-summer period, particulate organic matter (POM) and 38 taxa, ranging from suspension feeding copepods and salps to omnivorous euphausiids, pelagic fish and higher, land-based predators including fur seals, penguins and flying birds. Spatial variation in δ15N of POM correlated well with nutrient availability and primary productivity. Latitudinal differences in δ13C of POM were closely linked to variations in temperature, nutrients and productivity depending on the frontal region sampled. This translated to equivalent (although smaller) regional δ13C differences among higher trophic levels. The trophic positions of species based on isotope values broadly agreed with previously published dietary data with three important exceptions. First, the carnivorous amphipod Themisto gaudichaudii had anomalously low δ15N values. Second, Euphausia superba had δ15N values that were also surprisingly low, considering the abundant literature suggesting its omnivory. Third, the copepod Rhincalanus gigas, considered a suspension feeder, had unexpectedly high δ15N values rather more in keeping with omnivorous feeding. The consumer δ15N values ranged from 1.2‰ (min.) measured in Salpa thompsoni (designated here as trophic level (TL) 2 across all regions) to 15.2‰ (max.) measured in white-chinned petrels (Procellaria aequinoctialis, calculated as TL5 relative to the TL2 of salps). Excluding seabirds, the resulting food chain length of 3.7 TL (above POM at TL1) was lower than in most other Southern Ocean and temperate marine pelagic ecosystems. The majority (60%) of vertebrate predators occupied only 1–1.5 trophic levels above the herbivorous suspension feeders such as krill. This indicates the e

Mesozooplankton distribution and community structure was investigated during 3 cruises to the Scotia Sea in austral spring, summer and autumn. Three mesh sizes of Bongo nets were used during each cruise with a 53 μm mesh net yielding on average 1.76 times higher densities (median 923,000 ind. m−2, 0–400 m) than a 100 μm net and 7.42 times more than a 200 μm net across all cruises. Small copepods dominated numerically across all nets with Oithona spp., Oncaea spp., Ctenocalanus citer and Microcalanus pygmaeus being particularly abundant, with sample densities of up to 3.5×106 ind. m−2 recorded within the top 400 m. A more even distribution of biomass among net sizes was apparent, with median net ratios (1.15–1.25) smaller and more even than for abundance. To the south of the Scotia Sea plankton maxima occurred in autumn, consistent with a later spawning in many species, whereas further north, abundance in 53 and 100 μm nets varied little across seasons, although in the 200 μm net there was a clear summer maximum. Median biomass increased through summer and by autumn was twice than found during spring in all parts of the Scotia Sea. Cluster analysis indicated two main station groups in all 3 seasons. To the south of the Southern boundary of the ACC (SB-ACC), Group 1 contained stations, that lay within the seasonal sea-ice zone and where zooplankton abundance and biomass was persistently low. In contrast at Group 2 stations, north of the Southern Antarctic Circumpolar Current Front (SACCF) abundance and biomass was consistently higher. Differences between the two groups were largely apparent at the population rather than at the taxonomic level. LHPR hauls to 1000 m indicated that the large seasonal migrant copepods Calanoides acutus and Rhincalanus gigas were at a more advanced stage of development in the north in spring and summer where they were generally present in the upper water column. In autumn, at all stations, C. acutus was dominated by later stages and was di

The kinetics of food processing by zooplankton affects both their energy budgets and the biogeochemical fate of their fecal pellets. We sampled 40 schools of krill across the Scotia Sea during spring, summer and autumn and found that in all 3 seasons, every aspect of their absorption and defecation varied greatly. The C content of fecal pellets varied from 0.85% to 29% of their dry mass (median 9.8%) and C egestion rates varied 75-fold. C:N mass ratios of pellets ranged from 4.9 to 13.2 (median 7.8), higher than values of 3.9 in the krill and 5.4 in their food, pointing to enhanced uptake of N. Pellet sinking rates equated to 27–1218 m d−1 (median 304 m d−1), being governed mainly by pellet diameter (80–600 μm, mean 183 μm) and density (1.038–1.391 g cm−3, mean 1.121 g cm−3). Pellets showed little loss of C or N in filtered seawater over the first 2 days and were physically robust. When feeding rates were low, slow gut passage time and high absorption efficiency resulted in low egestion rates of pellets that were low in C and N content. These pellets were compact, dense and fast-sinking. Conversely, in good feeding conditions much food tended to pass quickly through the gut and was not efficiently absorbed, producing C and N-rich, slow-sinking pellets. Such “superfluous feeding” probably maximises the absolute rates of nutrient absorption. Food composition was also important: diatom-rich diets depressed the C content of the pellets but increased their sinking rates, likely due to silica ballasting. So depending on how krill process food, their pellets could represent both vehicles for rapid export and slow sinking, C and N-rich food sources for pelagic scavengers. C egestion rates by krill averaged 3.4% of summer primary production (and ingestion rates would be 2–10-fold higher than this) so whatever the fate of the pellets, krill are an important re-packager within the food web. While salp pellets tend to sink faster than those of krill, it is the latter that tend

