Beaver Wetlands Moderate Diffuse Water Pollution in Agricultural Catchments

ABSTRACT

Most streams in agricultural catchments across Eurasia and North America are in a degraded ecological condition with high runoff concentrations of sediment and nutrients a major contributor. Previous studies have shown that beavers can reduce such diffuse pollution in small streams and rivers by building dams, creating wetlands in which deposition and nutrient cycling can occur. Results are variable between studies; however, some identify beaver wetlands as significant sinks, removing high proportions of influent loads, whereas others show source effects. In this study, we used a multi‐site control‐impact design to confirm whether three beaver sites in England reduced diffuse pollution and to identify the main sources of variation in TSS, DOC, NO ₃ , NO ₂ , NH ₃ and oP. We found overall site was the most important controlling variable. Within that, sites exhibited a source‐to‐sink nature of effects on suspended solids and nutrient concentrations, tending to be sources at low inflow concentrations and sinks at higher concentrations, particularly those exceeding Environmental Quality Standards. The beaver wetlands showed water regulation, particularly a reduction of higher winter flows, which resulted in reduction of downstream loads, especially of DOC and oP. A literature review showed that the observed source‐to‐sink continuum across influent concentrations extended across previously published studies, helping to explain variation across the results. This work advances our knowledge of the hydrological and nutrient‐cycling dynamics of beaver wetlands in a modern agricultural landscape into which beavers are returning through reintroduction after an absence of 400 years.