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Abstract

The present study examines phosphorus (P) release potential from fine-sediment in a gravel-bed river across a gradient of sediment pressures from landscape disturbance (roads, harvesting, wildfire, wastewater effluent) during environmentally sensitive periods of summer low-flow in the Crowsnest River, Alberta. A series of batch experiments were used to determine the equilibrium phosphorus concentration (EPC₀) and measure the release potential of fine sediment to adsorb or release P. Diffusive fluxes of soluble reactive phosphorus (SRP) from riverbed sediments were estimated at each site by measuring SRP gradients between pore-water in the bed and overlying water column. Vertical distributions of SRP in the gravel matrix were measured with pore-water peepers. SRP concentrations in pore-water were variable among depths and sites but were most elevated at a site located immediately below sewage outflow. Larger SRP concentration gradients were observed at sites that had either smaller substrate size or increased biofilm activity. The EPC₀ and diffusive pore-water flux data suggest that fine riverbed sediment was a source of SRP to the water column under low-flow conditions when the risk for eutrophication is higher and such conditions shift nutrient dynamics and degrade water quality. The observed large inter- and intra-site variability in the EPC₀ indicates heterogeneous responses to disturbance. Overlapping and varying proportions of historic and contemporary disturbance pressures (harvesting, roads, road-stream or culvert crossings, and OHV use) confound the apportionment of landscape impacts on P release from fine sediment stored in gravel bed of the Crowsnest River.