Cumulative disturbance effects on phosphorus mobility in a gravel-bed river at the catchment scale

Abstract

Landscape disturbance pressures in forested headwater regions can modify both the supply and transport of sediment from hillslopes to river networks. The effects of these pressures on phosphorus (P) mobility in rivers vary regionally depending upon the type and severity of the disturbance as well as interactions amongst other watershed scale controls such as climate, geology, hydrology and vegetation. The present study examines P dynamics in a gravel-bed river across multiple disturbances during environmentally sensitive periods of summer low-flow. Six study sites were selected to represent a gradient of sediment pressures from landscape disturbances (e.g., roads, harvesting, wildfire, sewage) in the Crowsnest River, Alberta. Interactions between fine bed sediments and soluble reactive phosphorus (SRP) were examined using equilibrium phosphorus concentrations (EPC0) and diffusive fluxes of SRP from the riverbed sediments. Diffusive fluxes at each site were estimated using gradients of SRP between pore-water in the bed and water column, determined from vertical distributions of SRP in the gravel matrix measured with pore-water peepers. SRP concentrations in pore-water were variable among depths and sites but were elevated downstream of the stream reach receiving primary sewage effluent outflow. Larger SRP concentration gradients were observed at sites that had either smaller substrate or increased biofilm activity. The EPC0 and diffusive pore-water flux data suggest that fine sediment in the riverbed acted as a source of SRP to the water column under low-flow conditions when the risk for eutrophication is higher and such conditions favor the growth of biofilms. EPC0 concentrations showed large inter- and intra-site variability indicating heterogeneous responses to disturbance. Furthermore, overlapping, and varying proportions of historic and contemporary harvesting, roads, road-stream or culvert crossings, and OHV use confounds the apportionment of landscape impacts. This study provides insight into the potential for the regulation of P by sediments in gravel-bed rivers following a range of landscape disturbance effects.