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Using fatty acid biomarkers and stable isotopic signatures, we investigated the feeding strategies and dietary preferences of four dominant copepod species (Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas) sampled during the late austral summer in Prydz Bay, Antarctica. Our results show that diatoms, dinoflagellates and ciliates dominated copepod food sources (hypothesized to be phytoplankton and particulate organic matter) in the inner bay regions more than in the oceanic regions of Prydz Bay. Regional differences in the composition and abundance of food sources were also reflected in the fatty acid biomarkers and stable isotopic values. In the inner bay region, the total fatty acid contents of these food sources were nearly twofold higher, including greater contributions from fatty acids of dinoflagellate origin; these samples also had higher d13C and d15N values. Fatty acid biomarkers and stable isotopic values in copepod species roughly mirrored the spatial patterns in food sources. As found in the primary producers, the concentrations of dinoflagellate fatty acids and d13C and d15N values were higher in copepods from the inner bay regions. Additionally, there were inter-species differences in the fatty acids and stable isotopic values of copepods. C. acutus and C. propinquus did not exhibit significant regional differences in their total fatty acid contents. In contrast, M. gerlachei from the inner bay region had higher fatty acid values. C. acutus and C. propinquus had higher compositions of the long chain fatty acids 20:1n-9, 22:1n-9 and 22:1n-1, while docosahexaenoic acid (DHA) was higher in M. gerlachei. The d15N values indicate that C. acutus occupies a higher trophic level than the other copepod species. Similarly, higher fatty acid ratios in M. gerlachei, including DHA/EPA(eicosapntemacnioc acid) and 18:1n-9/18:1n-7, indicate that this species feeds more opportunistically and prefers a carnivorous diet. Insights from this combined fatty acid and stable isotopic approach suggest that the dominant copepod species in Prydz Bay, Antarctica, have flexible feeding strategies that vary by food source during the late austral summer.

Multicorer samples for meiofaunal study were obtained within the framework of the international project CeDAMar, at 21 stations occupied during the DIVA and ANDEEP campaigns (2000, 2002 and 2005) to the southern Atlantic Ocean (Guinea, Angola and Cape Basins) and the Southern Ocean (Weddell and Scotia Seas), respectively. A total of 311 adult Paramesochridae Lang, 1944 (Copepoda, Harpacticoida) was extracted from 437 cores obtained during 83 deployments of the corer at depths between 1107 and 5655 m. All specimens were determined to species based on morphological characteristics. They belonged to 19 species and four genera (Kliopsyllus Kunz, 1962, Leptopsyllus T. Scott, 1894, Paramesochra T. Scott, 1892 and Scottopsyllus Kunz, 1962). Eleven species were restricted to single regions,whereas the others showed a much wider distribution. For example, Kliopsyllus andeep Veit-Köhler, 2004 and Kliopsyllus diva Veit-Köhler, 2005, were both collected from Guinea, Angola and Weddell Abyssal Plains,and Kliopsyllus schminkei Veit-Köhler & Drewes, 2009 occurred in the three West-African basins. This study provides a first insight into the large-scale biogeography of deep-sea harpacticoids, represented by the Paramesochridae, and indicates that distribution ranges, at least in this family, may extend across South Atlanticand Southern Ocean Abyssal Plains.

The distribution and abundance of larval and postlarval fishes was investigated in the Lazarev Sea, Southern Ocean, in March and April 2004. The upper 200 m of the water column were sampled with an 8 m2 rectangular midwater trawl at 93 stations. The larval species community clustered in a diverse coastal community with high densities of Antarctic silverfish Pleuragramma antarcticum larvae and a less diverse offshore community dominated by Antarctic jonasfish Notolepis coatsi and the lanternfish Electrona antarctica. No postlarval fish were caught in coastal areas. The offshore community of postlarval fishes consisted of the deep-sea smelt Bathylagus antarcticus, and the lanternfishes Gymnoscopelus braueri, G. nicholsi and E. antarctica. The latter species clearly dominated, occurring at mean individual and wet mass densities of 0.17 individuals m–2 and 0.26 g m–2, respectively. A generalized additive model significantly related the density of postlarval E. antarctica to the abundance of Antarctic krill Euphausia superba, ocean depth and sea surface temperature. The diet of E. antarctica was dominated by copepods and euphausiid larvae. Mean energy density of E. antarctica in the upper 200 m was 2.8 kJ m–2, which is equivalent to 36% of the energy stored in Antarctic krill stocks and probably would be considerably higher if a greater depth range were considered. This suggests that E. antarctica is a major energy transmitter in the food web of the Lazarev Sea, challenging the classical krill-dominated food web paradigm of the Southern Ocean.

