Dynamics of microplankton communities at the ice-edge zone of the Lazarev Sea during a summer drogue study

Microzooplankton grazing and community structure were investigated in the austral summer of 1995 during a Southern Ocean Drogue and Ocean Flux Study (SODOFS) at the ice-edge zone of the Lazarev Sea. Grazing was estimated at the surface chlorophyll maximum (5-10 m) by employing the sequential dilution technique. Chlorophyll a concentrations were dominated by chain-forming microphytoplankton (>20 mu m) of the genera Chaetoceros and Nitzschia. Microzooplankton were numerically dominated by aloricate ciliates and dinoflagellates (Protoperidinium sp., Amphisolenia sp. and Gymnodinium sp.). Instantaneous growth rates of nanophytoplankton (<20 mu m) varied between 0.019 and 0.080 day(-1) equivalent to between 0.03 and 0.12 chlorophyll doublings day(-1). Instantaneous grazing rates of microzooplankton on nanophytoplankton varied from 0.012 to 0.052 day(-1). This corresponds to a nanophytoplankton daily loss of between 1.3 and 7.0% (mean = 3.76%) of the initial standing stock, and between 45 and 97% (mean = 70.37%) of the daily potential production. Growth rates of microphytoplankton (>20 mu m) were lower, varying between 0.011 and 0.070 day(-1), equivalent to 0.015-0.097 chlorophyll doublings day(-1). At only three of the 10 stations did grazing by microzooplankton result in a decrease in microphytoplankton concentration. At these stations instantaneous grazing rates of microzooplankton on microphytoplankton ranged between 0.009 and 0.015 day(-1), equivalent to a daily loss of <1.56% (mean = 1.11%) of initial standing stock and <40% (mean = 28.55%) of the potential production. Time series grazing experiments conducted at 6 h intervals did not show any diel patterns of grazing by microzooplankton. Our data show that microzooplankton grazing at the ice edge were not sufficient to prevent chlorophyll a accumulation in regions dominated by microphytoplankton. Here, the major biological routes for the uptake of carbon therefore appear to be grazing by metazoans or the sedimentation of phytoplankton cells. Under these conditions, the biological pump will be relatively efficient in the drawdown of atmospheric CO2.