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Most analyses of marine microbial systems in the seasonally ice covered areas of the Southern Ocean have been based on data from the major embayment areas of the Ross and Weddell Seas. In this study data were collected at stations covering a range of regimes from full ice cover through to open water in the Bellingshausen Sea. A major feature of the production system was a rapid retreat of the ice-edge, which uncoupled the marginal ice zone from a phytoplankton bloom which remained associated with a frontal system. This bloom was maintained, and probably initiated, in an unusual environment generated by the interaction between the marginal ice zone and the front. Size-based analyses of the microbial system were derived for ice-covered, recently ice-covered and open water sites. Estimates of standing stocks and key rate processes were combined to produce a single food web network for each station. The under-ice system was one of low production and low recycling but apparently high retention. As the ice retreated the microbial systems to the north began to develop, but these were constrained by grazing pressure. The bloom in the area appeared to be sustained even though estimated losses were far higher than production, although the high sedimentation losses expected were not observed. The carbon flow networks are discussed in relation to the environmental changes and the interaction of the marginal ice zone and the frontal system appears crucial to the phytoplankton. Microzooplankton grazing is implicated as a major controlling factor. The local microbial dynamics are strongly influenced by material which was produced at an earlier time and somewhere else in the Southern Ocean.

Microzooplankton from water samples collected from depths of between 10 and 50 m at four stations between 47° and 60°N along the 20°W line of longitude during each of three cruises were classified into taxonomic groups and measured to estimate their abundance and biomass. Metazoan members of the microzooplankton were rarely found and their overall biomass was negligible in comparison with protists. The mean numbers of protistan microzooplankton cells >15 μn ranged between 3 and 11 cells ml-1, with mean biomass values between 2 and 14 μgC 1-1. Numbers and biomass increased towards midsummer and then decreased again, except at the most northerly station where the values were highest in the early August sample. The ratio of dinoflagellate to ciliate biomass tended to increase through the summer to levels of three or four and then decrease again, with later changes further north; this ratio also tended to be higher at deeper levels than in the mixed layer. Gymnodinioid forms usually dominated dinoflagellate numbers and biomass, but they were overshadowed by Protoperidinium and others at the two southern stations in June and July samples. Strombidium species were the dominant ciliates, commonly providing 25–50% of total microzooplankton biomass, while predatory haptorid ciliates were occasionally numerous, but tintinnids never formed more than 10% of the biomass.

Microzooplankton and their herbivorous activity were studied at four stations between 60 and 47°N along the 20°W meridian in mid-summer, as part of the 1989 U.K. Biogeochemical Ocean Flux Study. Microzooplankton were abundant in the surface mixed layer at all stations, with concentrations ranging between 2000 and 14,000 organisms l−1. The community was dominated by Protozoa, which contributed > 98% of the numerical abundance of microzooplankton and > 80% of their standing stocks. Three families of Protozoa—the Strombidiidae (Ciliophora: Oligotrichida), Gymnodiniidae and Peridiniidae (both Sarcomastigophora: Dinoflagellida)—dominated the microzooplankton, contributing > 70% of the standing stocks. This varied between 0.6 and 20.3 μC l−1 and was positively correlated to ambient phytoplankton concentrations. Microzooplankton standing stocks within the mixed layer showed a distinct latitudinal gradient that varied between 6.5 and 9.8 μgC l−1 at 60 and 47°N, respectively. When integrated over the mixed layer depth, microzooplankton standing stocks ranged between 300 and 428 mgC m−2 at 47 and 60°N, respectively. Microzooplankton standing stocks were approximately one-quarter to one-third that of the phytoplankton. Microzooplankton herbivory, investigated using dilution experiments, was vigorous. Microzooplankton turned over between 2 and 45% of the phytoplankton standing stock daily, thereby consuming between 0.5 and 17 μg phytoplankton carbon l−1 day−1. The magnitude of this activity was positively related to seawater temperature and to phytoplankton concentration. Microzooplankton consumed the equivalent of 100–800% their body carbon each day. Analysis of phytogenic carbon flux in the surface mixed layer showed that microzooplankton herbivory was more than an order of magnitude higher than copepod herbivory. Microzooplankton in the mixed layer grazed between 288 and 589 mgC m−2 day−1 and accounted for between 39 and 115% of the phytoplankton production at 60 and 52°N, respectively. Microzooplankton are clearly important in determining the fate of phytogenic carbon in the northeastern Atlantic Ocean during the mid-summer period.