Beaver dams along an agricultural stream in southern Ontario, Canada: their impact on riparian zone hydrology and nitrogen chemistry

摘要

The hydrology and nitrogen biogeochemistry of a riparian zone were compared before and after the construction of beaver dams along an agricultural stream in southern Ontario, Canada. The beaver dams increased surface flooding and raised the riparian water table by up to 1·0 m. Increased hydraulic gradients inland from the stream limited the entry of oxic nitrate-rich subsurface water from adjacent cropland. Permeable riparian sediments overlying dense till remained saturated during the summer and autumn months, whereas before dam construction a large area of the riparian zone was unsaturated in these seasons each year. Beaver dam construction produced significant changes in riparian groundwater chemistry. Median dissolved oxygen concentrations were lower in riparian groundwater after dam construction (0·9-2·1 mg L~(-1)) than in the pre-dam period (2·3-3·9 mg L~(-1)). Median NO_3-N concentrations in autumn and spring were also lower in the post-dam (0·03-0·07 mg L~(-1)) versus the pre-dam period (0·1-0·3 mg L~(-1)). In contrast, median NH_4-N concentrations in autumn and spring months were higher after dam construction (0·3-0·4 mg L~(-1)) than before construction (0·13-0·14 mg L~(-1)). Results suggest that beaver dams can increase stream inflow to riparian areas that limit water table declines and increase depths of saturated riparian soils which become more anaerobic. These changes in subsurface hydrology and chemistry have the potential to affect the transport and transformation of nitrate fluxes from adjacent cropland in agricultural landscapes.