An assessment of the toxicity of some pesticides and their metabolites affecting a natural aquatic environment using the Microtox (TM) system

The conservation and preservation of aquatic ecosystems is of utmost import ance due to the high diversity and density of species and their complex foo d network. The evaluation of the potential adverse environmental impact cau sed by pesticides entering water bodies is an important parameter in aquati c toxicity. The toxicity of the insecticide, Fenitrothion and two of its metabolites, 3 -methyl-4 nitrophenol and 3-methyl-4-nitroanisole, and of the herbicides Th iobencarb and Molinate, commonly used in rice fields in Valencia near the p rotected area of lake Albufera, has been tested by using the Microtox(TM) s ystem. The 15 min EC50 Values obtained with the marine luminescent bacterium Vibri o fischeri showed that the Thiobencarb was the most toxic of the three test ed pesticides with an EC50 value of 0.03 mg/l. The EC50 values of the first two steps of the Fenitrothion hydrolysis indicated that while the first me tabolite, 3-methyl-4-nitrophenol, was as toxic as its parent compound, a de creased toxicity was observed for the second metabolite, 3-methyl-4-nitroan isole. In order to analyze the toxic effects of pesticides in complex mixtures the EC50 values of Fenitrothion, Molinate and Thiobencarb as pure substances a nd their mixtures were compared. The impact of the pesticides in the natura l ecosystem was also studied and a protective effect of lake water was obse rved.