QUANTITATIVE IMPORTANCE AND TROPHIC ROLE OF HETEROTROPHIC DINOFLAGELLATES IN A COASTAL PELAGIAL FOOD WEB

The quantiative importance and trophic role of heterotrophic dinoflagellates was studied at a permanent station in the Kattegat, Denmark. Heterotrophic dinoflagellates were most abundant during periods characterized by large phytoplankton forms (e.g. diatoms and dinoflagellates). During these periods the heterotrophic dinoflagellate biomass corresponded to between 13 and 77 % of the phytoplankton biomass. Large forms (> 20-mu-m) dominated the heterotrophic dinoflagellate biomass, but smaller forms were important during periods dominated by nanoplankton. Observations on the feeding mechanisms of the most abundant heterotrophic dinoflagellates revealed species which engulf prey directly as well as species which feed with a peduncle and a pallium - mechanisms which all allow the ingestion of large prey. The succession of heterotrophic dinoflagellates is also discussed in relation to the distribution of potential competitors and predators.