In-stream oxygenation to mitigate internal loading of phosphorus in lowland streams

Many lowland regions are afflicted with episodic high phosphorus (P) concentrations in streams during low flow periods. It has been hypothesised that these P peaks are due to internal loading after reductive dissolution of ferric iron (Fe(III)) minerals in sediments with a high P/Fe ratio. Here, we experimentally tested that hypothesis by measuring the sediment/water fluxes of P in streams and by testing responses to in-stream oxygenation. A phosphate stock solution with a bromide (Br-) tracer was administered during 3 h in four streams with varying sediment P/Fe ratios during summer (low DO concentration). The experiment was repeated in winter (high DO concentration) in one of the streams. The sediments were either net P sinks (three cases), net unreactive (one case) or a net source (one case), with the net source occurring in the stream with the highest sediment P/Fe and at lowest DO concentration. A direct oxygenation experiment in one stream increased downstream DO concentration with about 1 mg O-2 L-1 and decreased the P concentration with, on average, about 0.3 mg P L-1 over a two-week period. The efficiency of oxygenation to reduce P concentrations enhanced when flow rates were lower. In conclusion, this experimental study confirmed the hypothesis that internal P loading in streams is largest in high P/Fe sediments at low DO conditions. Lowering P concentrations in lowland streams should combine reduced P emission and strategies avoiding low DO concentration, i.e. reduced biological oxygen demand (BOD) and nutrient emissions.