Seasonal variation of resuspension-mediated aerobic release of phosphorus

In shallow lakes, sediment resuspension can occur frequently. Therefore, the surface sediment is often well oxidized, and the effect of aerobic phosphorus release on internal nutrient loading can be substantial as compared with anoxic phosphorus release. The effect of resuspension on the aerobic release of phosphorus was studied in the field using experimental water columns. The study included three experiments that were conducted in May, August, and October in the shallow Kirkkojärvi basin of Lake Hiidenvesi. The concentrations of suspended solids (SS) and total phosphorus (TP) increased substantially due to the 10-min resuspension event in all the experiments. However, the concentration of soluble reactive phosphorus (SRP) was strongly affected only in August, when the high pH levels promoted SRP release by ligand-exchange reactions. In August, the amount of resuspended TP remaining in the water column 4 hours after the resuspension event equalled the external phosphorus loading of 13 days. Moreover, 46% of the TP increase was in the form of SRP and thus significantly affected the availability of phosphorus for phytoplankton growth. The results demonstrated that resuspension brings substantial amounts of soluble phosphorus into the water column, providing that it occurs during algal blooms when pH in the water column is high. The effect of sediment resuspension on the availability of phosphorus in the shallow Kirkkojärvi basin thus depends on conditions in the water column, which vary with the phase of the growing season.