Experimental Evidence of Zooplankton Control of the Species Composition and Size Distribution of Marine Phytoplankton

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Abstract
Grazing by Acartia tonsa in large volume (1 m3) enclosures caused significant changes in the abundance and species composition of natural phytoplankton. Cell densities and biomass were reduced 20-34% in 2 separate experiments. Calculations suggest that grazing was responsible for all of the biomass reduction that occurred. Centric diatoms, especially Skeletonema costatum, were reduced in grazed tanks relative to controls, while microflagellates were greatly increased. Size distribution was also affected by zooplankton grazing. Grazed phytoplankton assemblages were composed of a much smaller dominant size class (< 7 .mu.m) than were control assemblages (10-15 .mu.m).