ANNUAL CYCLE OF PARTICLE-SIZE FRACTIONS AND PHYTOPLANKTON BIOMASS IN THE NORTHERN BALTIC PROPER

A large data set of particle concentrations in the range 1 to 1000-mu-m, chlorophyll a, and associated measurements from the surface waters of the northern Baltic proper during 1985 to 1989 was analyzed to construct spatially and temporally averaged annual time curves of particle size fractions and phytoplankton biomass. Chlorophyll a concentration was well correlated with the 28-73-mu-m size fraction and had 2 seasonal peaks: in April-May, corresponding to the diatom spring bloom, and in August, corresponding to the late-summer bloom of blue-green algae. The 2 maxima were separated by the early summer minimum when the 1-10-mu-m particle fractions corresponding to a mixed assemblage of small phytoplankton and microheterotrophs reached their maxima. The largest particles (163-1000-mu-m), corresponding to smaller mesozooplankton, reached their maximum in July-August before the blue-green maximum. Automated particle counting is well suited for routine monitoring of pelagic systems as it reveals all major shifts in the planktonic community.