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     

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     

Phytoplankton and particulate matter at the Weddell / Scotia Confluence (47°W) in summer 1989, as a final step of a temporal succession (EPOS project)

During January 1989, phytoplankton biomass and species composition were studied in a north / south transect at the Weddell / Scotia Confluence (47°W), between 57° and 61°30′S. Results showed a diatom bloom in the Scotia Sea (chlorophyll a 1.9 μg l⁻¹, particulate organic carbon 239 μg l⁻¹), dominated by Fragilariopsis cylindrus, Dactyliosolen antarcticus and Chaetoceros dichaeta. Low chlorophyll a / phaeopigments ratios (about 1.4) and silicate concentrations (15 μmol l⁻¹) suggested that this was an advanced bloom phase, probably linked to high grazing pressure. Minimum chlorophyll a values of 0.1–0.2 μg l⁻¹ and particulate organic carbon 46 μg l⁻¹ were found at the Weddell / Scotia Front and in a subsurface layer of the Weddell Sea Water. In the southern part of the transect (61°30′S), in the Weddell Sea, a second surface maximum was found (chlorophyll a 0.9 μg l⁻¹, particulate organic carbon 120 μg l⁻¹), but with a different species composition, with Cryptomonas sp. dominant. Our results show a succession within the diatom community in the Weddell / Scotia Confluence Waters when comparing the three EPOS legs. In the Weddell Sea from spring to summer, nanoflagellates, with only a minor contribution from diatoms, persist over a long period with little change in the community structure. We suggest that the frontal system, together with the receding ice edge and the grazing pressure of either krill or protozooplankton, are mainly responsible for the phytoplankton distribution patterns found. Received: 3 July 1996 / Accepted: 3 November 1996     

Bottom-up and top-down controls of the microbial food web in the Southern Ocean: experiments with manipulated microcosms

Summary We have studied bottom-up and top-down control of the Southern Ocean microbial food web by microcosm experiments. Water from the Weddell Sea and Weddell Scotia Confluence were used for the experiments. Microcosms were manipulated by nutrients and light, and by size-selective screening. Incubation at the higher light level doubled phytoplankton growth rates from 0.12 to 0.24 day^–1 in the Weddell experiment and from 0.15 to 0.30 day^–1 in the Confluence experiment. Nutrient enrichment had no significant effect on growth rates in either of the experiments, indicating that phytoplankton growth was not nutrient-limited. In the microcosms where dinoflagellate growth rate was different, high dinoflagellate numbers were reflected as depressed nanoflagellate growth as well as depressed growth of phytoplankton, suggesting that dinoflagellates controlled both heterotrophic nanoflagellates and autotrophic nanoplankton. Only during short periods, when dinoflagellate numbers were low, could exponential growth of nanoflagellates be demonstrated. Bacterioplankton growth rates were, on average, 0.26 day^–1 in the Weddell experiment and 0.22 day^–1 in the Confluence experiment. Bacteria were controlled by heterotrophic nanoflagellates. Potential growth rates up to 0.75 day^–1 were measured from batch cultures without predators. With the microcosm experiments, we could demonstrate a strong top-down control by dinoflagellates on phytoplankton and on heterotrophic nanoflagellates, and a control by heterotrophic nanoflagellates on bacteria. We could also demonstrate weak nutrient limitation on autotrophs and substrate limitation on heterotrophic bacteria. In the two study areas, biomass production and carbon flow were mediated mainly by organisms that passed through a 20 m net and had growth rates in the order of 0.20 to 0.30 day^–1.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Micronekton and macrozooplankton in the open waters near Antarctic ice edge zones (AMERIEZ 1983 and 1986)

Summary Micronekton and macrozooplankton assemblages (0–1000 m) were sampled from the open ocean in the vicinity of marginal ice zones in the southern Scotia and western Weddell Seas using midwater trawls. Small regional differences in species composition were found in the differing hydrographic settings with the Scotia Sea being slightly more diverse. Most species exhibited broad vertical ranges with no distinct pattern of vertical movement. Exceptions were mesopelagic fish and Salpa thompsoni which undertook diel vertical migrations. Biomass was high (2.4–3.1 g DW/m²), comparable to Pacific subarctic waters. Euphausia superba and Salpa tompsoni were the numerical and biomass dominants, representing over 50% of the total numbers and standing stocks. In terms of biomass, euphausiids were the most important group at shallow depths (0–200 m) but were surpassed by salps in the Scotia Sea and mesopelagic fish in the Weddell Sea when all depths down to 1000 m were considered. Pelagic fish biomass (3.3–4.4 g WW/m²) greatly exceeded published estimates for birds (0.025–0.070 g WW/m²), seals (0.068–0.089 g WW/m²) and whales (0.167 to 0.399 g WW/m²), making mesopelagic fish the most prevalent krill predators in the Antarctic oceanic system.     

