Using a freshwater amphipod in situ bioassay as a sensitive tool to detect pesticide effects in the field.

In situ testing represents an alternative to conventional laboratory toxicity testing of field samples. Juvenile Paramelita nigroculus (Crustacea: Amphipoda) were exposed in situ in two rivers downstream of fruit orchard areas in the Western Cape, South Africa. Exposure took place during six time intervals (3-7 d), of which three represented the first rainfall-induced edge-of-field runoff events of the wet season, which was about two months after the last pesticide application. Survival rates were significantly reduced (35-70%) during runoff events 1 and 2 at both sites. No difference was observed from the respective no-runoff survival rate (>90%) during event 3. Peak levels of total insecticides (azinphosmethyl, chlorpyrifos, endosulfan, and prothiofos) in samples taken with water level-triggered samplers during the runoff events were between 0.03 and 0.26 microg/L in filtered water and between 305 and 870 microg/kg in suspended particles during runoff events 1 and 2 and only up to 0.01 microg/L and 101 microg/kg during event 3. Total suspended solids (TSS) varied between 400 and 700 mg/L during all three runoff events but never exceeded 65 mg/L during no-runoff time intervals. A laboratory experiment revealed that uncontaminated TSS levels of 1,500 mg/L during a 7-d exposure caused insignificant mortality (<2.5%) in P. nigroculus. No acute toxicity was observed in standard 48-h toxicity tests with juvenile Daphnia pulex using the filtered water samples taken during runoff and no-runoff conditions. It is concluded that the observed mortalities were caused by particle-associated pesticides and that the present amphipod in situ bioassay represents a sound and sensitive tool to detect runoff-related insecticide effects under field conditions.