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

Abstract

Grazing by Acartia tonsa in large-volume (1 mSUP-3 ) enclosures caused significant changes in the abundance and species composition of natural phytoplankton. Cell densities and biomass were reduced 20-34% in two 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 generally increased. Accodingly, size distribution was also affected by zooplankton grazing. Grazed phytoplankton assemblages were composed of a much smaller dominant size class (<7 m) than were control assemblages (10-15 m).