Additive effects prevail: The response of biota to multiple stressors in an intensively monitored watershed

Abstract Freshwater ecosystemsare impacted by a range of stressorsarising from diverse human-caused land and water uses. Identifying the relative importance of single stressorsand understanding how multiple stressorsinteract and jointly affect biology is crucial for River Basin Management. This study addressed multiple human-induced stressorsand their effects on the aquatic flora and fauna based on data from standard WFD monitoring schemes. For altogether 1095 sites within a mountainous catchment, we used 12 stressorvariables covering three different stressorgroups: riparian land use, physical habitat quality and nutrient enrichment. Twenty-one biological metrics calculated from taxa lists of three organism groups (fish, benthic invertebrates and aquatic macrophytes) served as response variables. Stressorand response variables were subjected to Boosted Regression Tree (BRT) analysis to identify stressorhierarchy and stressorinteractions and subsequently to Generalised Linear Regression Modelling (GLM) to quantify the stressorsstandardised effect size. Our results show that riverine habitat degradation was the dominant stressorgroup for the river fauna, notably the bed physical habitat structure. Overall, the explained variationin benthic invertebrate metrics was higher than it was in fish and macrophytemetrics. In particular, general integrative (aggregate) metrics such as % Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa performed better than ecological traits (e.g. % feeding types). Overall, additive stressoreffects dominated, while significant and meaningful stressorinteractions were generally rare and weak. We concluded that given the type of stressorand ecological response variables addressed in this study, river basin managers do not need to bother much about complex stressorinteractions, but can focus on the prevailing stressorsaccording to the hierarchy identified.