Sediment and nutrient storage in a beaver engineered wetland

Abstract

Beavers, primarily through the building of dams, can deliver significant geomorphic modifications and result in changes to nutrient and sediment fluxes. Research is required to understand the implications and possible benefits of widespread beaver reintroduction across Europe. This study surveyed sediment depth, extent and carbon/nitrogen content in a sequence of beaver pond and dam structures in South West England, where a pair of Eurasian beavers (Castor fiber) were introduced to a controlled 1.8 ha site in 2011. Results showed that the 13 beaver ponds subsequently created hold a total of 101.53 ± 16.24 t of sediment, equating to a normalised average of 71.40 ± 39.65 kg m². The ponds also hold 15.90 ± 2.50 t of carbon and 0.91 ± 0.15 t of nitrogen within the accumulated pond sediment. The size of beaver pond appeared to be the main control over sediment storage, with larger ponds holding a greater mass of sediment per unit area. Furthermore, position within the site appeared to play a role with the upper-middle ponds, nearest to the intensively-farmed headwaters of the catchment, holding a greater amount of sediment. Carbon and nitrogen concentrations in ponds showed no clear trends, but were significantly higher than in stream bed sediment upstream of the site. We estimate that >70% of sediment in the ponds is sourced from the intensively managed grassland catchment upstream, with the remainder from in situ redistribution by beaver activity. While further research is required into the long-term storage and nutrient cycling within beaver ponds, results indicate that beaver ponds may help to mitigate the negative off-site impacts of accelerated soil erosion and diffuse pollution from agriculturally dominated landscapes such as the intensively managed grassland in this study. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Keywords: Eurasian beaver; ecosystem engineering; nutrient storage; sediment storage; soil erosion.

Figure 1

Schematic showing change in site structure between 2011 (immediately prior to beaver introduction) and 2016. Solid black lines signify dam position and extent while dark grey areas are impounded water and light grey areas are wet areas resulting from raised water table. Pond 8 was artificially constructed to allow for humane beaver release. Black and grey arrow indicates downstream flow direction through the site. Bottom graph illustrates age of ponds in years. Site schematics provided by South West Archaeology and included with permission. [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 2

Pond sediment survey results. (a) 2016 pond schematic with ponds numbered and arrow indicating flow direction. Provided by South West Archaeology and included with permission; (b) pond characteristics including sediment depth and surface area; (c) bulk density throughout the pond sequence; and (d) cumulative sediment throughout the sequence and normalised sediment per m² surface area. [Colour figure can be viewed at http://wileyonlinelibrary.com]

Figure 3

Pond sediment carbon and nitrogen content results. Top (a) 2016 pond schematic with ponds numbered and arrow indicating flow direction. Provided by South West Archaeology and included with permission; middle top (b) C:N ratios throughout the pond sequence and above the site (AB) and below the site (BB); (c) carbon and nitrogen concentrations throughout the pond sequence and above the site (AB) and below the site (BB); middle bottom (d) total carbon and nitrogen within each pond; bottom (e) cumulative total carbon and nitrogen throughout the pond sequence. [Colour figure can be viewed at http://wileyonlinelibrary.com]