| The middle and lower reaches of the Yangtze River are one of the central areas of shallow eutrophic lakes in China. An understanding of the mechanisms of nitrogen (N) and phosphorus (P) exchanges at the sediment-water interface of the shallow lakes is important for the control of eutrophication. In this thesis, I gave an overall review on N and P cycles, the relationship between N and P and lake eutrophication, and P retention and release in aquatic ecosystems, and reported a comprehensive study on: 1) the regulation mechanisms of internal P-loading in Lake Chaohu, 2) N mobility at the sediment-water interface in Lake Chaohu, 3) the distribution and pollution assessment of heavy metals in surface sediment in Lake Chaohu, 4) sedimentary record of N, P and heavy metal in Lake Chaohu, 5) influence of cage-culture on the P mobility at sediment-water interface in Lake Donghu, 5) comparative studies on the P and N in the sediments of shallow lakes in the middle and lower reaches of the Yangtze River. The main results and conclusions are described as follows.In Lake Chaohu, the spatial pattern of orthophosphate (o-P), particulate phosphorus (PP) and total phosphorus (TP) was due to different external P-loading from the surrounding cities through sewage drainage systems. There was a significantly positive correlation between PP and Microcystis biomass. In summer, Microcystis bloomsresulted in high pH, which might also have induced a massive release of dissolved P from the sediment to the interstitial water. Spatial pattern of P may be affected by various factors, e.g. human activities, soil geochemistry and mineral composition. Increase of Fe-P concentration in the sediment may result in the high risk of P release.Spatial pattern of N was consistent with that of P in Lake Chaohu. The predominant form of N was nitrate in the overlying water, but NH4-N in the pore water, indicating a strong oxidative nutrient regeneration occurring near the sediment-water interface. Seasonally, although NH4-N was sufficient in lake water, NO3-N might be an important source for phytoplankton which in turn influenced the seasonal variations of NO3-N concentrations in the water column. There was a high seasonal variation of NH4-N fluxes, with the maximal ammonium release in summer and autumn when the temperature of the water column was the highest. In addition, significantly positive correlation between NH4-N fluxes and the total biomass of Microcystis and Anabaena indicates that phytoplankton was an important factor driving ammonium fluxes. Through the calculation of diffusive ammonium fluxes, it is estimated that ammonium fluxes from sediment was responsible for approximately 9.9% of primary production in the lake, suggesting that the diffusive ammonium flux from surface sediment was an important component of the total inorganic N flow to the water column in the lake.In Lake Chaohu the spatial distribution of heavy metals in the sediment were consistent with the patterns of urban effluents (including industrial and municipal waste) through sewage drainage systems. Based on enrichment factor (EF) and geoaccumulation index (/geo)5 the intensity of sediment pollution by heavy metal was in the order of As > Ni > Pb.The sediment profiles of N, P and heavy metals in Lake Chaohuindicate that there have been increasing effluents of urban sewage and agriculture wastes from the surrounding areas of the lake since the 1940's. The increases in terrigenous and anthropogenic sediment inputs during the past five decades were associated with qualitative changes in sedimentary P forms, especially the proportions of Fe-P and Ca-P in total P changed from 16% and 33% in the 1940s' to 27% and 15% in recent of sediments. The geoaccumulation index (/geo) of heavy metals showed that the sediments in the western part of the lake were seriously polluted by Cd and moderately polluted by Pb since the 1940s'.The sediments immediately under the cage once rearing Oreochromis niloticus showed significantly higher contents of organic matter (OM), phosphatase hydrolyzable phosphorus (PHP), various P species, and alkaline phosphatase activity (APA) as well. In interstitial water, both APA and orthophosphate (o-P) concentrations were significantly higher, while PHP was significantly lower, at vertical and horizontal scales. These results suggest that the enhancement of o-P release from sediment, resulted from the enrichment of OM, might be controlled by an enzymatical mechanism. The Langmuir isotherm approach revealed that, in interstitial water under the cage, sorption capacity was significantly higher, while the corresponding sorption strength was significantly lower in the upper layers, which lends an additional explanation to the enhancement of o-P release in the sediments as affected by the cage culture. A remarkably seasonal variation in vertical patterns of kinetics of APA was observed in interstitial water. Therefore, the cage culture not only increased phosphorus levels of various forms, but also enhanced significantly alkaline phosphatase activity. It is likely that intensive cage culture of O. niloticus eventually saturates the sediment capacity of further P retention, thereby greatly promoting eutrophication of lake water.Spatial variations of TN, TP and organic matter content in surfacesediments of various shallow lakes in the middle and lower reaches of the Yangze River showed that eutrophication in urban lakes was very serious, due to increased human activities with the development of industry and agriculture. N, P and OM concentrations were the key factors affecting the potentially mobile P (i.e. Al-P and Fe-P) contents in the sediments. The significantly positive correlations between Chi.a and TN, TP, OM, Al-P and Fe-P indicate that phytoplankton biomass was influenced by concentrations of nutrient and potentially mobile P in the sediments. The EF and /geo revealed that the sediments of shallow lakes in the middle and lower reaches of the Yangze River were contaminated by Cd , As, Cu, Ni and Pb, among which pollution by Cd was the most serious. Lakes Sanliqi and Dongting were the two most seriously contaminated lakes by heavy metals.
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