BackgroundThe Register of Antarctic Marine Species (RAMS, De Broyer et al. 2015) is the regional component of the World Register of Marine Species (WoRMS Editorial Board 2015) in the Southern Ocean. It has been operating for the last ten years, with a special effort devoted towards its completion after the International Polar Year (IPY) in 2007-2008, in the framework of the Census of Antarctic Marine Life (CAML, 2005 - 2010). Its objective is to offer free and open access to a complete register of all known species living in the Southern Ocean, building a workbench of the present taxonomic knowledge for that region. The Antarctic zone defined by this dynamic and community-based tool has been investigated with a particular interest. The Sub-Antarctic zone was a secondary objective during the establishment of the RAMS and is still lacking the impulse of the scientific community for some taxa.New informationIn the present study, more than 13,000 occurrences records of Asteroidea (Echinodermata) have been compiled within the RAMS area of interest and checked against the RAMS species list of sea stars, using WoRMS Taxon Match tool. Few mismatches (basionym mistakes :i.e. original name misspelled or incorrect) were found within the existing list and 97 unregistered species are actually occurring within the RAMS boundaries. After this update, the number of Asteroidea species was increased by around 50%, now reaching 295 accepted species., SCOPUS: ar.j, info:eu-repo/semantics/published

Because of the unique conditions that exist around the Antarctic continent, Southern Ocean (SO) ecosystems are very susceptible to the growing impact of global climate change and other anthropogenic influences. Consequently, there is an urgent need to understand how SO marine life will cope with expected future changes in the environment. Studies of Antarctic organisms have shown that individual species and higher taxa display different degrees of sensitivity to environmental shifts, making it difficult to predict overall community or ecosystem responses. This emphasizes the need for an improved understanding of the Antarctic benthic ecosystemresponse to global climate change using amultitaxon approach with consideration of different levels of biological organization.Here, we provide a synthesis of the ability of five important Antarctic benthic taxa (Foraminifera, Nematoda, Amphipoda, Isopoda, and Echinoidea) to cope with changes in the environment (temperature, pH, ice cover, ice scouring, food quantity, and quality) that are linked to climatic changes. Responses from individual to the taxon-specific community level to these drivers will vary with taxon but will include local species extinctions, invasions ofwarmer-water species, shifts in diversity, dominance, and trophic group composition, all with likely consequences for ecosystemfunctioning. Limitations in our current knowledge and understanding of climate change effects on the different levels are discussed. © 2012 The Authors., IF: new journal, SCOPUS: ar.j, info:eu-repo/semantics/published

Particulate Ba profiles were measured in the Indian sector of the southern ocean. The largest fraction (>80%) of this barium is present as barite microcrystals. The profiles of total barium are characterized by a subsurface maximum between 200 and 500 m depth in the vicinity of the oxygen minimum. Highest barium values are found just south of the Polar Front, while lowest values occur close to the Antarctic Divergence. Between the divergence and the Polar Front a tight inverse relationship is observed between oxygen in the oxygen minimum and barium in the barium maximum. This relationship disappears north of the Polar Front. Since suspended barite is known to be of biological origin, the correlation of barite with oxygen suggests that the observed decrease of oxygen in the oxygen minimum, between the divergence and the Polar Front is due to local consumption of oxygen. It is proposed that deep low oxygen water is advected towards the Divergence where upwelling occurs and where this water subsequently partly spreads out to the north, north-east, as entrained by the Antarctic Circumpolar Current. Oxidation of locally produced organic matter, with which barite crystals are associated, consumes oxygen and sets free individual discrete barites. As a result, oxygen decreases and barite increases away from the divergence, with barite integrating former biological processes.

[1] An empirical method to reconstruct distributions of mixing water masses is developed and applied to two Southern Ocean (Australian sector) data sets. The method is a parametric extension of the well-known Optimum Multiparameter (OMP) analysis, therefore called POMP (parametric OMP). The main development consists of parameterizing the mixing fractions as a function of position (e.g. latitude and depth) and estimating the parameters of the resulting function, instead of estimating the mixing fractions for every sampling point individually. Because this enables to reduce the total number of unknowns to be estimated, the proposed adjustments result in more robust estimates of the mixing fractions, as illustrated on the Southern Ocean CLIVAR-SR3 data set (early spring 2001). An additional consequence of the new working scheme is that the mixing distributions are smoothed. It is emphasized that the degree of smoothing should be chosen with care in order not to neglect any significant features. Statistical rules to do this are applied. Application of the POMP method to the second Southern Ocean data set (SAZ'98, summer 1998) revealed the importance of a careful selection of the mixing source water types. This study was limited to the subantarctic region and upper 1000 m, where variations in water mass characteristics and distributions are most important. Indeed, it seems necessary to redefine the source water characteristics for each data set. The direction of these changes suggests that interannual variability or long term changes of water masses overwhelm the seasonal variability.

