Seasonal variations in the sensitivity of Lake Geneva phytoplankton community structure to atrazine

Natural phytoplankton assemblages from Lake Geneva were cultivated in outdo or microcosms contaminated with 10 mu g/l of the PS II inhibitor herbicide, atrazine. The 12 experiments were performed in the spring and early and la te summer. Physico-chemical parameters and the changes in the densities of algal species were followed. The algae in the phytoplankton community were inhibited, stimulated, or apparently unaffected by the herbicide. The growt h of 3 species (Fragilaria cratonensis KITTON, Nitzschia sp. and Oscillator ia limnetica LEMMERMANN) was Stimulated by atrazine contamination in microc osms, but showed no such response in monoculture, whereas Chlorella vulgari s BEIJER, which was inhibited in the outdoor microcosm experiments, was als o inhibited by the herbicide in laboratory monoculture. Atrazine influenced the interspecies interactions, which could explain the greater development of some phytoplanktonic species in the contaminated microcosms. Hence, the herbicide may act as a supplementary factor disturbing algal succession in lakes. The effect of atrazine on species development and community structu re varied. The algal communities were most sensitive to restructuring by th e PS II inhibitor herbicide in the clear water phase, whereas the robust sp ring algal communities were the least sensitive. Seasonal changes in enviro nmental parameters, species composition and interactions may affect the res ponse of algal communities to the toxicant. These interactions between herb icides and seasonal successions may reduce or increase the consequences of pollution in aquatic systems. This may be ecologically important, especiall y when the herbicide contamination from runoff also varies seasonally.