Fast phosphorus loss by sediment resuspension in a re-established shallow lake on former agricultural fields

Lake restoration on fertilized agricultural fields can lead to extensive nutrient release from flooded soils which can maintain a poor ecological quality in the new lake. The period with high sediment release is poorly understood due to few detailed lake restorations studies. We conducted such a study of phosphorus (P) mass balance and sediment dynamics in the large (889 ha) and shallow (avg. 1.03 m) Lake Filsø, Denmark to evaluate when and how fast P was removed from the lake during the first three years after establishment in autumn 2012. The results revealed that P release was high and temporally dynamic due to strong wind and wave exposure, frequent sediment resuspension and a short water retention time (WRT: 20-183 days). To quantify sediment pools across the entire lake, we performed broad-scale sonar measurements of sediment density and established close relationships to organic carbon and P contents. This enabled estimate of the loss of sediment P with high spatial resolution by comparison with the initial homogeneous P content in the soils. The approach was advantageous as P pools were highly heterogeneously distributed depending on water depth and wave exposure. We found that 65 tons P of the initial 163 tons P in the top 9 cm of the sediment had been lost from the lake between October 2012 and September 2015. Detailed measurements of P stream input and output, initiated 7 months after establishment of the lake, showed that sediment release peaked during winter when mean WRT was much shorter (40 days) than during summer (92 days). Phosphorus concentrations in the water were closely related to wind speed which caused resuspension of P-containing sediment particles particularly from shallow areas. The excess stream output relative to stream input from May 2013 to November 2015 was much smaller than the broad-scale estimate from sediment analysis suggesting that extensive P pools had already been lost from the lake during the first winter after lake establishment. We conclude that high wind exposure, shallow water and short water retention time can be a useful combination in future lake restorations on former agricultural land by facilitating fast and profound sediment P loss lowering subsequent internal loading in the following years. Keeping the water level low during the earliest phases could increase P loss by enhancing both sediment resuspension and hydraulic flushing.