Seasonal Dynamic Patterns Of The Water Quality And Sediment Sorption And Release Of Daming Lake

Daming Lake is a shallow, urban lake located in Jinan, China. The lake area is 46 ha, and the average depth is 2m. The average volume capacity is 830,000 m³. The lake was polluted by the domestic sewage and became a eutrophic lake. Half of the lake water was replaced by clean fountain water from December 2003 to January 2004, and the water quality became better. The concentrations of chlorophyll a (Chl.a), total nitrogen (TN), total phosphorus (TP) and other physicochemical parameters in lake, inflow and outflow were measured monthly from March 2004 to February 2005. The distribution and concentration of Pb, Zn, Cr, Cu, total phosphorus (TP), organic carbon (OC) and the thickness of surface sediments were investigated, and simulation experiments on sediment phosphorus release and sorption were carried out for researching the sediment eutrophication and sediment phosphorus pollution to the lake water. The results are as follow:In the research period, the phosphorus concentration in the lake was at the range of 0.080-0.746 mg/L (Average, 0.269 mg/L), and TN were at the range of 1.28-6.76 mg/L (Average, 4.39 mg/L). The max and average values of TN and TP concentrations in lake water (TN_1ake and TP_1ake) were higher than that in Grade V in China's surface water quality standard. The concentration of Chl. a in lake water was high in summer (max, 318.79 μg/L) and the lake was categorized as hypereutrophic by the OECD classification. The concentration of TN_1ake became low and stayed until late summer, which showed that TN_1ake was reduced the water replacement and surface sediment was flushed. The increasing pattern of TN_1ake in lake water was followed by logistic curve, and the concentration of TN_1ake increased to high levels which were close to the concentration of inflow TN. It shows that the concentration of TN_1ake was influenced by external loadings. TP in the lake water was low in spring and increased sharply in summer, but dropped to low levels in winter. A significant positive correlation was found between TP in lake water and that in inflow, TP in lake was influenced by external loading and probably mostly from non-point pollution. A significant positive correlation was found between Chl. a and TP, and phosphorus may be the limiting factor in the lake. However, the ratio of TN and TP was low in summer (TN/TP(?)7) when algal biomass concentration was high, and nitrogen co-limitation or nitrogen limitation may occur in summer. Nitrogen limitation may be caused by sufficient phosphorus and nitrogen depletion in summer. The average concentration of OC in the lake was 103.1 mg/g, and TP was 1.76 mg/g, and nutrient pollution in Daming lake surface sediment was serious. The research indicated that the lake bottom sediments were polluted by Pb, Zn, and Cu and partly with Cr. The concentrations of heavy metals and TP in the sediment near the south bank of the lake were higher than the other part of the lake, which should be influenced by the external pollution. The surface sediments in north and west part of the lake were thicker than the other part, and the total volume was approximately 3.27×10⁵m³.Phosphorus fractions in sediment were divided into loosely adsorbed P (NH₄Cl-P), redox-sensitive P (BD-P), metal oxides bound P (NaOH-RP), calcium bound P (HC1-P), organic P (NaOH-NRP) and residual P (Res-P). The rank order of the different P extracts in lake center and outlet was HCl-P >Res-P >BD-P > NaOH-NRP> NaOH-RP> NH₄Cl-P, and inlet was HCl-P >Res-P >NaOH-RP >BD-P >NaOH-NRP >NH₄Cl-P. The ratio of potentially mobile phosphorus fractions (NH4Cl-RP, BD-RP, NaOH-RP) to total phosphorus (TP) was high (26%) in the sediments of lake center which means that the potential pollution was serious. In the simulation experiments, phosphorus sorption could be neglected in the natural conditions because the sorption occurred at a high phosphorus concentration (>2 mg/L) of overlying water, which was much higher than lake water phosphorus concentration. The lake center sediment was able to release 0.152-0.189 mg/g, and phosphorus release was increasing with the increase of temperature. The initial pH of overlying water can be neglected in the experiments and sediment can influenced the pH of surface water.In summary, in this study, the trophic state and seasonal patterns of Daming Lake were discussed and lake limiting nutrients on eutrophication in a whole year was analyzed. Sediment analysis and simulation experiments were carried out for deeper research of the relationship between sediment and lake water. Meanwhile, the results can offer scientific information on the management of Daming Lake for local government.