Likely controls on dissolved reactive phosphorus concentrations in baseflow of an agricultural stream

Purpose High baseflow-phosphorus (P) concentrations increase the likelihood of periphyton blooms. Several physical and chemical factors can control baseflow-P concentrations such as hydraulic exchange with groundwater, particle size-sorting, redox chemistry and different sediment sources. We investigated which of these sources dominated the control of baseflow-P concentrations and whether concentrations could be predicted using the equilibrium P concentration at zero net sorption or desorption (EPC0) of sediments. Materials and methods At four locations along an agricultural stream we measured dissolved reactive P (DRP), pH, iron, manganese, sulphate, nitrate and dissolved oxygen in streamflow and hyporheic water at 0-200, 200-400 and 400-800-mm depths, and P-fractions and EPC0 in sediment samples from the 0-200, 200-400 and 400-800-mm depths. Results and discussion Concentrations of DRP in streamflow and shallow hyporheic zone water increased downstream and were mirrored by concentrations in shallow sediment, EPC0 measurements and deeper hyporheic waters. Groundwater samples and the EPC0 in deeper sediments did not show a pattern nor residence time consistent with the supply of P to baseflow despite deeper sediment being anoxic and less likely to sorb upwelling P. There was also no change in pH nor particle size downstream ruling out the degassing of groundwater or sediment size-sorting as an influence. However, the composition of sediment and underlying lithology of the catchment pointed to sediment downstream that was different to upstream sediment in that it could store and release more P. Conclusions Given the strong influence of sediment source on baseflow-P concentrations, efforts to decrease the likelihood of periphyton blooms under baseflow should focus on reducing the erosion of P-rich sediment. However, predicting the likely release of P from anoxic sediments will require EPC0 to be measured under reducing conditions.