EN
The toxic effect of the organophosphorus (OP) insecticide dimethoate on the structure and function of natural phytoplankton assemblage of brackish lake water of the Darss-Zingst bodden chain was tested in a laboratory under both short- and long-term incubation. A decrease in photosynthetic activity, an increase of photosystem II (PS II) fluorescence emission and acceleration of respiratory O2 consumption were observed as immediate responses (1 h treatment) by the sample to the OP insecticide taken at concentrations of up to 1 mg/1. Long-term exposure (8 days) of the phytoplankton assemblage to the chemical caused elimination of the sensitive species, a change in phytoplankton density and diversity, a decrease of photosynthetic activity and differential fluorescence responses of PS II. A direct relation to the number of eliminated species with insecticide concentrations and days of incubation was observed. All the tested concentrations were found to be toxic to the phytoplankton assemblage albeit acceleration of growth of certain species at 0.01 mg/1 of the insecticide. The monitoring of the fluorescence response cf PS II and respiratory O2 consumption were considered convincing parameters for short-term toxicity assessment while photosynthetic efficiency and phytoplankton diversity were found to be better indicators of long-term toxicity exposure.