Foodweb models provide a useful framework for compiling data on biomass, production, consumption and feeding relationships. They are particularly useful for identifying gaps and inconsistencies in the data, and for exploring plausible scenarios of change. We compiled data on the pelagic foodweb of the South Georgia shelf, which is one of the most intensively studied areas in the Southern Ocean. The data suggest that current average annual copepod production is three times that of Antarctic krill and that flying seabirds and fish are, respectively, responsible for 25% and 21% of local krill consumption. The most striking inconsistency was that estimated consumption of fish was 5 times their estimated production. We developed a static mass balance model of the foodweb representing one of many possible solutions to the inconsistencies in the data. The model included sufficient fish biomass to balance the original consumption estimate, and consequently fish became the main krill consumers. Nonetheless, only 74% of local krill production was consumed by predators, suggesting that there are additional mortality sources that we did not explicitly model. We developed further models to explore scenarios incorporating plausible climate-driven reductions in krill biomass. In scenarios with unchanged predator diets, an 80% reduction in krill biomass resulted in a 73% reduction in vertebrate biomass. However, when predators with diverse diets were able to switch to feeding on alternative zooplankton prey, total vertebrate biomass was maintained at current levels. Scenarios in which 80% of krill biomass was replaced with copepod biomass required 28% more primary production because the estimated consumption rate of copepods is higher than that of krill. The additional copepod biomass did not alter the consequences for vertebrates. These scenarios illustrate the wide range of potential consequences of a shift from a krill to a copepod dominated system in a warming climate. They sug

Bioregionalisation, the partitioning of large ecosystems into functionally distinct sub-units, facilitates ecosystem modelling, management and conservation. A variety of schemes have been used to partition the Southern Ocean, based variously on frontal positions, sea ice, productivity, water depth and nutrient concentrations. We have tested the utility and robustness of ecosystem bioregionalisation for the Scotia Sea, by classifying spring, summer and autumn stations on the basis of nutrient concentrations, phytoplankton taxa, meso- and macrozooplankton, fish catches and acoustic data. Despite sampling across different seasons and years, at different spatial scales and taxonomic resolutions, cluster analysis indicated basically consistent spatial divisions across this wide range of trophic levels. Stations could be classified into two main groups, lying broadly to the north and south of the Southern Antarctic Circumpolar Current Front (SACCF). In some aspects the 2 station groups were similar, with both having variable but often high phytoplankton biomass as well as similar biomass of fish. However, the colder water southern group, most of which was covered by seasonal sea ice, had a fundamentally different food web structure to that in the northern Scotia Sea. The cold water community had a depleted, cold-adapted fauna characterised by low zooplankton biomass, Euphausia superba and the fish Electrona antarctica and Gymnoscopelus braueri. In contrast the northern group was richer with higher mesozooplankton biomass and a fauna of warmer or more cosmopolitan species such as Themisto gaudichaudii, Euphausia triacantha and the fish Protomyctophum bolini, Kreffticthys anderssoni and Gymnoscopelus fraseri. The position of the food web transition, broadly consistent with the position of the SACCF, supports a recent circumpolar-scale bioregionalisation. However, there is little evidence that this relatively weak frontal transition represents a significant barrier either he

