Growth and herbivory by heterotrophic dinoflagellates in the Southern Ocean, studied by microcosm experiments

Abstract

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.