Phytoplankton distribution in relation to sea ice,hydrography and nutrients in the northwestern Weddell Sea in early spring 1988 during EPOS

Summary Phytoplankton biomass and distribution of major phytoplankton groups were investigated in relation to sea ice conditions, hydrography and nutrients along three north-south transects in the north western Weddell Sea in early spring 1988 during the EPOS Study (European Polarstern Study), Leg 1. Three different zones along the transects could be distinguished: 1) the Open Water Zone (OWZ) from 58° to 60°S with high chlorophyll a concentrations up to 3.5 g l^–1; 2) the Marginal Ice Zone (MIZ) from 60° to about 62.5° with chlorophyll a concentrations between 0.1 and 0.3 g l^–1, and 3) the closed pack-ice zone (CPI) from 62.5° to 63.2°S with chlorophyll a concentrations below 0.1 gl^–1. Nutrient concentrations increased towards the south showing winter values under the closed pack-ice. Centric diatoms such as Thalassiosira gravida and Chaetoceros neglectum forming large colonies dominated the phytoplankton assemblage in terms of biomass in open water together with large, long chain forming, pennate diatoms, whereas small pennate diatoms such as Nitzschia spp., and nanoflagellates prevailed in ice covered areas. Fairly low concentrations of phytoplankton cells were encountered at the southernmost stations and many empty diatom frustules were found in the samples. The enhanced phytoplankton biomass in the Weddell-Scotia-Confluence area is achieved through sea ice melting in the frontal zone of two different water masses, the Weddell and the Scotia Sea surface waters.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

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     

Phytoplankton composition in the Weddell-Scotia Confluence area during austral spring in relation to hydrography

During the EPOS leg 2 cruise of the RV Polarstern, carried out in late austral spring of 1988–1989, the composition of phytoplankton in relation to the distribution of hydrographic parameters was studied in four successive transects carried out along 49°W and 47°W, across the Weddell-Scotia Confluence (WSC) and the marginal ice zone (which overlapped in part). In all transects, a maximum of phytoplankton biomass was found in the WSC, in surface waters stabilized by ice melting. Different phytoplankton assemblages could be distinguished. North of the Scotia Front (the northern limit of the WSC) diatoms with Chaetoceros neglectus, Nitzschia spp. and (Thalassiosira gravida) dominated the phytoplankton community. This assemblage appeared to have seeded a biomass maximum which occupied, during the first transect, an area of the WSC, south of the Scotia Front. The southernmost stations of the first transect and all the stations to the south of the Scotia Front in the other transects were populated by a flagellate assemblage (with a cryptomonad, Pyramimonas spp. and Phaeocystis sp.) and an assemblage of diatoms (Corethron criophilum and Tropidoneis vanheurkii among others) associated to the presence of ice. During the last three transects, the flagellate assemblage formed a bloom in the low salinity surface layers of the WSC zone. The bulk of the biomass maximum was formed by the cryptomonad which reached concentrations up to 4×10⁶ cells l^–1 towards the end of the cruise. Multivariate analysis is used to summarize phytoplankton composition variation. The relationships between the distribution of the different assemblages and the hydrographic conditions indicate that the change of dominance from diatoms to flagellates in the WSC zone was related to the presence of water masses from different origin.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Factors controlling phytoplankton ice-edge blooms in the marginal ice-zone of the northwestern Weddell Sea during sea ice retreat 1988: Field observations and mathematical modelling

