The seasonal cycle of phytoplankton, macronutrients, and the microbial community in a nearshore antarctic marine ecosystem

Seawater chlorophyll a concentration (size-fractionated at 20, 2, and 0.2 µm) and temperature have been measured weekly from December 1988 to August 1994 at a nearshore shallow-water station at Signy Island in the maritime Antarctic. Macronutrients (N, P, Si) were measured monthly, and the microbial community enumerated during a 15-month period. The duration of winter sea-ice, summer seawater temperature, and peak chlorophyll biomass all exhibited strong interannual variability. The summer peak of microplankton (>20 µm) chlorophyll was dominated by large diatoms and colonial forms, and there were late summer-early winter blooms in two of the six seasons examined. Peak concentrations of nanoplankton (20–2 µm) chlorophyll, predominantly from flagellates, were much lower but the bloom lasted longer and winter biomass was higher than in the microplankton. Seasonal macronutrient patterns suggested a preferential utilization of ammonium at the start of spring followed by a major utilization of nitrate by the summer diatom bloom. The microbial community was complex and showed a clear seasonality in all components, although the timing of the summer bloom differed between taxa. Cell numbers were low in winter, increasing in summer with flagellate taxa especially important in late summer. This study emphasizes the complex nature of the nearshore marine ecosystem and highlights the need for year-round oceanographic studies in the highly seasonal polar marine environment.

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