Chronic toxicity of pulse‐dosed fenoxycarb toDaphnia magnaexposed to environmentally realistic concentrations

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Abstract
Effects of fenoxycarb on growth and reproduction ofDaphnia magnawere 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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