GROWTH AND GRAZING RATES OF THE HERBIVOROUS DINOFLAGELLATE GYMNODINIUM SP FROM THE OPEN SUB-ARCTIC PACIFIC-OCEAN

Growth, grazing and cell volume of the small heterotrophic dinoflagellate Gymnodinium sp. isolated from the open subarctic Pacific Ocean were measured as a function of food concentration using 2 phytoplankton food species. Growth and ingestion rates increased asymptotically with increasing phytoplankton food levels, as did grazer cell volume; rates at representative oceanic food levels were high but below maxima. Clearance rates decreased with increasing food levels when Isochrysis galbana was the food source; they increased with increasing food levels when Synechococcus sp. was the food source. There was apparently a grazing threshold for ingestion of Synechococcus: below an initial Synechococcus concentration of 20-mu-gC l-1 ingestion rates on this alga were very low, while above this initial concentration Synechococcus was grazed preferentially. Gross growth efficiency varied between 0.03 and 0.53 (mean 0.21) and was highest at low food concentrations. Results support the hypothesis that heterotrophic dinoflagellates may contribute to controlling population increases of small, rapidly-growing phytoplankton species even at low, oceanic phytoplankton concentrations.