Abstract
The annual cycle of 21 species of Protoperidinium, the dominant genus among the thecate heterotrophic dinoflagellates in the inner Oslofjord, was investigated by ca. monthly sampling between March 1994 and April 1995. Protoperidinium cell counts were obtained by a rapid filtration procedure allowing a 167× concentration of cells from one-litre samples, before counting in an inverted microscope. Concentration by this method was shown to yield counts equal to those obtained by sedimentation from equally large water volumes. The cell counts were compared to biomass estimates of potential food for Protoperidinium, in the form of estimated carbon standing stocks (in εg C I-1) of diatoms and dinoflagellates in the microplankton (> 20 εm) size range. Several Protoperidinium species reached their maximum abundance during or immediately after peaks in autotrophic biomass concentration. Thus, in 1994, P. pellucidum and P. conicoides were associated with the spring diatom 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 the diatom Pseudo-nitzschia pseudodelicatissima in October. The observations are discussed in relation to known food requirements of individual Protoperidinium species. Food availability is more important than physical variables (temperature) in regulating population growth in these microplankton grazers.
