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Seasonal Dynamics of Chrysochromulina Species (Prymnesiophyceae) in a Coastal Area and a Nutrient-enriched Inlet of the Northern Baltic Proper

Phytoplankton belonging to the prymnesiophycean genus Chrysochromulina were studied during 4 years (1985-1988) in a coastal area and an inlet differing in Hydrographie and nutrient regimes in the northern Baltic proper. In both areas Chrysochromulina species were regular components of the phytoplankton community and often made up a large fraction (up to 65%) of the nanophytoplankton abundance in summer and early autumn, and constituted occasionally over half of the total phytoplankton biomass. We found distinctive differences in the occurence of Chrysochromulina spp. between the two stations. In the coastal station, abundance maxima usually occurred in July whereas in the nutrient enriched inlet, maxima occurred in early summer or early autumn. In the inlet, a maximum of 9 X 10 cells L" were found as compared to 3.0 x 10 cells L" in the coastal area. Abundance maxima often followed a period of low concentrations of inorganic phosphorus in the trophogenic layer. Our data indicate nutrient enriched conditions favoured the observed species, either directly by higher nutrient availability or indirectly through increased microbial activity. In a study of the vertical distribution of Chrysochromulina spp. at the coastal station, three distinctive size groups were distinguished, which differed in their depth distributions within the trophogenic layer. Critical identification of Chrysochromulina to species level is rarely possible without examination of the scales by transmission electron microscopy. In samples from the summer of 1992, nine species were identified on bases of the structures of their scales. Three species were abundant, Chrysochromulina polylepis, C. minor and Chrysochromulina sp. 2. not yet described in the literature. Six other species, C. brachycylindra, C. hirta, C. simplex, C. leadbeateri, C. ericina and C sp. 1. were rare. Five species (C. hirta, C. leadbeateri, C. simplex, C. sp. 1. and C. sp. 2.) are new to the northern Baltic proper. Scale morphology of C. polylepis differed from specimens collected in the Kattegat. A detailed description of C. minor scales and a short characterisation of the new species are given.