Impact of beaver re-colonization on aquifer recharge and water quality in two topographically contrasting lowland agricultural catchments 

Increasing hydroclimate extremes and land degradation have intensified concerns over aquifer recharge, water quality, and climate resilience in lowland continental regions across Western and Central Europe, as these are vital areas for agricultural production. Concurrently, the re-colonization of beavers in Germany and across Europe has revived interest in their function as natural ecosystem engineers and their ability to support ecosystem restoration. Despite mounting evidence of beavers’ impacts on restoration of wetlands and natural riparian areas, evidence from agriculturally impacted lowlands remains limited. In this study, we explore how catchment structure, hydrogeology, and land use mediate beaver activity, and how in turn beavers impact hydrology and water quality, as well as drought resilience, in two topographically contrasting lowland agricultural catchments; the Sophienfliess and the Demnitz Mill Creek in eastern Brandenburg, Germany.Integrative assessments of water quality, water sources and flowpaths, as well as landscape settings have revealed differential impacts of beavers in both catchments. In the Sophienfliess catchment, extensive beaver dam cascades in the lower catchment significantly impacted downstream water quality, indicating strong denitrification and reduction of organic carbon, as well as fertilizer-based nutrients. Furthermore, increased water retention and storage in beaver ponds has resulted in strong surface-groundwater connectivity and increased aquifer recharge. In contrast, lower dam density and spatially diverse dynamics of nutrient fluxes in den Demnitz Mill Creek catchment revealed a strong influence of local hydrological conditions on mobilization processes and water quality dynamics. As a result, changes in water quality and groundwater recharge could only be partially linked to beaver activity. Ultimately, understanding these relationships is crucial for evaluating the role of beavers in ecosystem restoration, how their engineering efforts modify catchment hydrology, infiltration, and groundwater recharge across different topographic settings, and whether they can sustainably contribute to groundwater management, drought resilience and ecosystem restoration of degraded agricultural lowland catchments under ongoing climate change.