Water and mass budgets for estimating phosphorus sediment–water exchange in Lake Taihu (China P. R.)

Since a few decades, Lake Taihu, one of the largest fresh water lakes in China, has suffered from eutrophication problems, with total P and N level values up to 0.15 mg P/l and 2.5 mg N/l. To show the evident role of P sediment–water exchange in the lake, annual P budgets of Lake Taihu were set up for the period 1998–2000. Reliable water budgets could be derived as a basis for the subsequent P mass budgets. For the latter, the P accumulation rates by the sediment, as a 'fitting term' of the P mass budgets, were found to be between 1000 and 1060 × 103 kg P/yr (= 1.2 mg P/m2 day). This P accumulation amounts to around 65% of the total P inputs into the lake; it represents both physico–chemical processes (adsorption/precipitation, etc.) as well as physical sedimentation of particulate matter. No significant seasonal trends in the P sediment–water exchange were observed here. The particulate P profiles in the upper sediment layers of the lake indicate that, especially for the relatively-polluted north-western sub-lakes, the sediment P uptake capacity has now gradually been depleted. Restoration measures, such as sediment dredging in the NW sub-lakes, will be necessary to improve the Lake Taihu water quality in the coming years.