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea were investigated during the EPOS (Leg 2) expedition (1988). Measurements were made of physical and chemical processes and biological activities associated with the process of ice-melting and their controlling variables particularly light limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and process studies show that the initiation of the phytoplankton bloom, dominated by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 mg Chla m^–3) with a relatively narrow geographical extent (100–150 km). The rôle of trace constituents, in particular iron, was minor. The importance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scenarios of meteorological and biological forcing, making use of a one-dimensional coupled physicalbiological model. The analysis of simulations clearly shows that wind mixing events — their duration, strength and frequency — determines both the distance from the iceedge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combined action of meteorological conditions and grazing pressure. In the absence of grazers, a maximum ice-edge bloom of 7.5 mg Chla m^–3 is predicted under averaged wind conditions of 8 m s^–1. Extreme constant wind scenarios (4–14 m s^–1) combined with realistic grazing pressure predict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chla m^–3. Persistent violent wind conditions ( 14 m s^–1) are shown to prevent blooms from developing even during the brightest period of the year.     

Phytoplankton dynamics in relation to hydrography, nutrients and zooplankton at the onset of sea ice formation in the eastern Weddell Sea (Antarctica)

The quantitative and qualitative distribution of phytoplankton was investigated along five North–South transects in the eastern Weddell Sea during the transition from late autumn to winter. Relationships with the regional hydrography, progressing sea ice coverage, nutrient distribution and zooplankton are discussed and compared with data from other seasons. To the north of the Antarctic Slope Front (ASF) a remnant temperature minimum layer was found above the primary pycnocline throughout summer. Surface waters had not entirely acquired typical winter characteristics. While temperature was already in the winter range, this was not the case for salinity. Highest biomass of phytoplankton, with the exception of the first transect, was found in the region adjoining the ASF to the north. Absolute chlorophyll a (Chl a) concentrations dropped from 0.35 to 0.19 g l^–1 . Nutrient pools exhibited a replenishing tendency. Ammonium concentrations were high (0.75–2 mol l^–1), indicating extensive heterotrophic activity. The phytoplankton in the ASF region was dominated by nanoflagellates, particularly Phaeocystis spp.. North of the ASF the abundance of diatoms increased, with Fragilariopsis spp., F. cylindrus and Thalassiosira spp. dominating. Community structure varied both due to hydrographical conditions and the advancing ice edge. The phytoplankton assemblage formed during late autumn were very similar to the ones found in early spring. A POC/PON ratio close to Redfield, decreasing POC concentration and a high phaeophytin/Chl a ratio, as well as a high abundance of mesozooplankton indicated that a strong grazing pressure was exerted on the phytoplankton community. A comparison between primary production (PP) in the water column and the sea ice showed a shift of the major portion of PP into the ice during the period of investigation.     

Build-up and decline of summer phytoplankton biomass in the eastern Weddell Sea,Antarctica

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the first half of the study (15 Jan–13 Feb) in an area off Vestkapp, favourable irradiance/mixing regimes initiated net phytoplankton growth in ice-free waters on the shelf and in stretches of open water over the partially ice-covered deep ocean. Chi a concentrations in the upper water column were moderate (0.2–0.8 g l^–1), but significantly above winter values. Later in the season (16 Feb–11 March), a phytoplankton bloom with surface Chl a concentrations ranging from 1.6–2.3 g l^–1 was encountered in an area further to the east. We suggest that the upper water column must have been stratified in this region for time scales of weeks to faciliate bloom development. Bacterial biomass and productivity generally paralleled the seasonal development of the phytoplankton. Nitrate concentrations in the upper mixed layer were substantially lower than would be expected from the existing phytoplankton standing stock, suggesting that heterotrophic consumption of organic matter by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by the passage of a storm, resulting in a homogeneous distribution of phytoplankton biomass over the entire water column, followed by sedimentation and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high, amounting to more than half of the primary production (range: 10%–120%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values <1 g Chl a l^–1. The combination of low incident irradiances and incessant deep mixing prevented the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A major fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankton in the eastern Weddell Sea.     