Shallow marine benthic communities around Antarctica show high levels of endemism, gigantism, slow growth, longevity and late maturity, as well as adaptive radiations that have generated considerable biodiversity in some taxa1. The deeper parts of the Southern Ocean exhibit some unique environmental features, including a very deep continental shelf2 and a weakly stratified water column, and are the source for much of the deep water in the world ocean. These features suggest that deep-sea faunas around the Antarctic may be related both to adjacent shelf communities and to those in other oceans. Unlike shallow-water Antarctic benthic communities, however, little is known about life in this vast deep-sea region2,3. Here, we report new data from recent sampling expeditions in the deep Weddell Sea and adjacent areas (748-6,348m water depth) that reveal high levels of new biodiversity; for example, 674 isopods species, of which 585 were new to science. Bathymetric and biogeographic trends varied between taxa. In groups such as the isopods and polychaetes, slope assemblages included species that have invaded from the shelf. In other taxa, the shelf and slope assemblages were more distinct. Abyssal faunas tended to have stronger links to other oceans, particularly the Atlantic, but mainly in taxa with good dispersal capabilities, such as the Foraminifera. The isopods, ostracods and nematodes, which are poor dispersers, include many species currently known only from the Southern Ocean. Our findings challenge suggestions that deep-sea diversity is depressed in the Southern Ocean and provide a basis for exploring the evolutionary significance of the varied biogeographic patterns observed in this remote environment.

BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) is a longterm project aiming at applying and developing process-level studies, geochemical proxy tools and numerical tools for assessing and understanding the present-day functioning of the CO2 biological pump in the iron-limited Southern Ocean (S.O.) and for predicting its evolution in response to scenarios of increasing atmospheric CO2. Over the last four years BELCANTO implemented and developed several multi-proxy approaches for assessing nutrient consumption and carbon fluxes, including 234Thdeficit and natural silicon isotopic composition. High quality results were obtained for whole water column δ29Si-silicate and δ29Si-opal, including first ever results for the low silicate Subantarctic region. Results show that the δ29Si signature of diatoms appears to be homogeneous in the mixed layer and between diatom species. The preferential uptake of light isotope by diatoms is well reflected in the vertical distribution of silicate δ29Si, as well as in the upper ocean silicate δ29Si values which increase northwards in parallel with the decrease of silicate concentration. However, the relationship between δ29Si and silicate concentration appears complex and depends on variations in vertical and horizontal supply of silicate. Results based on other proxy tools (Baxs 234Th-deficit, f-ratio & new production) indicate relatively high particulate carbon export and absence of strong mesopelagic mineralization in the Subantarctic Zone, but relatively low export and enhanced mesopelagic mineralization further south in the Polar Front Zone and the southern ACC. Furthermore, remineralization was clearly enhanced during summer as compared to spring. Our observations of 234Th-based carbon export and new production along cross frontal transects also appear to challenge the widely accepted idea of enhanced primary and export production in the Subantarctic Zone and the Polar Frontal Zone, compared to more southern areas. Laboratory-controlled experiments on two widespread phytoplankton species (diatoms-Thalassiosira gravida and Phaeocystis Antarctica) showed an effect of Fe addition on the morphological form, enhancing the presence of the colonial form compared to the free-living cells for Phaeocystis and increasing the appearance of long chains of diatoms vs. free living cells. Under Fe enriched conditions, the maximum photosynthesis and growth rates increased. In addition the quality of the organic matter was modified, enhancing bacterial remineralization. Consequently, Fe addition may impact on the fate of the phytoplankton in the planktonic food web and the resulting efficiency of the biological pump. Accordingly, during the EIFEX large scale iron enrichment experiment, iron addition fosters a diatom bloom which appeared to broke up rather fast and to sink rapidly accordingly to 234Th measurements. This in turn triggered mesopelagic mineralization as evidenced in Baxs measurements. N-uptake experiments with Fe-limited and Fe-replete natural algal communities indicate that the effects of ammonium and iron on f-ratio and new production are not simply cumulative and that the enhancement of the f-ratio due to Fe addition depends on the ambient ammonium concentration. However the relationship between this enhancement and ammonium concentration is at present not fully understood. Our results imply that there is no simple relationship between export production and iron availability. Ammonium appears to counter the effects of iron addition on export production, particularly for HNLC areas such as the Southern Ocean. The meso-scale iron enrichment experiment performed during the EIFEX cruise, clearly showed that iron addition induced a diatom bloom but that the latter did not persist for long and collapsed soon after, thereby inducing a massive export of carbon as witnessed from significant deficits in 234Th activity. This export in turn triggered mesopelagic organic carbon mineralization as evidenced by excess Baxs contents. Measurements of pCO2, scaled using remote sensing data (sea surface temperature, chlorophyll and wind stress) and related CO2 fluxes, suggest that previous budgets of atmospheric CO2 uptake in the S.O. may be overestimated. However, our observations indicate that sea ice cover can act as an additional CO2 sink not taken into account in previous CO2 budgets. This sea-ice CO2 sink function results from physical and biogeochemical processes prevailing within the sea ice itself. Results obtained with SWAMCO-4, an idealized 1-D model of the marine planktonic system calculating C, N, P, Si, Fe cycling within the upper ocean, the export production and air-sea CO2 fluxes, suggest that the sink for atmospheric CO2 will increase in response to the raising atmospheric CO2 concentration. SWAMCO-4 simulations show that the amplitude of the predicted CO2 sink displays large regional and inter-annual variations which are related to local hydrodynamics and the dominant phytoplankton species. This is particularly acute in the Phaeocystis-dominated marginal ice zone of the Ross Sea. There the predicted annual CO2 sink appears positively related to the length of the sea ice cover period. This results from the accumulation of iron within the ice and its sudden release in the water column at the time of ice melting, favoring algal growth. The past and future evolution of the sea ice cover was conducted using the ORCA2- LIM and ECBILT-CLIO 3D ice-ocean models. The influence of the Southern Annular Mode on zonal integrated sea surface temperature and ice concentration seems to be small due to counteracting effects. Over the last 250 years, the annual mean ice coverage decreased in response to both natural and anthropogenic forcing, with the impact of the latter forcing clearly being enhanced over the last 150 years. Nowadays, the decrease of ice cover is more acute in the northern hemisphere as compared to the southern hemisphere, and this difference is due to thermal inertia of the S.O. However, model outputs predict a more abrupt decrease of S.O. sea ice extent in the future, resulting in similar decreases of annual mean sea ice extents in both hemispheres by the end of the century. In parallel, the seasonal amplitude of sea ice extent will increase. Preliminary runs with the coupled ice-ocean-biogeochemical model ORCA-LIMSWAMCO- 4, predict that the efficiency of the S.O. biological pump will be very sensitive to changes in the seasonal amplitude and the mean extent of the ice cover.

