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

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 micrograms/L and was not significantly different (p less than 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 to Daphnia magna was between 130 micrograms/L and 200 micrograms/L. Chlorothalonil exposure of Daphnia to concentrations as low as 32 micrograms/L significantly (p less than 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 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)