Meiofauna communities along a depth transect off Halley Bay (Weddell Sea-Antarctica)

Summary Meiofauna communities from 10 stations along a depth transect from approximately 500 to 2,000 m off the Halley Bay Station (Weddell Sea) are investigated. Representatives of about 30 smallsized taxa of higher category are found, most of them belonging to the meiofauna. Loricifera are recorded for the first time for the Southern Ocean. At one of the stations a maximum of 22 taxa occur, the mean number of taxa ranges from 7 to 16. Nematoda, Harpacticoida, Ostracoda, Polychaeta and Bivalvia are present at all sampling sites. Nematodes are always dominant representing more than 90% of the individuals per sample, followed by harpacticoids (3%) and kinorhynchs (1.2%). Important fractions of the meiofauna (an average of more than 50%) occur in strata below the top 0–1 cm layer. Maximal density is 3,800 individuals (10 cm^–2), the mean abundance per station ranges from 790 to 3,720 individuals (10 cm^–2) and the overall mean is 1,700 individuals (10 cm^–2). Multivariate analysis (TWINSPAN, Cluster analysis, DCA) discriminates between three communities which are correlated with depth and sediment characteristics: the near shelf-ice, the slope and the deep-sea communities.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Biological activity in the antarctic zooplankton community

Summary Zooplankton biomass and respiratory ETS activity were studied along a transect at 49°W, running from open Scotia Sea water (57°S) into the Weddell Sea pack ice (62°S) in November–December 1988. Zooplankton biomass and respiratory activity were relatively high in the ice edge region and in the frontal zone separating the Scotia Sea from waters in the south; low biomasses and activities were encountered under the pack ice. The higher activities in the frontal zone were mainly attributed to locally higher water temperatures, while in the ice edge region they were probably related to changes of the Zooplankton population into more active developmental stages. This development in life stages is possibly a response to ice edge generated phytoplankton blooms. A comparison of the most abundant Zooplankton taxa indicated that amphipods and euphausiids had relatively the highest weight specific ETS activities. The developmental stages of Euphausia superba, from juvenile to sexually mature adults, showed a large variability in ETS activity; only the most inactive stages were found under the pack ice. The difference in respiration between the most active and inactive stages was at least a factor of 6 and is of importance to the overwintering of the species in the under ice habitat.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Spatial distribution of bacterioplankton production across the Weddell-Scotia Confluence during early austral summer 1988–1989

Summary We applied two methods to measure bacterio-plankton production, the [³H]-thymidine (TTI) and the [³H]-leucine (LEU) incorporation into cold trichloro-acetic acid precipitate. Both methods gave similar results of the distribution of production in time and space (r ²=0.82, n=66). Using empirically determined conversion factors the TTI gave production values from 21 to 125 mg Cm^–2 day^–1, which are within the range reported earlier from the Southern Ocean. Highest production rates were associated with the open water in the Confluence area (59°S–60°S) and with the Scotia Sea front. Low production rates were recorded from the ice covered areas in the Weddell Sea and in the open Scotia Sea waters. Good correlation on an areal basis was found between bacterioplankton production and other measures of heterotrophy, including ETS (r²=0.93, n=9) and NH₄(r²=0.50, n=21). Good correlation was also found between bacterioplankton and phytoplankton production (r²=0.63, n=19). Bacterioplankton production seems to be driven by products from photosynthesis and heterotrophic processes, most likely grazing, which are tightly coupled to autotrophy. Quantitatively, bacterioplankton production was on an average 11 % of net primary production, which is clearly a lower value than the 30% based on a review from temperate freshwater and marine ecosystems, but is comparable with values reported from the spring period in subarctic ecosystems. In comparison with the measurements of ETS, bacterioplankton contribution to community respiration was also lower than predicted from results from temperate ecosystems. We concluded from these results and the results obtained from microcosm experiments (Bjørnsen and Kuparinen 1991b) that the flux of organic matter to eucaryote heterotrophs via bacterioplankton during spring and early summer periods in the Southern Ocean is of considerable, but not of equivalent importance as in temperate waters.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Abundance,horizontal and vertical distribution of fish in eastern Weddell Sea micronekton

Summary The spatial distribution and species composition of high-Antarctic ichthyonekton was investigated during the EPOS 3 cruise by RV Polarstern in the eastern Weddell Sea during January–February 1989. A multiple rectangular midwater trawl was used to collect samples from the surface to near the sea floor at 11 stations along a 245 kra transect off Halley Bay. Early larval stages of 18 species, representing about 24% of the known Weddell Sea ichthyofauna, were present in the water column. The Antarctic silver-fish, Pleuragramma antarcticum, over-whelmingly dominated the catches comprising 84.5% of the 5022 specimens caught. The abundance of this species markedly increased towards the offshore end of the transect with the highest numbers occurring near the shelf-break front associated with the westerly current of the southern limb of the Weddell Gyre. The increased abundance of P. antarcticum in continental slope waters was attributed to deflection of the East Weddell Coastal Current beyond the shelf/slope break by fringing ice shelves. Most larval and juvenile fish were found in the seasonally warmed upper 0–70 m layer of the Antarctic Surface Water where conditions occurred that appeared to be favourable to both feeding and growth. Cluster analysis indicated that inner-, central-and outer-shelf assemblages were represented and that the species composition was most effectively described by reference to water mass and depth.     