Our knowledge of the biodiversity of the Southern Ocean (SO) deep benthos is scarce. In this review, we describe the general biodiversity patterns of meio-, macro- and megafaunal taxa, based on historical and recent expeditions, and against the background of the geological events and phylogenetic relationships that have influenced the biodiversity and evolution of the investigated taxa. The relationship of the fauna to environmental parameters, such as water depth, sediment type, food availability and carbonate solubility, as well as species interrelationships, probably have shaped present-day biodiversity patterns as much as evolution. However, different taxa exhibit different large-scale biodiversity and biogeographic patterns. Moreover, there is rarely any clear relationship of biodiversity pattern with depth, latitude or environmental parameters, such as sediment composition or grain size. Similarities and differences between the SO biodiversity and biodiversity of global oceans are outlined. The high percentage (often more than 90%) of new species in almost all taxa, as well as the high degree of endemism of many groups, may reflect undersampling of the area, and it is likely to decrease as more information is gathered about SO deep-sea biodiversity by future expeditions. Indeed, among certain taxa such as the Foraminifera, close links at the species level are already apparent between deep Weddell Sea faunas and those from similar depths in the North Atlantic and Arctic. With regard to the vertical zonation from the shelf edge into deep water, biodiversity patterns among some taxa in the SO might differ from those in other deep-sea areas, due to the deep Antarctic shelf and the evolution of eurybathy in many species, as well as to deep-water production that can fuel the SO deep sea with freshly produced organic matter derived not only from phytoplankton, but also from ice algae.

