GROWTH AND HERBIVORY BY HETEROTROPHIC DINOFLAGELLATES IN THE SOUTHERN-OCEAN, STUDIED BY MICROCOSM EXPERIMENTS

Growth and herbivory of heterotrophic dinoflagellates (Gymnodinium sp.) from the Weddell Sea and the Weddell/Scotia Confluence were studied in 1988 in 100-liter microcosms. The microcosms were screened through 200-mu-m or 20-mu-m mesh nets and incubated for 12 d at 1-degrees-C under artificial light. Mean cell volume of dinoflagellates was 1000 to 1500-mu-m3, and that of their phytoplankton prey 360 to 430-mu-m3. Dinoflagellate growth rate followed a Holling type II functional response, with a maximum growth rate of 0.3 d-1 and half-saturation food concentrations of 1.0-mu-g chlorophyll a l-1, 50-mu-g C l-1, or 1 500 cells ml-1. Carbon budgets based on (CO2)-C-14 assimilation and biomasses of phytoplankton and heterotrophic dinoflagellates suggested a balance between phytoplankton grazing loss and dinoflagellate consumption, assuming a dinoflagellate carbon conversion efficiency of 40%. Applying this to the functional response yielded estimates of maximum ingestion rate (0.8-mu-g C mu-g-1 C d-1, or 6 pg C dinoflagellate-1 h-1) and maximum clearance (0.8 to 1.2 x 10(5) body volumes h-1, or 80 to 120 nl ind.-1 h-1). The microcosm experiments suggested that heterotrophic dinoflagellates may contribute significantly to maintenance of low phytoplankton biomass in the Southern Ocean.