Whole season net community production in the Weddell Sea

Depletions of total CO₂, nitrate, phosphate and silicate in the surface layer were calculated for cruise ANT XXII/3 with FS Polarstern in March 2005 for estimating the annual net community production. East-west across the Weddell Sea the variation of all depletions is similar, but this holds to a lesser extent for silicate. Depletions in March 2005 are 2–3 times larger than those in January 1993 for the same transect. Very low N:P and C:P depletion ratios seem to point to dominance of diatoms, in the central Weddell Sea more than in the margin. Estimates of annual net community productions are about 1.8 and 3.5 mol C m⁻² year⁻¹ for the interior Weddell Sea and a near-margin region, respectively. The region does not comply with the classical view of a marginal ice zone with high productivity. Net community production is similar to annual export production, implying that remineralization in the ensuing winter be minor.     

Distribution and activity of bacteria in the headwaters of the Rhode River Estuary,Maryland, USA

A transect along the axis of the headwaters of a tidal estuary was sampled for microbial, nutrient, and physical parameters. Chlorophylla averaged 42g 1^–1 and phytoplankton comprised an estimated 80% of the total microbial biomass as determined by adenosine triphosphate (ATP). Bacterial concentrations ranged from 0.3–53.9×10⁶ cells ml^–1 and comprised about 4% of the total living microbial biomass. Bacterial production, determined by³H-methyl-thymidine incorporation was about 0.05–2.09× 10⁹ cells 1^–1 h^–1, with specific growth rates of 0.26–1.69 d^–1. Most bacterial production was retained on 0.2m pore size filters, but passed through 1.0m filters. Significant positive correlations were found between all biomass measures and most nutrient measures with the exception of dissolved inorganic nitrogen nutrients where correlations were negative. Seasonal variability was evident in all parameters and variability among the stations was evident in most. The results suggest that bacterial production requires a significant carbon input, likely derived from autotrophic production, and that microbial trophic interactions are important.     

Dynamics of nutrients and phytoplankton,and fluxes of carbon,nitrogen and silicon in the Antarctic Ocean

Summary Four major functional units have been identified in the Southern Ocean and the mechanisms that control the dynamics of nutrients and phytoplankton are detailed for the different sub-systems. The very productive Coastal and Continental Shelf Zone (CCSZ, 0.9 M km ²) can experience severe macronutrient depletion paralleling intense diatom-dominated phytoplankton blooming (maximum > 8 mg Chl a m^–3) at the ice edge. In the Seasonal Ice Zone (SIZ, 16 M km ²), dramatic variations in the hydrological structure occur in surface waters during the spring to summer retreat of the pack-ice, changing from a well-mixed system to a stratified one within the reaches of the ice edge. Grazing activity of euphausiids limits phytoplankton biomass to a moderate level (Chl a maximum around 4 mg m^–3). A shift from new production to a regenerated production regime has been demonstrated during spring, along with the key role played by protozoans in controlling high ammonium concentrations (maximum > 2 M) in the surface layers. The well-mixed Permanently Open Ocean Zone (POOZ, 14 M km ²) is characterised by variable N/Si ratios in surface waters along a north-south transect: at the northern border of the POOZ (N/Si = 0.25) silicate concentrations as low as < 10 M could help limit the phytoplankton growth. Although favourable conditions have been demonstrated for the initiation of blooms in spring in the Antarctic Circumpolar Current, it appears that critical-depth/ mixing-depth relationships control maximum chlorophyll a concentrations < 1 g l^–1 during summer. The POOZ is usually not influenced directly by euphausiids, except for the Scotia Sea and Drake Passage where migrations of krill from the adjacent SIZ are usual. Mesoscale eddies are typical of the Polar Front Zone (FPZ, 3 M km ²): significant increases in phytoplankton biomass have been reported in this frontal area (maximum Chl a = 2 mg m^–3). Food web and biogeochemical cycles in this sub-system are poorly documented. The question of limitation of the primary production by eolian-transported trace-metals in these different sub-systems is still a matter of debate, although clear iron limitation has been evidenced for offshore waters of the Ross Sea.Data presented here were partly collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Photosynthesis-irradiance relationships and carbon metabolism of different ice algal assemblages collected from Weddell Sea pack ice during austral spring (EPOS 1)