Since the Discovery Investigations of the 1920s and 1930s, seawater temperatures have increased in the Atlantic sector by ∼1 °C; greater than the global mean rise. The aims of this paper were first to rescue the Discovery macrozooplankton data, second to provide quantitative “baseline” distribution maps, relating these to indices of temperature and food. Our third aim was to use the relationships we derived between abundance and temperature to project the potential affect of a 1 °C warming on the Discovery era distribution patterns. Based on the 1 m ringnet data retrieved from 615 stations (Nov–March), four taxa comprised >90% of the Antarctic macrozooplankton abundance: Rhincalanus gigas, Thysanoessa spp., Euphausia superba, and Chaetognaths. Most of the taxa, especially the more abundant ones, were warm water species penetrating into Antarctica and thus total macrozooplankton abundance decreased about 100-fold from 50°S to 70°S. While temperature correlated best with distribution at this large scale, food availability (proxied by a present-day satellite-based Chlorophyll a climatology) had a secondary effect, with the major euphausiids Euphausia superba and Thysanoessa spp. concentrated in high chl a areas. A modelled uniform 1 °C temperature rise produced a poleward shift for all taxa, but the Antarctic continent blocked this re-adjustment for the high latitude species, constricting their predicted range. More widespread polar/sub-polar species were predicted to increase their penetration into Antarctica by 4–12° in latitude, whereas the poleward shift in potential range of sub-Antarctic taxa were limited by the steep temperature gradient across the Antarctic Polar Front (APF). However, within the Scotia Sea the relatively warm temperatures of the northern Antarctic Zone, abundant food due to iron fertilisation and intense eddy activity provide a “gateway” for northern species to penetrate south of the APF. Our model predictions, based on measured distributional

The fronts in the Antarctic Circumpolar Current (ACC) comprise the Southern ACC Front (SACCF), the Polar Front (PF) and Sub-Antarctic Front (SAF), which, together with the Southern Boundary (SB), separate zones with distinct water mass properties in the Southern Ocean. They are important for describing habitat ranges of oceanic species over the range of trophic levels and are also key habitat zones in their own right for a variety of species. Frontal positions were studied along a transect running S to N across the Scotia Sea during three hydrographic cruises in spring 2006, summer 2008 and autumn 2009. Subsurface data from these cruises and Argo data are here combined with Aviso satellite absolute dynamic topography data to locate these fronts, facilitating study of their ecological significance within the Scotia Sea. Northern and southern dynamic height limits are found for each front, as opposed to the more usual one dimensional line. Thus the SB was found between −132 to −116 dyn cm, the SACCF between −115 to −99 dyn cm and the PF between −71 to −45 dyn cm. Argo data allow the sensitivity of the dynamic heights found to be tested against seasonal, interannual and longitudinal effects and each is found to be small. Although the SAF lay beyond the northern extent of the transect we sampled along, Argo profiles allowed us to determine its limits as lying between −5 and +29 dyn cm. The dynamic height limits found are thus suitable for assessing the location, in relation to fronts, of any sample or tracked animal within this area. The chlorophyll distribution across the Scotia Sea is assessed relative to frontal positions and known iron sources. The physical conditions (sea surface temperature, sea ice and mixed layer depth) along the sampling transect are also presented, allowing the oceanographic conditions of the stations occupied across the Scotia Sea to be assessed.

The Scotia Sea area has high productivity relative to the Southern Ocean as a whole, but this displays strong latitudinal and longitudinal gradients. Elucidating the extent of these from a single cruise is problematic, given the high variability of bloom timing and location in this region. Therefore, this study used data from transects across the central Scotia Sea in spring, summer and autumn of 2006, 2008 and 2009, combined with satellite data, to obtain a larger-scale appreciation of the latitudinal contrasts in phytoplankton standing stock and primary production across the region. Concentrations of nitrate, phosphate and particularly silicic acid increased towards the south of the transect with the latter showing a step change at the Southern Antarctic Circumpolar Current Front (SACCF). Changes in seasonal nutrient concentrations indicated increasing phytoplankton uptake north of ∼57°S that peaked at ∼53°S in the Georgia Basin. Based on seasonal depletions of nitrate relative to phosphate, the highest relative nitrate uptake occurred northwest of South Georgia on the periphery of the Georgia Basin, indicating efficient nitrate use here due to iron-replete conditions. An integrative approach to examine these gradients was with the use of 10-year satellite climatology data. These showed that the lowest mean chlorophyll a (chl-a) values were in the central/northern Scotia Sea, but these were still substantial values, 67% of values within the Georgia Basin bloom. Cruise data on chl-a and on microplankton biomass from cell counts support this finding of substantial biomass in the central Scotia Sea; since these averaged half of values in the iron-fertilised bloom of the Georgia Basin downstream of South Georgia. Given that our transect was nearly 1000 km long and in parts was land remote with low iron concentrations, the relatively high production in the central and northern Scotia Sea is surprising. Iron levels may be maintained here by efficient recycling and irreg

