The toxicity of chlorothalonil to aquatic fauna and the impact of its operational use on a pond ecosystem

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Abstract
Chlorothalonil is a fungicide whose heavy use in eastern Canada gives it the potential for significant aquatic contamination. Laboratory bioassays and field treatments of a pond system were undertaken to determine the toxic effects of Chlorothalonil on aquatic fauna. The 96-h LC50 of technical Chlorothalonil for rainbow trout (Oncorhynchus mykiss) was 76 μg/L and was not significantly different (p < 0.05) from that of the formulated product (Bravo® 500). The 96-h LC50 of Bravo 500 for blue mussels (Mytilus edulis) and clams (Mya arenaria) was 5.9 mg/L and 35.0 mg/L respectively, while its 48-h LC50 toDaphnia magna was between 130 μg/L and 200 μg/L. Chlorothalonil exposure ofDaphnia to concentrations as low as 32 μg/L significantly (p < 0.05) increased the time to production of first young, but there were no delayed effects on number of young produced or growth at concentrations of 180 μg/L or less. Chlorothalonil was initially accumulated by blue mussels to concentrations approximately ten times greater than exposure concentrations; however, tissue concentrations returned to the same level as exposure concentrations within 96 h. Spraying of ponds resulted in mortality of caged water boatmen (Sigara alternata) and threespine stickleback (Gasterosteus aculeatus) which could be related to chlorothalonil exposure, however, caddisfly larvae (Limnephilus sp.), freshwater clams (Psidium sp.), water beetles (Haliplus sp.), scud (Gamarus spp.) and midge larvae (Chironomidae) did not suffer substantial chlorothalonil-induced mortality. Changes in endemic benthic invertebrate abundance after sprays were not remarkable or related to treatment. Faunal impacts in the pond were generally of a smaller magnitude than were predicted by bioassay results. Factors such as dilution, adsorption to suspended particles and microbial degradation are thought to have attenuated the initial pond concentrations of Chlorothalonil, thereby reducing their toxicity.