Water pH and sediment resuspension regulating internal phosphorus loading in a shallow lake – field experiment on diurnal variation

Resuspension-mediated aerobic release of P can be crucial to the phosphorus cycling of a shallow lake. The effect of resuspension on the aerobic release of phosphorus was studied in the field with experimental water columns. The study included four experiments that were conducted in May and August during the day and following night in the shallow Kirkkojärvi basin of Lake Hiidenvesi. In all the experiments, the concentrations of suspended solids (SS) and total phosphorus (TP) increased substantially, due to the 10-min resuspension treatment. However, the concentration of soluble reactive phosphorus (SRP) was strongly affected only at night in August, when high pH levels promoted SRP release by ligand-exchange reactions. During the day experiment in August, no SRP release was detected, due to high assimilation by phytoplankton. The results pointed out that coupling of resuspension and high algal blooms induced pH in the water column can liberate significant amounts of soluble P into the water column. The importance of this phenomenon for the lake studied was emphasized by the fact that the P pools susceptible to pHdependent P release (Al-P and Fe-P) formed a large part of the total extractable P of the surface sediment. Additionally, the conditions of high pH are common from early July to late September and the water pH can stay at a high level also at night during strong phytoplankton blooms.