The Scotia Sea has been a focus of biological- and physical oceanographic study since the Discovery expeditions in the early 1900s. It is a physically energetic region with some of the highest levels of productivity in the Southern Ocean. It is also a region within which there have been greater than average levels of change in upper water column temperature. We describe the results of three cruises transecting the central Scotia Sea from south to north in consecutive years and covering spring, summer and autumn periods. We also report on some community level syntheses using both current-day and historical data from this region. A wide range of parameters were measured during the field campaigns, covering the physical oceanography of the region, air–sea CO2 fluxes, macro- and micronutrient concentrations, the composition and biomass of the nano-, micro- and mesoplankton communities, and the distribution and biomass of Antarctic krill and mesopelagic fish. Process studies examined the effect of iron-stress on the physiology of primary producers, reproduction and egestion in Antarctic krill and the transfer of stable isotopes between trophic layers, from primary consumers up to birds and seals. Community level syntheses included an examination of the biomass-spectra, food-web modelling, spatial analysis of multiple trophic layers and historical species distributions. The spatial analyses in particular identified two distinct community types: a northern warmer water community and a southern cold community, their boundary being broadly consistent with the position of the Southern Antarctic Circumpolar Current Front (SACCF). Temperature and ice cover appeared to be the dominant, over-riding factors in driving this pattern. Extensive phytoplankton blooms were a major feature of the surveys, and were persistent in areas such as South Georgia. In situ and bioassay measurements emphasised the important role of iron inputs as facilitators of these blooms. Based on seasonal DIC

The spawning success of Antarctic krill (Euphausia superba) is generally assumed to depend on substantial winter sea ice extent, as ice biota can serve as a food source during winter/spring and the seasonal ice melt conditions the upper water column for extensive phytoplankton blooms. However, direct observations during spring are rare. Here we studied krill body condition and maturity stage in relation to feeding (i.e. stomach fullness, diet, absorption of individual fatty acids and defecation rate) across the Scotia Sea in November 2006. The phytoplankton concentrations were low at the marginal ice zone (MIZ) in the southern Scotia Sea (Stn. 1, 2, and 3), high in open waters of the Southern Antarctic Circumpolar Current Front (SACCF) in the central Scotia Sea (Stn. 5), and moderate further north (Stn. 6 and 7). Krill had low lipid reserves (∼6.5% of dry mass, DM), low mass:length ratios (∼1.7 mg DM mm−1), and small digestive glands (∼7% of total DM) near the ice edge. The stomachs contained lithogenic particles, diatom debris, and bacterial fatty acids, but low proportions of diatom-indicating fatty acids, which suggest that these krill were feeding on detritus rather than on fresh ice algae. In the SACCF, krill had higher lipid reserves (∼10% of DM), high mass:length ratios (∼2.2 mg DM mm−1), and large digestive glands (∼16% of total DM). Stomach content and tissue composition indicate feeding on diatoms. In the north, moderate food concentrations co-occurred with low lipid reserves in krill, and moderate mass:length ratios and digestive gland sizes. Only in the phytoplankton bloom in the SACCF had the mating season already started and some females were about to spawn. Based on the way krill processed their food at the different stations, we indicate two mechanisms that can lead to fast regeneration of body reserves and oocyte maturation in E. superba. One is “superfluous” feeding at high food concentrations, which maximises the overall nutrient gain. The other i

