Polychlorinated biphenyls at concentrations of 1 to 10 micrograms per liter reduced phytoplankton biomass and size in natural estuarine phytoplankton communities grown within dialysis bags in situ in an estuarine marsh. In polychlorinated biphenyls-contaminated waters, these changes could increase the number of trophic levels and divert the flow of biomass from harvestable fish to jellyfish and other gelatinous predators.

Chlordane and heptachlor at 50 microgram/1 reduced cell density, chlorophyll a per unit volume of culture, 14C uptake per cell and carbon fixation per unit of chlorophyll a in the marine dinoflagellate Exuviella baltica Lohmann. The concentration of chlorophyll a per cell was not reduced, however, by treatment with either compound. Chlordane was more toxic than heptachlor at this concentration, and caused the disintegration of many cells, thus affecting particle size distribution in the cultures. In nature, such an inhibition and shift in size class distribution could affect the availability of food for particle-feeding herbivores.