Relevance of pesticides for macroinvertebrate communities in agricultural streams – a re-evaluation

Attributing ecological impairment in agricultural streams to individual stressors is a common challenge in freshwater monitoring because pesticide exposure often co-occurs with habitat degradation, nutrient enrichment, oxygen depletion, and other environmental pressures. The German ‘small water bodies monitoring project’ (‘Kleingewässermonitoring’, KgM) provides an opportunity to examine this challenge using a large-scale field dataset. The KgM was conducted in 2018–2019 to assess the chemical and biological condition of small agricultural watercourses. At 101 stream sites, water samples were analysed for pesticide active substances, pharmaceuticals, and other urban contaminants. Aquatic macroinvertebrates were sampled to determine community composition. The authors of the KgM concluded that pesticide exposure was the dominant stressor affecting vulnerable insects and the macroinvertebrate community based primarily on analyses using the SPEARPesticides (SPEcies At Risk) index. Because conclusions about stressor relevance may depend on biological indicators, reference conditions and analytical methods, we re-evaluated the robustness of the original interpretation using the publicly available KgM dataset. Based on our re-evaluation, we argue that the choice of index can strongly influence the interpretation of stressor relevance. In addition, the SPEARPesticides results were affected by the inclusion of forest-dominated sites, which differed from agricultural sites not only in pesticide exposure but also in other chemical pollutants and abiotic factors. Analysis of full macroinvertebrate communities using multivariate ordination methods, rather than indices designed to identify sensitivity to single stressors, indicated associations with several environmental factors affecting ecological quality. Our findings demonstrate that inferences about the relative importance of pesticides depend on study design, reference conditions and analytical methods. More broadly, the study highlights challenges that are relevant to freshwater monitoring projects internationally whenever ecological responses are evaluated along correlated multi-stressor gradients and supports the use of integrated multi-stressor assessment frameworks.