USING A FRESHWATER AMPHIPOD IN SITU BIOASSAY AS A SENSITIVE TOOL TO DETECT PESTICIDE EFFECTS IN THE FIELD

摘要

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―7d), 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 μg/L in filtered water and between 305 and 870 μg/kg in suspended particles during runoff events 1 and 2 and only up to 0.01 蘥/L and 101 μg/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.