In this study we examined the hypothesis that, under conditions of replete macronutrients and iron in the Southern Ocean, phytoplankton abundance and specific N uptake rates are influenced strongly by the processes of grazing and NH(4) regeneration. NH4 and NO(3) uptake rates by marine phytoplankton were measured to the northeast and northwest of the island of South Georgia during January-February 1998. Mean specific uptake rate for NO(3) (vNO(3)) was 0.0026 h(-1) (range 0.0013-0.0065 h(-1)) and for NH(4) (vNH(4)) was 0.0097 h(-1) (0.0014-0.0376 h(-1)). vNH(4) was related positively with NH(4) availability, which ranged from 0.1 to 1.5 mmol m(-3) within the upper mixed layer. Ambient NH(4) concentrations and vNH(4) were both positively related to local krill biomass values, computed from mean values along acoustic transect segments within 2 km of the uptake measurement stations. These biomass values ranged from similar to 1 g krill fresh mass m(-2) in the northwest to >4 kg krill wet mass m(-2) in the northeast. In contrast to the variability found with NH(4) concentrations and uptake rates, vNO(3) was more uniform across the sampling sites. Under these conditions, increasing NH(4) concentration appeared to represent an additional N resource. However, high vNH(4) tended to be found for stations with lower phytoplankton standing stocks, across a total range of 0.24-20 mg chlorophyll a m(-3). These patterns suggest a coupling between phytoplankton biomass, vNH(4) and krill in this region of variable but high krill biomass. Locally high concentrations of krill in parts of the study area appeared to have two opposing effects. On the one hand they could graze down phytoplankton stocks, but on the other hand, their NH(4) excretion supported enhanced uptake rates by the remaining, ungrazed cells. (C) 2011 Elsevier Ltd. All rights reserved.

Swarming is a fundamental part of the life of Euphausia superba, yet we still know very little about what drives the considerable variability in swarm shape, size and biomass. We examined swarms across the Scotia Sea in January and February 2003 using a Simrad EK60 (38 and 120 kHz) echosounder, concurrent with net sampling. The acoustic data were analysed through applying a swarm-identification algorithm and then filtering out all non-krill targets. The area, length, height, depth, packing-concentration and inter-swarm distance of 4525 swarms was derived by this method. Hierarchical clustering revealed 2 principal swarm types, which differed in both their dimensions and packing-concentrations. Type 1 swarms were generally small (<50 m long) and were not very tightly packed (<10 in.m(-3)), whereas type 2 swarms were an order of magnitude larger and had packing concentrations up to 10 times greater. Further subdivisions of these types identified small and standard swarms within the type 1 group and large and superswarms within the type 2 group. A minor group (swarm type 3) was also found, containing swarms that were isolated (>100 km away from the next swarm). The distribution of swarm types over the survey grid was examined with respect to a number of potential explanatory variables describing both the environment and the internal-state of krill (namely maturity, body length, body condition). Most variables were spatially averaged over scales of similar to 100 km and so mainly had a mesoscale perspective. The exception was the level of light (photosynthetically active radiation (PAR)) for which measurements were specific to each swarm. A binary logistic model was constructed from four variables found to have significant explanatory power (P < 0.05): surface fluorescence, PAR, krill maturity and krill body length. Larger (type 2) swarms were more commonly found during nighttime or when it was overcast during the day, when surface fluorescence was low, and when the kri

During Austral summer 2001/02 the spatial distribution of phytoplankton and zooplankton communities and associated production processes were investigated in waters to the north of the island of South Georgia. Nearest neighbour cluster analysis of phytoplankton and zooplankton data sets indicated the presence of 3 major station groupings within each analysis that collectively had great geographic integrity. Thus a shelf station group was characterised by low phytoplankton biomass and primary production rates (median values 69 mg chl a m−2; 418 mg C m−2 d−1, respectively) and a dominance of nano- and pico-phytoplankton, on the one hand, and low overall mesozooplankton levels (median 50,135 ind. m−2), which was also coincident with elevated krill biomass, on the other. At the western end of the island high mesozooplankton abundance (median >250,000 ind. m−2) in oceanic waters was co-incident with a phytoplankton bloom that originated over the North West Georgia Rise (NWGR). These ‘bloom’ stations were characterised by high phytoplankton biomass and primary production rates (median 218 mg m−2 and 916 mg C m−2 d−1, respectively) and were dominated by microphytoplankton. This elevated production was associated with an anticyclonic eddy which was considered to have been initiated where the Southern Antarctic Circumpolar Current Front (SACCF) interacted with the bathymetry of the NWGR. SeaWiFS and drifter buoy data indicated that this production was subsequently entrained and transported cyclonically around the Georgia Basin downstream from the island. In contrast, at the eastern end of the island, in waters south of the SACCF, phytoplankton biomass and primary production rates were lower (median 40 mg m−2 and 214 mg C m −2 d−1, respectively) although median zooplankton abundance was intermediate (not, vert, similar157,000 ind. m−2). Of the environmental variables measured in this study the proportion of microphytoplankton within samples, rather than for example, phytoplank

