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The roles of inorganic nutnents and grazing by crustaceans and microprotozoans as factors shaping the planktonic community of the Gulf of Finland were studied in a 6 d laboratory experiment. The removal of metazoans > l00 pin greatly enhanced microprotozoan growth during the last 3 d of the experiment. When inorganic N and P additions were combined with the removal of metazoans, the growth of protozoans was stimulated even further. On the other hand, addition of metazoans >200 pm kept the protozoan populations low, even in the presence of extra N and P. In connection with high protozoan numbers, the growth of phytoplankton > l 0 pm was enhanced, while the 1-10 pm phytoplankton populations decreased towards the end of the experiment. With abundant metazooplankton and nutrient additions (and only few protozoans), phytoplankton in the size range of 1 to 10 pm flourished, while > l 0 pm phytoplankton biomass did not rise above the initlal level. Whether the grazer community was dominatcd by protozoans or metazoans did not have much effect on picoalgal ( 2 pm depends on the composition of the grazer populations The bioinass and composition of microprotozoan communities also appear largely to depend on the shaping effect of metazoan grazers. Nutrient recycling by crustaceans seems to enhance bactenal production, and possibly also picoalgal growth