Source or sink? Quantifying beaver pond influence on non-point source pollutant transport in the Intermountain West

Non-point source (NPS) pollution remains high in many watersheds despite strategies aimed at reducing such pollution. Beaver (Castor canadensis) activity converts lotic systems to semi-lentic by impounding stream flow and trapping sediments, which have a high affinity for NPS pollutants such as nitrogen (N), phosphorus (P), and heavy metals. This study aimed to identify environmental conditions under which beaver ponds influence the fate and cycling of NPS pollutants. Dissolved and particulate nutrients were sampled upstream and downstream of three headwater beaver ponds differing in age and character through the summer season. Sedimentation rates and sediment concentrations of nutrients and metals were also determined. Results from this study suggest that beaver ponds can attenuate heavy metals at a rate 2 to 4 times greater than a riffle reach (p < 0.05). Metal sequestration scaled with pond age and sediment organic matter content. The oldest and youngest ponds had no significant effect on dissolved nutrients (NO₃^-, TDN and SRP) or total P (TP). The middle age pond was a significant TN sink in summer (0.6-0.8 g N m^-2 d^-1 [p = 0.03]) and influenced dissolved nutrient concentrations differently in spring (21% NO₃^- sink [p = 0.03], 61% SRP source [p = 0.05]) compared to summer (34% NO₃^- source, 7% SRP sink). This pond had little apparent effect on TP loads during the study period but accumulated a total of 146 g m^-2 of phosphorus in the sediments suggesting that beaver ponds may reach their phosphorus sequestration potential within the first few years of pond development and then subsequently act as a weak SRP source. We use a theoretical relationship describing sediment-water interactions to show that biogeochemical processing in a beaver pond is optimized at intermediate levels of pond nutrient supply and residence time. If beaver ponds are to be considered as an option for landscape scale restoration, this theoretical relationship may be useful for predicting the effects of beaver ponds on water chemistry, and aid in the interpretation of variable water quality results from inherently heterogeneous environments.