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-µm or 20-µm mesh nets and incubated for 12 d at 1 °C under artificial light. Mean cell volume of dinoflagellates was 1 000 to 1 500µm3, and that of their phytoplankton prey 360 to 430µm3. 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 1.0µg chlorophylla l−1, 50µg C l−1, or 1 500 cells ml−1. Carbon budgets based on14CO2 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 (0.8µg Cµg−1 C d−1, or 6 pg C dinoflagellate−1 h−1) and maximum clearance (0.8 to 1.2 × 105 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.