Transient effects following peak exposures towards pesticides - An explanation for the unresponsiveness of in situ measured functional variables.

Invertebrate-mediated leaf litter decomposition is frequently used to assess stress-related implications in stream ecosystem integrity. In situ measures such as the mass loss from leaf bags or the feeding of caged invertebrates deployed for days or weeks may, however, fail to detect transient effects due to recovery or compensatory mechanisms. We assessed the relevance of transient effects using the peak exposure towards an insecticide (i.e., etofenprox) as a model scenario at three levels of complexity. These were 1) the assessment of the decomposition realised by invertebrate communities in stream mesocosms over 21 days via leaf bags, 2) 7-days lasting in situ bioassays quantifying the leaf consumption of Gammarus fossarum, and 3) a laboratory experiment determining the daily feeding rate of the same species over 7 days. Etofenprox did not trigger a significantly altered decomposition by invertebrate communities during the leaf bag assay, while in situ bioassays detected a significant reduction in gammarids' feeding rate at the highest tested concentration. The laboratory bioassay suggests that observed mismatches might be explained by recovery and post-exposure compensation. As leaf-shredding invertebrates are likely in a vulnerable state following transient effects, biomonitoring for implications of peak exposures and other pulsed stress events must happen at an adequate temporal resolution.