Icefishes of the perciform suborder Notothenioidei dominate the Antarctic ichthyofauna. These species originated from a benthic ancestor and do not possess a swimbladder. However, some notothenioids have achieved neutral buoyancy through skeletal reductions as well as storage of lipids to reduce body mass relative to seawater. These adaptations enable them to exploit the highly productive pelagic realm. Mobilizing these lipid reserves in periods of critically low food intake may lead to buoyancy problems. Accumulating and conserving these reserves may slow down the development of somatic and reproductive tissues and hence future reproductive output. We constructed a dynamic state variable model to investigate how ingested energy is partitioned over 3 state variables: lipid reserves, structural protein body mass and egg development. Two forms of the model differed in that lipid reserves were either included in or excluded from the total metabolic energy budget of an individual. The model was parameterised for the Antarctic silverfish Pleuragramma antarcticum, a key species in the pelagic food web of the high Antarctic zone of the Southern Ocean. In Pleuragramma, lipids are stored in unique extracellular lipid sacs, which are thought to serve as buoyancy aids and energy reserves. The model predicts optimal habitat selection and an optimal energy allocation strategy by maximizing future reproductive output. The environment is simulated using vertical gradients in water temperature, optical properties, food availability and predation risk. The form of the model that considers lipids as metabolically inactive reserves best replicates field measurements of fat content and yields high values for fitness in Pleuragramma. Uncoupling fat reserves from metabolism, through the development of extracellular lipid sacs, probably represents a key adaptation in the evolution towards a pelagic lifestyle in a fish species with a low scope for activity.

Different deep-sea sites are frequently characterized by a high number of species belonging to the same genus. In addition, some deep-sea genera (e.g. Acantholaimus, Monhystera, Halalaimus) seem to be dominant and present world­wide. The genus Manganonema is present in all oceans in low to medium densities « 2%) and, apparently, restricted to the deep sea (600 m and deeper). The present study deals with a revision of the genus Manganonema based upon known species and nine new species, where only 5 were named due to the lack of males (M. media sp. nov., M. pitilica sp. nov., M. robus­tus sp. nov., M. antarctica sp. nov., M. bussauensis sp. nov.). For the species identification, important characters are sum­marized in a pictorial key. The material studied was collected from six different sites: two in the North Atlantic (Europe), two in the Southern Ocean (Antarctic), one in the South-western Atlantic (Brazil), and one in Western Indian Ocean (Kenya). The species distribution suggests a high turnover of species between sites. Only one Manganonema species is recorded from two different sites in the Atlantic Ocean. These results suggest a high degree of endemism of species within the genus.

With more than 820 different species, among which about 75% endemics, the amphipod crustaceans form one of the richest animal group of the Southern Ocean. They have colonized most habitats and exhibit very diverse life styles and trophic types. They moreover show a broad size spectrum, with numerous giant species. Despite their importance in terms of biodiversity, very few is known about the role of amphipods in Antarctic trophodynamics. Based on an exhaustive literature survey (more than 300 references), we tried to delineate their importance as potential food for higher trophic levels. About 200 different predators were recorded: 33 invertebrates (from 12 orders), 101 fishes (19 families), 48 birds (11 families) and 10 mammals. Using this vast dataset (total amount of citations close to 1500) and published values about predators' standing stocks and feeding rates, an attempt was made to build up a small model, distinguishing between benthic and pelagic species of both amphipods and predators. The total amount of consumedamphipods was estimated to 60 millions of tons per year for the whole Southern Ocean, i.e. the second animal group in importance after euphausiids.

We present water column profiles of excess particulate Ba (Baxs, an estimate of biogenic Ba from total particulate Ba after small corrections for lithogenic Ba) along a transect in the Australian sector of the Southern Ocean from the Subantarctic Zone (SAZ) to the Sea Ice Zone during spring (October-December 2001). Surface water Baxs contents appear related to phytoplankton derived particles. Below, in the twilight zone, mesopelagic Baxs records the changes in plankton biomass from the mixed layer over a time scale of a few weeks and confirms its usefulness as an indicator of the carbon remineralisation process. In comparison to the SAZ, the mesopelagic Baxs accumulation is larger and begins at shallower depths south of the Polar Front Zone (PFZ), in the Antarctic Zone (AZ), where diatoms are the dominant component of the phytoplankton community. Summer results from 1998, when mesopelagic Baxs accumulations were larger, also show this latitudinal trend. In contrast, as observed also for deep particulate organic carbon fluxes, the flux of Baxs to moored deep sea sediment traps was larger in the nano-phytoplankton dominated SAZ than the diatom dominated PFZ. Overall, the results suggest relatively high particulate carbon export to the deep sea in the absence of strong remineralisation in the SAZ, and relatively low export to the deep sea in the presence of strong remineralisation further south. Mesopelagic carbon remineralisation is higher in summer than in spring as also observed on deep sediment traps carbon fluxes. Our findings are supported by 234Th and N-uptake proxies from the same transect. This study expands the utility of Ba as an indicator of biogeochemical processes in the twilight zone and supports its usability as a paleoceanographic proxy for deep C export.

