Hyporheic exchange in an urban beaver pond mediates high nutrient groundwater inflow and pond productivity

Abstract

The rebound of beaver populations across North America has resulted in new understanding of how their alteration of stream channels affects hydrology and biogeochemical cycling, but their increased impact on urban areas has not been well investigated. Urban beaver ponds have the potential to slow and retain runoff, reconnect surrounding floodplains, and reduce nutrients. Where management allows, beaver ponds may partially ameliorate water quality problems in urban streams and enhance the ecological function of the broader riparian zone. In this study we measured water chemistry in surface (over ten months) and hyporheic (over three months) waters, including nitrate, ammonium, and dissolved organic carbon (DOC), to assess the impact of urban beaver dams on water quality. Pond water is sourced from groundwater, with anoxic conditions in the surface water at the top of the pond that become increasingly oxic moving longitudinally through the pond. Large diel dissolved oxygen swings in the pond indicate high sestonic productivity. There are distinct hyporheic geochemical signatures above and below the dam as well as across the dam indicating the dam is driving downwelling above the dam and upwelling below. Hyporheic ammonium concentrations and surface nitrate concentrations are high above the dam and decrease below the dam, showing that shifting oxic conditions drive nitrogen speciation. The δ15N and δ18O values of nitrate point to diffuse sewage leaks as the main source of nutrients to the pond instead of overland runoff. In addition, surface waters have lower δ15N and δ18O of nitrate and DOC concentrations than groundwaters, indicating denitrification may drive the decrease in nitrate concentration. With increasing presence of beaver in urban streams, surface water-groundwater interactions through urban beaver dams have the potential to remediate urban sources of nutrients from ponds.