Twelve epipelagic copepod species were reviewed to compare their adaptations to the short primary production season and low temperatures which characterise the Southern Ocean. The species show a spectrum of adaptations, but three broad life cycle strategies were defined: (1) herbivorous in summer, a short reproductive period and winter diapause at depth (Calanoides acutus and possibly Ctenocalanus citer); (2) predominantly omnivorous/detritivorous diet, an extended period of feeding, growth and reproduction and less reliance on diapause at depth (Metridia gerlachei, Calanus propinquus, Calanus simillimus, Oithona similis, Microcalanus pygmaeus, and possibly Oncaea curvata and Oithona frigida); (3) overwintering and feeding within sea ice as early nauplii or copepodids (Stephos longipes and Paralabidocera antarctica). The large species Rhincalanus gigas appears to be intermediate between strategies (1) and (2). Contrasting species from groups (1) and (2), namely C. acutus and O. similis, were selected for more detailed comparison. For C. acutus, maximum (probably food saturated) feeding and egg production rates are well below equivalent values for Calanus spp. at lower latitudes. Likewise, summer growth and moulting rates are slower, and the growth season of this herbivore is only 2–4 months. Therefore, both the low summer temperatures and short primary production season seem to dictate a long (∼1 year) life cycle for C. acutus. A collation of data on O. similis revealed that its abundance increases about tenfold from the Antarctic shelf northwards to the Polar Frontal Zone, where abundances are similar to those in temperate and tropical shelf seas. In contrast to C. acutus, O. similis appears to remain in the epipelagic and reproduce there year-round, although the food sources which sustain this are still uncertain.

During Austral spring 1992, R.R.S. James Clark Ross worked a five station transect in the Bellingshausen Sea. The transect spanned unproductive waters under solid pack ice to an open-water bloom in the north, three weeks later. This paper addresses the ontogenetic development of the copepod community, and from grazing experiments on females of five species investigates their trophic response to a spring bloom. Copepods dominated the mesozooplankton in both numbers and biomass. Their mean biomass in the top 600 m was low (0.85–1.5 mg drymass m−3), which is similar to other high latitude oceanic localities in the Southern Ocean. Almost all the major copepod species underwent an ontogenetic seasonal ascent, from mainly below 250 m under the ice to the top 250 m at the open-water bloom stations. Based on the timings of migration, feeding and reproduction, the species appeared to fall into two broad categories. Firstly, the pronounced seasonal migrants, Calanoides acutus and Rhincalanus gigas, ascended from below 250 m into the top 50 m to coincide with the bloom. The few individuals of C. acutus in the surface layers prior to the bloom do not appear to have been feeding. Reproduction of R. gigas was later than that of C. acutus. Production of the second group, namely Calanus propinquus, Oithona spp. and possibly Metridia gerlachei, appeared to have been less keyed to the bloom. Their seasonal migration was less, and individuals were actively feeding prior to the bloom, albeit at rates about half of those measured during the bloom. Mass specific feeding rates of the species in this group were greater than those of C. acutus and R. gigas. In the top 250 m, carbon:nitrogen ratios of C. propinquus and M. gerlachei were less than those of R. gigas and C. acutus, which suggests less reliance on depot lipids at this time of year. Despite the cold temperatures, the mass-specific feeding rates of the five species measured were broadly comparable to summer values from more northe

A net sampling survey was conducted around the island of South Georgia in the austral summer 1981/1982 and again in winter 1983. This paper uses Principal Components Analysis to elucidate the mesoscale features of zooplankton distribution within the two survey grids. During both surveys the major contrast was between the plankton of the shelf and the surrounding ocean. However, within the oceanic part of each grid, localized areas of warmer water were inhabited by a fauna characteristics of warmer latitudes to the north. The biological and oceanographic evidence suggests that areas represents core rings or similar intrusions from north of the Polar Front. Not all of the characteristic warm- and cold-water species showed a clear correlation with water origin. For example, the swarming behaviour of Euphausia superba limited its usefulness as a mesoscale tracer. Likewise, the distribution of the Antarctic copepod Calanoides acutus was poorly related to water temperature during the summer survey because the timing of sampling coincided with its annual spawning cycle. Large-scale movement of oceanic fronts has already been identified as being important in dictating overall species composition around the island, but at a smaller scale these intrusions of warmer water may be an important intermediary mechanism of community advection into the area.