We analyzed d29Si of dissolved silicate for eight water column profiles across the Southern Ocean (south of Australia in spring 2001) from the Seasonal Ice Zone (SIZ) north to the Subantarctic Zone (SAZ), including the first isotopic compositions measured for Si-depleted seawaters. All profiles display mixed layer enrichments in heavy Si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms. As silicate levels decrease from the SIZ northward across the Polar Front Zone (PFZ) to the SAZ, surface and mesopelagic d29Si signatures generally become progressively heavier, but the most Si-depleted SAZ waters do not exhibit d29Si values heavier than in the PFZ. This intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters, and by applying a steady state open system model, we estimate a fractionation factor, 29E, between diatoms and seawater of −0.45 ± 0.17‰, independently of zones and phytoplankton community. Though encouraging, these results are related to latitudinal changes in mesopelagic d29Si values, complexity in surface silicate− d29Si correlations, and differences from previous studies, which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations.

234Th deficit and excess were examined in the upper 500 m of the Southern Ocean from Sub-Antarctic to Seasonal Ice Zones (Australian sector) during austral spring 2001. 234Th fluxes at 100 m indicate that particle export was low in the North (46.9-51.0°S), minimal in the Polar and Inter-Polar Frontal Zones and high in the South (≥61°S). These results are in tight agreement with new production estimates from the same cruise. Our data indicate that Polar and Inter-Polar Frontal Zones were not zones of intense export in the Australian sector at this time of year, in contrast with other sectors of the Southern Ocean. Also, we highlight the usefulness of 234Th excess below the mixed layer as a tool to study mesopelagic remineralization.

N uptake data obtained from 15N tracer techniques were surveyed in surface waters of the Subantarctic and Polar Frontal Zones southwest of Tasmania during summer 1998. The N flux rates and f ratio were computed using inverse modeling. The approach provides a general framework for the handling of random and systematic variations. Furthermore, factorial experiments based on controlled ammonium additions were used to study the sensitivity of the f ratio relative to perturbations of the regenerated nutrient supply. This was achieved in distinct regions: the Subtropical Convergence Zone, the Subantarctic Zone, the Subantarctic Front, and the Polar Frontal Zone. Overall, the stability of the f ratio appeared closely related to the availability of both dissolved iron and ammonium. The effect of one factor depended on the level of the other one, and the interaction influenced the f ratio patterns. The relationship between f ratio and ammonium supplementation ranged from linear to convex curves, with the steepest slopes in water masses rich in dissolved iron and with low ammonium content. The magnitude of this variation may have important ecological and geochemical consequences. Our results suggest that only versatile systems are able to exhibit events of high new production.

A la fin de l'ete austral 1994. les eaux antarctiques sont caracterisees par une faible biomasse phytoplanctonique. Le long de la radiale 62°E, entre 49°S et 67°S, les concentrations en chlorophylle a sont de l'ordre de 0.2 mg m -3 dans la couche 0-100 m. Cependant, au sud, dans la Zone Saisonniere des Glaces, les biomasses sont plus elevees avec un maximum chlorophyllien profond marque. La plus forte valeur (0.6 mg Chl. a m -3 ) a ete enregistree au niveau de la divergence antarctique (64°S) a une profondeur de 100 m correspondant au minimum thermique. Ce maximum chlorophyllien s'enfonce en profondeur vers le nord en diminuant d'intensite et disparait dans la Zone Oceanique libre de glace. Au voisinage du Front Polaire, la biomasse augmente legerement, avec des valeurs homogenes, voisines de 0.3 mg Chl. a m -3 , dans les 150 premiers metres. En bordure du Continent Antarctique, dans la Zone Saisonniere des Glaces ou la couche de melange est reduite et la biomasse elevee, la fraction phytoplanctonique > 10 μm est predominante. Dans cette zone, les fractions de taille > 10 μm, 2-10 μm et < 2 μm representent respectivement 46%, 25.1% et 28.9% de la biomasse totale integree entre 0 et 100 m. Les populations naturelles y sont essentiellement composees de grosses diatomees et de dinoflagelles qui representent respectivement 42.7% et 33.1% de la biomasse carbonee autotrophe. Vers le nord, parallelement a la diminution de la biomasse, l'importance des algues picoplanctoniques et des flagelles nanoplanctoniques autotrophes (Cryptophycees essentiellement) augmente fortement. Dans la Zone Oceanique, la fraction picoplanctonique represente 47.4% de la biomasse chlorophyllienne totale. Il y a une opposition entre la Zone Oceanique libre de glace caracterisee par une biomasse faible dominee par le picoplancton et la Zone Saisonniere des glaces ou la biomasse relativement plus elevee est dominee par les grosses diatomees. Cette dualite reflete la variabilite spatiale et temporelle qui prevaut dans l'ensemble de l'ecosysteme pelagique antarctique.

