Evaluation of the partial renewal of in situ phytoplankton microcosms and application to the impact assessment of bentazon and dimethenamid

► The system allows multiple environmental toxicity assessment tests in microcosms in situ. ► Partial renewal of microcosms is required, using natural seawater containing cells. ► Phytoplankton in microcosms exhibited characteristics with the same ranges as those of the surrounding water. ► Results suggest that herbicides could affect phytoplankton in polluted coastal areas.