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 laborator y and field studies. However, the ecological relevance of in situ studies h ad not previously been assessed by direct comparison with field data. In th is study, an in situ bioassay employing Gammarus pulex L. (Crustacea: Amphi poda) and Limnephilus lunatus Curtis (Insecta: Trichoptera) was used in two agricultural tributaries and the connecting headstream. Simultaneously, th e short-term insecticide contamination and the stream population dynamics o f both species were monitored. During transient insecticide inputs (duratio n: about 1 h; peak concentrations: 6.2 mu g/L fenvalerate; 0.6 mu g/L parat hion-ethyl), mortality of both species in the in situ bioassay was signific antly higher in the contaminated tributary than in the uncontaminated contr ol tributary (analysis of variance [ANOVA], Fisher's protected least signif icant difference [PLSD]; p < 0.05). Limnephilus lunatus gave comparable res ponses 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, downs tream drift, is possible in the field. During runoff events, G. pulex migra ted from the potentially contaminated headstream section into the uncontami nated tributary, which can be regarded as a refuge and source for recoloniz ation. Significantly lower coefficients of variance in the bioassay (greate r than or equal to 0.22 compared to greater than or equal to 0.55 in the he ld samples) allow for a better detection of adverse effects of pesticide wi th this method. Hence, although the bioassay is valuable for identifying in secticide input events, supplementary field studies are necessary for an ec ological interpretation of the results.