Phytoplankton nitrogen uptake is studied in relation to the biomass and structure of phytoplankton community in the Atlantic and Indian sectors of the Southern Ocean. Two scenarios of seasonal evolution of uptake regime and phytoplankton community structure are described. The first scenario includes the Marginal Ice Zone areas of the Weddell Sea and adjacent areas where a predominantly nitrate based, diatom dominated assemblage, thriving in a stable water column at the beginning of the season was transformed into a mainly ammonium based, flagellate dominated assemblage, towards the end of the season. The change in phytoplankton community structure was caused by selective grazing by large grazers and reduced stability of the water column and the shift in uptake regime was due to increased ammonium availability and changes in community structure. In the second scenario, in the Coastal and Continental Shelf Zone (CCSZ) and Open Oceanic Zone (OOZ) of the Indian sector, a shift in uptake regime occurred without a big change in phytoplankton community structure. These areas were sampled late in the growth season and were characterized by prolonged water column stability, less grazing pressure on large diatoms and high ammonium availability. Diatoms dominated the assemblage and about 80% phytoplankton biomass was on the >10 µm size fraction. Unlike the first scenario, diatoms were largely based on ammonium. Thus, in areas of persistent water column stability and less selective grazing pressure, a shift in uptake regime can occur without change in community structure. The dominance of diatoms under regenerated production provides a physiological evidence for the excess net removal of silicate over nitrate occurring in certain provinces of the Southern Ocean.

The separation in Southern Ocean provinces of silicate excess at nitrate exhaustion and of nitrate excess at silicate exhaustion was already introduced by Kamykowski and Zentara (Kamykowski, D., Zentara, S.J., 1985. Nitrate and silicic acid in the world ocean: patterns and processes. Mar. Ecol. Prog. Ser. 26, 47–59; and Kamykowski, D., Zentara, S.J., 1989. Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes. Mar. Ecol. Prog. Ser. 58, 101–111) and our investigations of the silicate to nitrate uptake ratios confirm the earlier distinction. Oligotrophic antarctic waters mainly exhibit proportionally higher silicate removal what induces a potential for nitrate excess. The nitrogen uptake regime of such areas is characterised by low absolute as well as specific nitrate uptake rates throughout. Maximal values did not exceed 0.15 µM d-1 and 0.005 h-1, respectively. Corresponding f-ratios ranged from 0.39 to 0.86. This scenario contrasts strikingly to the more fertile ice edge areas. They showed a drastic but short vernal increase in nitrate uptake. Absolute uptake rates reached a maximum value of 2.18 µM d-1 whereas the maximal specific uptake rate was 0.063 h-1. In addition to an optimal physical environment for bloom development, accumulation of ammonium stimulated nitrate uptake in a direct or indirect way. Since ammonium build-up in surface waters traces enhanced remineralisation, release of other essential compounds during degradation of organic matter might have been the main trigger. This peak nitrate utilisation during early spring led to the observed potential for silicate excess. With increasing seasonal maturity the nitrate uptake became inhibited by the presence of enhanced ammonium availability (up to 8% of the inorganic nitrogen pool), however, and after a short period of intensive nitrate consumption the uptake rates drop to very low levels, which are comparable to the ones observed in the area of nitrate excess at silicate exhaustion.

A checklist, with synonymical bibliography, of all benthic, supralittoral and pelagic Amphipoda (Gammaridea, Caprellidea and Hyperiidea) occurring in the Southern Ocean is drawn up, mostly from taxonomical literature checked until 31 December 1992. 883 taxa have been recorded: 711 spp. and subspp. of Gammaridea, 28 spp. of Caprellidea, 69 spp. and subspp. of Hyperiidea as well as 75 unidentified spp. (73 Gammaridea, 2 Caprellidea). Distribution in the East or West Antarctic sub-regions, in the Subantarctic Islands sub-region, in the Magellanic sub-region and in the Tristan da Cunha district is mentioned. Bathyal and abyssal benthic occurrence is indicated as well as the general bathymetrical distribution of the pelagic species occurring south of 45°S. The Barnard & Barnard (1983) coded geographic system for reporting distribution of taxa is revised for the Southern Ocean and a new list of geographic codes of general application for Antarctic and Subantarctic benthos is provided. The benthic Amphipod fauna of the Southern Ocean comprises 702 species (85 % endemic) of which 451 are distributed in the Antarctic region (78.4% endemic) and 342 in the Subantarctic region (50.8 % endemic). Endemicity at the genus level attains 36.7 % for the whole Southern Ocean, 26.2% for the Antarctic and 13.5% for the Subantarctic region respectively.

