Abstract-Effects of fenoxycarb on growth and reproduction of Daphnia magna were examined under realistic exposure conditions as defined by predictions from laboratory fate studies and observations in the field. Initial concentrations were administered in a single pulse dose and altered to mimic the reduction of fenoxycarb that occurs following field application to natural waters (halflife Ӎ 10 hours). Potential effects on the developmental stage of test organisms were determined by exposure of four age groups: less than 24 h, 4 to 6 d, 8 d, and 11 d. There were no significant effects on survival or time to first brood of first-and secondgeneration daphnids in any age group. The number of young per daphnid was significantly lower than controls only among daphnids that were less than 24 h old at test initiation and exposed to the highest initial measured concentration of fenoxycarb (45 g/L). The estimated maximum acceptable toxicant concentration (MATC) was 26 g/L, calculated from exposure of the most sensitive age group (less than 24 h old) to environmentally realistic levels of fenoxycarb. This represents a substantial reduction in toxicity when compared to the MATC of 0.0016 g/L previously reported from a standard, constant-exposure study.
Nitrate is one of the most commonly detected contaminants found in aquatic systems with other pesticides such as atrazine. The current study examined potential combined effects of nitrate and atrazine on adults of the freshwater amphipod Hyalella azteca, using survival and precopulatory guarding behavior as toxic endpoints. Although significant differences in acute toxicity with nitrate alone and in binary combination with atrazine (200 µg/L) in water-only tests were not consistently observed for each time point, potential biologically relevant trends in the data were observed. Posttest growth and behavioral observations (10-day period) conducted after 96-hour exposure suggested that atrazine and nitrate at these concentrations did not result in delayed effects on H. azteca. However, when test conditions were modified from standard toxicity tests by feeding amphipods, nitrate was found to be more toxic, with a reduction in median lethal concentration (LC50) values of approximately 80%. We also demonstrated that nitrate exhibits a dose-response effect on precopulatory guarding behavior of H. azteca, suggesting that reproductive effects may occur at environmentally relevant concentrations.
Effects of fenoxycarb on growth and reproduction of Daphnia magna were examined under realistic exposure conditions as defined by predictions from laboratory fate studies and observations in the field. Initial concentrations were administered in a single pulse dose and altered to mimic the reduction of fenoxycarb that occurs following field application to natural waters (half‐life ⋍ 10 hours). Potential effects on the developmental stage of test organisms were determined by exposure of four age groups: less than 24 h, 4 to 6 d, 8 d, and 11d. There were no significant effects on survival or time to first brood of first‐ and second‐generation daphnids in any age group. The number of young per daphnid was significantly lower than controls only among daphnids that were less than 24 h old at test initiation and exposed to the highest initial measured concentration of fenoxycarb (45 μg/L). The estimated maximum acceptable toxicant concentration (MATC) was 26 μg/L, calculated from exposure of the most sensitive age group (less than 24 h old) to environmentally realistic levels of fenoxycarb. This represents a substantial reduction in toxicity when compared to the MATC of 0.0016 μg/L previously reported from a standard, constant‐exposure study.
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