Organism-group-specific sensitivity to hydrologically relevant environmental gradients in perennial lowland streams

Climate change is intensifying hydrological variability in streams, but organism groups may respond to different environmental pathways associated with low-flow and high-flow conditions. We assessed how hydrologically relevant environmental gradients were associated with five biological response groups (diatoms, macrophytes, herbaceous riparian vegetation, macroinvertebrates and cotton-strip decomposition) in perennial lowland streams in Germany. Predictors included shear stress, nutrient concentrations (total N, total P, DOC), water temperature, solar radiation, shading and elevation above water level. Community composition and functional metrics were analysed using Canonical Correspondence Analyses and regression models. Predictor associations varied among groups, with macrophytes showing the highest explained variance (42%) and riparian vegetation the lowest (10.1%). Macrophyte composition was mainly associated with shear stress and total N, while diatom assemblages were most strongly associated with solar radiation. Riparian vegetation reflected local light and moisture gradients, with shading and elevation above water level correlated with Ellenberg indicator values. Macroinvertebrate metrics were associated with shear stress, total P and temperature, while cotton-strip decomposition increased with DOC and maximum temperature. Overall, our results indicate organism-specific sensitivity pathways rather than uniform responses to hydrological variability. Multi-taxa assessments can therefore help identify which environmental gradients and ecosystem components are most relevant for anticipating ecological responses to climate-driven hydrological change.