The Role of Beaver Dams in Modulating Hydrological Connectivity and Nutrient Dynamics in Agricultural Catchments With Intermittent Streams

Abstract

Beaver dams and associated wetlands can significantly alter hydrological connectivity and biogeochemical processes in catchments, but their combined influence on nutrient dynamics remains understudied. This research investigated the combined effects of beaver-impacted ditches and ponds on macronutrient (nitrogen (N), phosphorus (P), and carbon (C)) dynamics in an intermittent agricultural stream network. Building on long-term catchment monitoring, this study provides the first spatially extensive, site-scale assessment of nutrient dynamics across multiple water body types in this catchment, using comparisons between beaver dam impacted and non-impacted sites across multiple water body types to infer beaver-related effects under variable hydrological connectivity. Continuous monitoring of water presence and water levels across the stream network revealed distinct seasonal patterns of hydrological connectivity, which strongly influenced stream water quality. When hydrological connectivity was highest in the wet season, agricultural drainage elevated nitrate nitrogen (NO₃-N) concentrations, creating extreme stoichiometric imbalances. In contrast, during dry periods, as hydrological connectivity declined and the stream network became intermittent, beaver ditch-pond complexes demonstrated significant capacity for nutrient mitigation through enhanced N removal, coupled with increased concentrations of soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC). Beaver activity also contributed to maintaining hydrological connectivity and sustaining stream flows, prolonging the time for nutrient processing. Beaver-related ponds and wetlands significantly reduced NO₃-N while elevating total P and DOC concentrations, resulting in a more balanced nutrient stoichiometry closely linked to hydrological connectivity. These findings demonstrate the potential effectiveness of beaver dams and associated wetlands as nature-based solutions for agricultural nutrient management, though their benefits were most evident in spring and autumn when water fluxes are low and channel connectivity, supported by groundwater inputs, linked catchment nutrient inputs to the river. In addition, the impacts remain relatively localised, with downstream inputs from agricultural runoff rapidly offsetting benefits for catchment-scale water quality.