Summary Photosynthesis-irradiance relationships and the carbon metabolism of different ice algal assemblages collected from Weddell Sea pack ice were investigated during the EPOS 1 cruise. Infiltration- and interstitial assemblages exhibited the photosynthetic characteristics of high-light adapted ice algae with a mean assimilation number of 1.81±0.93 mg C (mg Chl a)^–1 h^–1. A higher light harvesting efficiency under light limited conditions (alpha^B-value), as well as a lower light intensity for light saturation (I_K-value) was determined for the interstitial assemblage. An increase in light intensity from 3.5 to 106 mol m^–2s^–1 resulted in increased synthesis of polymeric carbohydrates (presumably reserve material) in a band assemblage. However, the absolute incorporation of radiolabel into lipid- and amino acid fractions remained essentially constant over this range of photon flux densities. Light-saturated rates of photosynthesis of three infiltration assemblages under hypersaline conditions (approx. 50 and 110%) decreased by 13–55% (controls: approx. 32–34%). The adverse effect of salinity treatment was much less pronounced under hyposaline conditions (approx. 20), where maximal photosynthetic rates were only slightly decreased (-9%) or even stimulated (14–22%). These observations suggest that sea ice microalgae in the ice edge region of the Weddell Sea during spring, being in a metabolically active stage, may have the potential to initiate or contribute to phytoplankton blooms upon release into the water column.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Phyto- and protozooplankton biomass during austral summer in surface waters of the Weddell Sea and vicinity

Summary Phyto- and protozooplankton were sampled in the upper 10 m of the water column in austral summer during a cruise of RV Polarstern from January 6 to February 20 1985 in the eastern Bransfield Strait vicinity and in the northern, southeastern (off Vestkapp, twice: I and II) and southern Weddell Sea (Vahsel Bay across the Filchner Depression to Gould Bay). The plankton assemblages are discussed in relation to physical, chemical and biological factors in the different geographical areas in summer. Phytoplankton biomass (Phytoplankton carbon, PPC) ranged from 4–194 g carbon/l and consisted on average of 65% diatoms and 35% autotrophic flagellates. Whereas in the northwest phytoplankton assemblages were dominated by small nanoflagellates (78% of PPC), higher biomass of diatoms (54–94% of PPC) occurred at the other sampling sites. In general autotrophic flagellates and small pennate diatoms dominated at oceanic stations; in neritic areas large centric diatoms prevailed. Chlorophyll a concentrations ranged from 0.25–3.14/g chl a/l with a mean of 1.13/gmg chlorophyll a/l and an average phytoplankton carbon/chlorophyll a ratio of 39. Protozooplankton biomass (Protozooplankton carbon, PZC) ranged from 0–67 g carbon/l and consisted of 49% ciliates, 49% heterotrophic dinoflagellates and 2% tintinnids. Heterotrophic dinoflagellates were more important in the northern investigation areas (58%–84% of PZC). Ciliates dominated the protozooplankton in the southeast and south (56%–65% of PZC); higher abundances of tintinnids were observed only in the south (11% of PZC). The most remarkable feature of the surface waters was the high protozooplankton biomass: protozooplankton amounted to 25% on an average of the combined biomass of PPC plus PZC for the entire investigation period. Protozoan biomass in the southeastern and southern Weddell Sea occasionally exceeded phytoplankton biomass. Temperature, salinity, and inorganic nutrients were generally lower in the southern regions; at most of these stations a meltwater layer occurred in the upper meters of the water column. We suggest that this physical regime allows a well developed summer system with a high proportion of heterotrophic microplankton. In the eastern Bransfield Strait, in the northern Weddell Sea and close to the coast off Vestkapp (I), however, early summer conditions occurred with less protozooplankton contribution.Contribution no. 427 from the Alfred-Wegener-Institute for Polar and Marine Research