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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 hypothesized that of these sources, anoxic sediments would allow P-rich groundwater to influence baseflow P concentrations the most and that the measurement of the equilibrium P concentration (EPC 0 ) of sediments under oxic conditions would not predict P release in anaerobic sediment or baseflow P concentrations. 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 EPC 0  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, EPC 0  measurements of oxic sediments and deeper hyporheic waters. Groundwater samples and the EPC 0  in deeper sediments did not show a pattern or 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 or 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. Furthermore, the presence of oxic conditions in surface sediment meant that there was a relationship between EPC 0  and hyporheic water P concentrations. However, mixed oxic/anoxic conditions in deeper layer may require EPC 0 , or release rates, to be measured under reducing conditions.

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