The location of the main Southern Ocean frontal systems, identified from surface water data for salinity and temperature during INDIGO 3, determines the surface water values for nitrate, silica and barium. For nitrate the strongest gradient coincides with the Subtropical Convergence (STC; 42°S). with zero concentration north of this front and concentrations from 17 to 27 µmol/l south of it. North of the STC silica never is completely exhausted, while dissolved barium concentrations decrease to 40 nmol/l, the lowest values observed. Along the watercolumn within the Circumpolar Current the following watermasses are identified (from surface to bottom) : Antarctic Surface Water, Winter Water, Circumpolar Deep Water, Weddell Sea Deep Water and Antarctic Bottom Water. Here, typical profiles for dissolved barium show surface values of 77 nmol/l and bottom values of 100 nmol/l. Along Antarctica some stations show a decrease in dissolved barium and silica in bottom waters. This indicates local bottom water formation, confirmed by freon-11 data for two stations in Wild Canyon (west of Amery Basin). The maxima of particulate barium in the euphotic layer (-35m) and the oxygen minimum layer (-200m) suggest that both, active (the living phytoplankton cell) and passive (the desintegration of organic detritus microenvironments) processes control the production of barite.

The distribution of chlorophyll-a obtained by HPLC along vertical and horizontal profiles during Indigo I11 cruise in the Indian sector of the Southern Ocean was used to correlate the phytoplankton and watermasses patterns. They confirm that the Southern Ocean is not ecologically uniform as is implied by the general term "Antarctic and Subantarctic ecosystem" (Hempel,1985). Highest chlorophyll-a standing crops are associated with the main frontal systems (the Subtropical Convergence, the Antarctic Polar Front and the Antarctic Divergence), In these particular areas, significant amounts of living phytoplankton are found as far as 70 meters deep, suggesting a downwards transport of chlorophyll-a along isopycnals, in relation with the frontal hydrodynamics. In interfrontal areas, chlorophyll-a concentrations are generally very low and restricted to the upper layers of the water column. Therefore, the vertical distribution of phytoplankton in the Southern Ocean is not as uniform as expected, referring to El Sayed (1978) or to Jacques et Minas (1981) . These preliminar results show that a more intensive and detailed study of the Antarctic frontal systems might lead to a better understanding of the phytoplankton distribution and productivity.

An upgraded version of the biogeochemical model SWAMCO is coupled to the ocean-sea-ice model NEMO-LIM to explore processes governing the spatial distribution of the iron supply to phytoplankton in the Southern Ocean. The 3-D NEMO-LIM-SWAMCO model is implemented in the ocean domain south of latitude 30° S and runs are performed over September 1989–December 2000. Model scenarios include potential iron sources (atmospheric deposition, iceberg calving and continental sediments) as well as iron storage within sea ice, all formulated based on a literature review. When all these processes are included, the simulated iron profiles and phytoplankton bloom distributions show satisfactory agreement with observations. Analysis of simulations points to the key role played by continental sediments as a primary source for iron. Iceberg calving and melting contribute by up to 25% of Chl a simulated in areas under influence of icebergs while atmospheric deposition has little effect at high latitudes. Activating sea ice-ocean iron exchanges redistribute iron geographically. Stored in the ice during winter formation, iron is then transported due to ice motion and is released and made available to phytoplankton during summer melt, in the vicinity of the marginal ice zones. Transient iron storage and transport associated with sea ice dynamics stimulate summer phytoplankton blooming (up to 3 mg Chl a m−3) in the Weddell Sea and off East Antarctica but not in the Ross, Bellingshausen and Amundsen Seas. This contrasted feature results from the simulated variable content of iron in sea ice and release of melting ice showing higher ice-ocean iron fluxes in the continental shelves of the Weddell and Ross Seas than in the Eastern Weddell Sea and the Bellingshausen-Amundsen Seas. This study confirms that iron sources and transport in the Southern Ocean likely provide important mechanisms in the geographical development of phytoplankton blooms and associated ecosystems., info:eu-repo/semantics/published