Catchment structure and functioning mediates beaver activity in contrasting lowland agricultural landscapes

The reintroduction of beavers in Germany and across Europe has revived interest in their function as ecosystem engineers and their associated impacts on landscapes, hydrological regimes, and water quality. Despite mounting evidence of effects on the hydrology and water quality of rivers, beaver distribution and the links to landscape heterogeneity are still poorly understood. In this study, an extensive intercomparison of two topographically contrasting lowland agricultural catchments was conducted-the Sophienfliess (SOFL) and the Demnitzer Mill Creek (DMC), both in Eastern Brandenburg, Germany. The goal of this study was to assess how catchment structure and functioning mediate beaver activity in terms of distribution and dam density, and how this in turn affects water and nutrient fluxes. A survey of beaver dams in terms of dam location, density, and attributes in both catchments revealed significant differences in the evolution of beaver habitat over time, the number of dams present, and their effects on water flow and nutrient cycling. Most notably, in the lower SOFL catchment, dam density has increased steadily over the last decade and was almost 10 times higher compared to the DMC catchment. In contrast, the recent multi-year drought in Eastern Germany, as well as differential catchment functioning, has led to reduced beaver activity in the DMC catchment, despite recent wetland rehabilitation efforts. Furthermore, physical differences in dam structure and size could be linked to landscape settings (i.e., valley gradient), surface-groundwater connectivity, and channel characteristics (straightened vs. meandering). In SOFL, a beaver dam cascade system in the lower catchment propagated significant denitrification, reduction in dissolved organic carbon, and phosphorus concentrations. In contrast, the spatial dynamics of nutrient fluxes in the DMC revealed more localized mobilization processes and strong responses of solutes to hydrological connectivity, which could not be solely linked to beaver activity. Overall, the study showed that catchment structure, hydrological conditions, and stream channel characteristics influence how and where beaver dams are established, and how they in turn influence the physical, chemical, and biological characteristics of surface waters.