Ecology of the heterotrophic dinoflagellate genus Protoperidinium in the inner Oslofjord (Norway)

The annual cycle of 21 species of Protoperidinium, the dominant genus among the thecate heterotrophic dinoflagellates in the inner Oslofjord, was inve stigated by ca. monthly sampling between March 1994 and April 1995. Protope ridinium cell counts were obtained by a rapid filtration procedure allowing a 167x concentration of cells from one-litre samples, before counting in a n inverted microscope. Concentration by this method was shown to yield coun ts equal to those obtained by sedimentation from equally large water volume s. The cell counts were compared to biomass estimates of potential food for Protoperidinium. in the form of estimated carbon standing stocks (in mug C l(-1)) of diatoms and dinoflagellates in the microplankton (> 20 mum) size range. Several Protoperidinium species reached their maximum abundance dur ing or immediately after peaks in autotrophic biomass concentration. Thus, in 1994. P. pellucidum and P. conicoides were associated with the sprint: d iatom bloom in March-April, P. divergens with a mixed dinoflagellate-diatom bloom in August, and P. granii and several other species with a bloom of t he diatom Pseudo-nitzschia psuedodelicatissima in October. The observations are discussed in relation to known food requirements of individual Protope ridinium species. Food availability is more important than physical variabl es (temperature) in regulating population growth in these microplankton gra zers.