A field study of the effects of agriculturally derived insecticide input on stream macroinvertebrate dynamics

Information about the effects of agriculturally derived insecticide input o n the aquatic fauna in the field is sparse. Runoff-related insecticide inpu t and the resulting toxicological effects on the aquatic fauna (abundance, drift, emergence) at three sampling sites at different distances (450-1150 m) from the input location of contaminated edge-of-field runoff (erosion ri ll) were investigated. By means of event-triggered sampling it was shown th at the transient (about 1 h) insecticide contamination in the stream was ve ry high (e.g. 6 mu g l(-1) parathion-ethyl in water. 302 mu g kg(-1) fenval erate in suspended particulates). The rates of loss of applied substance ca lculated on the basis of the application rate, the concentrations measured in the stream, and discharge data for both insecticides were below 0.1%, th e value usually reported in the literature. The measured insecticide contam ination in the stream had a strong negative effect on the aquatic macroinve rtebrate community. Eight of the 11 common species vanished from 19 May on for a period of 3-6 months, although their life cycles include much longer aquatic stages. The remaining three species were significantly (P < 0.001) reduced in their population density, although in their normal life cycle th e population density would increase due to summer reproduction. These effec ts were observed for the entire longitudinal stream section investigated (u p to 1150 m from the edge-of-field runoff location). In contrast, the incre ase of drift rate (mainly G. pulex) and of the mortality in the drift (main ly caddisfly species), declined with increasing distance from the runoff si te. The comparison of runoff events with and without insecticide contaminat ion substantiates the crucial importance of the insecticide contamination f or the observed effects. There was no correlation between the hydraulic com ponent (discharge), turbidity or nutrient content and the observed toxicolo gical effects (decrease in number of species, increased drift; increased mo rtality in the drift) on the aquatic fauna. Considering all these facts, it can be concluded that agricultural insecticide input may play an important role in the dynamics of macroinvertebrate communities in agricultural stre ams. (C) 1999 Elsevier Science B.V. All rights reserved.