Beaver Pond Geomorphology Influences Pond Nitrogen Retention and Denitrification

Beaver ponds are increasingly promoted as a strategy for physical stream restoration, and more recently recognized as natural solutions for excess nonpoint source pollutants, such as nitrogen (N). Beaver ponds facilitate N transformations on the streamscape by increasing sediment-water interactions and creating low and high redox conditions in close proximity. Because beaver ponds vary widely in their geomorphology, the spatial extent of morphological units should dictate the degree of biogeochemical processing. We explore the role of beaver pond geomorphic units in facilitating sediment N transformations using a sediment mass balance approach. We quantified input, output, and transformation of N species within the sediments and water of the riffle, backwater, and margin geomorphic units of a beaver pond in Northern Utah. Our interpretations were supplemented using sediment oxygen demand (SOD) and sediment C and N stable isotopes. The backwater performed most of the biogeochemical work, including increased sedimentation (2.1 g N m(-2) d(-1)), ammonification (1.3 g N m(-2) d(-1)), nitrification (0.07 g N m(-2) d(-1)), and denitrification (0.1 g N-2 m(-2) d(-1)), the latter facilitated by high sediment OM (14%) and SOD (-0.95 +/- 0.2 g O-2 m(-2) d(-1)). Backwater sediment isotopes, supported by N flux measurements, indicated increased benthic microbial activity and denitrification. The margin and backwater surface sediments had low C:N ratios, indicating sediment input from autochthonous production. These findings suggest the representation of geomorphic units in a beaver pond may predict whether the pond can facilitate N removal through sediment sequestration and denitrification. Plain Language Summary Beaver dams may be a natural method for reducing excess N concentrations within streams. Beavers build earthen dams, causing water and sediment to accumulate, creating a pond. Within a pond, the distribution of sediment and water is not uniform, instead forming distinct geomorphic units (e.g., backwater, margin, riffle). A pond is comprised of a mosaic of geomorphic units that containing different levels of sediment oxygen content, temperature, and organic matter, all of which influence how different forms of N are cycled. We hypothesized that N transformations, such as the permanent removal of N through denitrification, will covary with a pond's physical structure. To test this hypothesis, the physical structure of one pond was cataloged using geomorphic unit criteria. Within three units we conducted experiments to quantify different N cycle steps from sedimentation to denitrification. Sediment characteristics related to N cycling such as organic matter concentration and C and N isotope composition were also measured in each unit. Backwater areas facilitated the storage of sediment N and, importantly, denitrification. The inflow riffle facilitated N transport rather than transformation, while nitrification rates were highest in the margin. The geomorphic composition of a beaver pond can inform whether beaver activity can provision water quality remediation.