Validity and ecological relevance of an active in situ bioassay using gammarus pulex and limnephilus lunatus

In situ bioassays using aquatic organisms serve as a link between laboratory and field studies. However, the ecological relevance of in situ studies had not previously been assessed by direct comparison with field data. In this study, an in situ bioassay employing Gammarus pulex L. (Crustacea: Amphipoda) and Limnephilus lunatus Curtis (Insecta: Trichoptera) was used in two agricultural tributaries and the connecting headstream. Simultaneously, the short-term insecticide contamination and the stream population dynamics of both species were monitored. During transient insecticide inputs (duration: about 1 h; peak concentrations: 6.2 mg/L fenvalerate; 0.6 μg/L parathion-ethyl), mortality of both species in the in situ bioassay was significantly higher in the contaminated tributary than in the uncontaminated control tributary (analysis of variance [ANOVA], Fisher's protected least significant difference [PLSD]; p < 0.05). Limnephilus lunatus gave comparable responses in the stream; however, no mortality of G. pulex was observed in the stream. The reason for this difference is presumably that, in contrast to the bioassay, a typical avoidance behavior of G. pulex to pesticides, downstream drift, is possible in the field. During runoff events, G. pulex migrated from the potentially contaminated headstream section into the uncontaminated tributary, which can be regarded as a refuge and source for recolonization. Significantly lower coefficients of variance in the bioassay (≤0.22 compared to ≥0.55 in the field samples) allow for a better detection of adverse effects of pesticide with this method. Hence, although the bioassay is valuable for identifying insecticide input events, supplementary field studies are necessary for an ecological interpretation of the results.