Eurasian Beaver (Castor Fiber) Reintroduction: A Nutrient Mitigation Solution for Lowland Chalk Streams?

Abstract Globally scarce and ecologically valuable, the lowland chalk streams of southern and eastern England experience extensive eutrophication pressures arising from intensive agriculture. Mitigation measures are required to restore natural chalk stream functioning and the reintroduction of the Eurasian beaver ( Castor fiber ) could provide a solution. Here, we investigated the impact upon chalk stream hydrochemistry of the reintroduction of beavers to a 6-ha enclosure on the headwaters of the River Glaven, Norfolk. Over a two-year period (March 2022 – May 2024), 190 river water samples were collected across control and impact sites displaying minor-to-major hydrological disturbance from beaver activity, with samples analysed for nitrate (NO 3 ), phosphate (PO 4 ), and dissolved organic carbon (DOC). Results revealed significant reductions in nutrient concentrations downstream of the beaver enclosure (NO 3 = -43%; PO 4 = -51%), as well as significantly lower concentrations relative to the agricultural control site (NO 3 = -64%; PO 4 = -86%). Conversely, DOC concentrations were significantly higher downstream of the enclosure (+ 94%) and compared to the control (+ 272%). Greater reductions in nitrate and phosphate during the summer (NO 3 = -47%; PO 4 = -61%) compared to winter (NO 3 = -37%; PO 4 = -38%) indicated biological assimilation within the beaver wetland as a likely causal mechanism, whilst lower dissolved oxygen concentrations within the beaver ponds indicated a role for denitrification in nitrate removal. Overall, these results demonstrate that beaver wetlands can significantly mitigate eutrophication risk in agriculturally impacted chalk streams and provides further evidence in support of their wider reintroduction to English catchments.