Macroinvertebrate community structure in agricultural streams: impact of runoff-related pesticide contamination.

This study aims to assess the runoff-related pesticide contamination, among other environmental factors, that contributes to differentiation in the macroinvertebrate communities inhabiting six streams in northern Germany (Braunschweig, Lower Saxony). A total of 91 macroinvertebrate taxa were sampled three times (April, May and June 2000) during the main period of pesticide application to cereals and sugar beets in the catchment. Thirteen environmental variables (e.g., nutrients, discharge, in-stream structure), including runoff-related pesticide concentrations expressed as a sum parameter based on toxicity units (TU(SUM)), were analyzed using canonical correspondence analysis (CCA) and redundancy analysis. CCA based on species-level data and environmental variables explained 48% of the variance, with TU(SUM), stream width, percentage of detritus in the stream bed, and temperature being the most important variables. The sampling dates were of minor importance for the classification. The community composition in three streams exposed to maximum total pesticide levels between 0.2 x and 0.01 x acute toxicity to Daphnia (48-h LC(50)) was clearly distinct from that at three control sites. Redundancy analysis using average values of the environmental variables and taxonomic orders explained 95% of the variance, with TU(SUM) being the only significant variable. The results show that the present pesticide levels affect the invertebrate community structure in the field. Furthermore, they underline the potential for effects in the field at pesticide concentrations greater than 0.01 x acute toxicity to Daphnia and are thus in agreement with the assumptions underlying the standards set by the European Union uniform principle.