Interplay of Aquatic and Riparian Variables in Two Anthropogenically-Influenced Small Stream Catchments

Small streams are important ecosystems in many areas globally, playing a crucial role in conserving biodiversity and providing ecosystem functions. However, owing to their high connectivity with surrounding landscapes, small streams are particularly susceptible to drivers of biodiversity decline. We report here the results of a monitoring program conducted between 2018 and 2021 at 18 sites in two stream catchments in southwest Germany, characterized by near-natural to highly anthropogenic land use. The monitoring program included 26 environmental, biological, and anthropogenic stressor variables describing the landscape context, aquatic ecosystem compartments, and riparian areas. We used ordination analysis and factor analysis of mixed data (FAMD) to analyse the relationship between variables and the effects of anthropogenic stressors on ecosystem structure and functioning. Results showed that near-natural upstream sites are substantially less impacted by anthropogenic stressors (e.g., pesticide pollution, morphological alterations) compared with agricultural and urban downstream sites. Biotic parameters, such as abundances and taxa richness of aquatic macroinvertebrates, fish, and carabid beetles, as well as leaf litter decomposition, exhibited overall high variations between sites. FAMD assigned the stream sites into four groups (i.e., forest, grassland, agriculture, viticulture), primarily based on land use and anthropogenic stressor influences and found anthropogenic stressors to often co-occur. Lower aquatic macroinvertebrate abundances and diversity, as well as reduced leaf litter decomposition, were observed at sites with higher anthropogenic disturbances. Overall, our findings suggest that small streams are subjected to multiple anthropogenic stressors, resulting in negative effects on both aquatic and riparian ecosystem health.