The present communication deal with the analysis of water samples from Bhoj wetland to detect three pesticides (chlorpyrifos, monocrotophos and endosulfan) and to assess their individual toxicity to the phytoplankton community. Higher concentration of most of the pesticides was found to be in the pre-monsoon (March to May) 0.9 µg/l (monocrotophos) and post-monsoon (September to December) 1 µg/l (monocrotophos and chlorpyrifos) period. Among the aquatic organisms, phytoplankton communities are the key targets for the pesticides because of their ecophysiological similarities with terrestrial plants. A standard 96 h static algal bioassay was followed to determine pesticide effects on the population growth rate of phytoplankton. At higher concentrations of all the pesticides elicited a significant effect on population growth rate by maximum inhibition of the cell division, but toxicity would not be expected at typical environmental concentrations. The population growth rate EC50 average values determined for Chlorophyceae, Cyanophyceae and Bacillariophyceae varied in the range of 16.1 to 32.3 µg/l for chlorpyrifos, 8.6 to 14.3 µg/l for monocrotophos and 4.2 to 15 µg/l for endosulfan, respectively. Therefore, decrease in phytoplankton populations resulting from pesticide exposure could occur at higher concentrations in aquatic systems where pesticides are present in mixture. Detrimental effects on phytoplankton population growth rate could impact nutrient cycling rates and food availability to higher trophic levels. Characterizing the toxicity of chemical mixtures likely to be encountered in the environment may benefit the pesticide registration and regulation processes.
Key words: Pesticides, monsoon, phytoplankton, toxicity.
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