Environmental pollution of marine ecosystems affects growth and productivity in all prokaryotic and eukaryotic organisms. Phytoplankton is the major producer in the oceans and its productivity rivals that of all terrestrial ecosystems taken together. Pollution is more severe in coastal ecosystems than in the open oceans, but the latter are also stricken by the accumulation of plastic material which has been calculated to amount to 250,000 t distributed over the oceans. Coastal ecosystems are affected by terrestrial run-off which includes municipal and industrial effluents, and fertilizers and pesticides from agriculture. The weed killer atrazine inhibits the photosynthetic electron transport chain and has been found to impair productivity in phytoplankton. This effect can be quantified monitoring chlorophyll a fluorescence e.g., by pulse amplitude modulated fluorescence (PAM). Photosynthesis assessed by chlorophyll fluorescence, this volume). However, natural phytoplankton populations have been shown to develop an induced community tolerance to atrazine. The molecular mechanism of this resistance based on a genetic adaptation of the phytoplankton organisms has been clarified by Chamovitz et al. with the herbicide norflurazon.