The impact of beaver dams on distribution of waterborne Escherichia coli and turbidity in an agricultural landscape

Globally, freshwater environments are threatened by point source and diffuse pollution, habitat loss, and climate change. Enhancing water quality and reducing microbial pollution are priorities to realise their ecosystem services potential but challenging to achieve and require creative solutions. Beavers are receiving increasing attention as ecosystem engineers, their dams benefitting aquatic ecosystems via improved biodiversity, water quality, and flow regulation. However, effects on microbial water quality remain uncertain. Here, we investigated the influence of engineering by Eurasian beaver (Castor fiber L.) on variation in Escherichia coli concentrations and turbidity in an agricultural stream. Water samples were collected over a period of two years (2017-2019, encompassing 11 sampling dates), from a sequence of 14 beaver dams and associated ponds to quantify fluxes of turbidity and E. coli. On average, dam structures were a source whereas ponds acted as a sink for both turbidity and E. coli. The sink effect of ponds strengthened with upstream load, increasingly outweighing the source effect of dams while being moderated by season and antecedent flow and rainfall. To complement these findings, in 2023, an in-situ pollution event was simulated by adding a slurry of livestock manure (25 l) to two nearby closely comparable streams, one beaver-engineered, the other not (control), and tracking the downstream distribution of waterborne E. coli. Consistent with our field sampling campaign, E. coli was strongly attenuated in beaver ponds, which reduced peak concentrations by >95 % and slowed the flushing of E. coli compared to the control stream. Our study demonstrates that beaver dams exert a range of effects on microbial and associated pollution but, importantly, under peak loading can significantly decrease pollution reaching downstream receptors. Beaver dams, and potentially their analogues, could therefore support environmental management strategies in agricultural systems as part of a suite of nature-based approaches.