Linking insecticide contamination and population response in an agricultural stream

The study aims to evaluate the impact of insecticides associated with rainf all-induced surface runoff from arable land on macroinvertebrate population s. These effects of insecticides were distinguished from the hydraulic stre ss also associated with surface runoff. Transient increase in discharge and insecticide contamination (maximum 6 mu g/L parathion-ethyl in stream wate r, 302 mu g/L fenvalerate in suspended particulates) was observed in a head water stream subsequent to surface runoff from arable land. In the aquatic macroinvertebrate community, eight of the eleven abundant species disappear ed, and the remaining three were reduced significantly in abundance followi ng the insecticide-contaminated runoff. Recovery within 6 months was observ ed for four species and recovery within 11 months for nine species. Two spe cies remained at a low population density for over a year. The effects of i nsecticides were distinguished from other parameters, such as hydraulic str ess associated with surface runoff, as well. The causal connection between insecticide contamination and biological response was established by elimin ating increased hydraulic stress during surface runoff using in-parallel by pass microcosms containing the dominant species Gammarus pulex and Limnephi lus lunatus. The mortality of these species was similar to that of the same species in the stream. Additional microcosms, disconnected from the stream during runoff events, served as a control. Thus, the toxic potential of th e runoff water is considered to be responsible for the observed effect on t he macroinvertebrates. It is concluded that agricultural insecticide input may alter the dynamics of macroinvertebrate communities in streams.