| Lake eutrophication, a global environmental problem, is harmful for the aquatic ecosystems and drinking water safety. When the external sources are gradually being controlled, the eutrophication and algal blooms were mainly caused by the release of phosphorus (P) from the sediments. However, the characteristics of internal P transformation at the water-sediment interface and the knowledge of biogeochemical effects on P transition process, especially concerning the role of chemical environment and microorganisms, are still lacking. In this thesis, a typical shallow city lake, West Lake, was chosen as the study area. The spatial and temporal variations in forms of P fractions and compositions of phosphate releasing bacteria (PSBs) were investigated, and the possible source of P was estimated by the Standards Measurements and Testing Program (SMT protocol). We also studied the significance of physic-chemical and biological effect on P transformation process at the water-sediment interface, and the variations of biological effect under different conditions of pH, light, and temperature conditions were discussed. Moreover, some PSBs strains had been isolated and identified, and laboratory tests on P release ability by these PSBs were also conducted. The purpose of this study lied in providing theoretic basis for effectively control of nutrients and clarification of the mechanisms of eutrophication. The main results were as follows:The seasonal variations of phosphorus contents in water and the temporal and spatial variations of different P fractions in sediments were investigated by SMT method. The concentrations of total P (TP) in water were around 0.10 mg L-1 in four seasons. The lowest value was observed in the Lake Xiaonan, while water TP in the Lake Outer and Beili had maintained relatively high levels throughout the year Moreover, TP contents were relatively high in spring and summer, but low in autumn and winter. Particulate P, which was hardly utilized by phytoplankton, was the main form of TP in lake water in most time of the year. The concentrations of TP in seven sediments ranged from 0.21 mg g-1 to 1.48 mg g-1,which was moderate compared to other lakes. In general, TP contents showed a downward variation with sediment depth within each season, and were high in winter and low in summer. HCl-P was the main component of TP in sediments and it showed significant correlation with TP. Organic P (OP) was the secondary component while NaOH-P was the smallest proportion of TP. P in sediment of the West Lake was mainly of anthropogenic origin, such as domestic effluents and agricultural runoff, and the inflow streams were the main medium of nutrients transfer.The characteristic of P sorption by sediment was studied. The sorption process was divided into fast step and slow step, and the reaction was almost completed in the first 12 hours. Using the modified Langmuir adsorption model could fit well with the P sorption process, and according to the fitting results, the maximum P adsorption capacity (Smax) in different sediments were ranged from 0.61 mg g-1 to 1.13 mg g-1, and this capacity was positively correlated with the proportion of smaller particles size in surface sediment. The zero equilibrium P concentration values (EPC0), ranging from 0.038 mg L-1 to 0.193 mg L-1, were significantly higher than concentrations of orthophosphate in overlying water, which indicated that sediments in the West Lake tended to act as "source" and kept the release of P to the overlying waterThe microbial diversity and the distribution of PSBs in sediments were analyzed. The differences in microbial diversity and population among seven sampling sites in the West Lake were not obvious, but it showed significant spatial differences between the West Lake and Xixi Wetland. The lowest value of Shannon index was appeared in W1 site, and the index showed a decreasing trend from north to south in lake. Enumeration and molecular identification of PSB strains indicated that these bacterial groups were abundant in the lake ecosystem and various kinds of bacteria participated in P release process. Twelve PSBs, including eight organic P-solubilizing bacteria (OPBs) and four inorganic P-solubilizing bacteria (IPBs), which belonged to three different families, were isolated and identified. Cupriavidus basilensis was found for the first time to have the ability to mineralize OP. Laboratory tests on P release ability revealed that IPBs were more effective at releasing P than OPBs.The biological effect on P transformation process at the water-sediment interface was revealed, and the variations of biological effect under different conditions of pH, light, and temperature were discussed. A customized integrated equipment was designed and employed in study in order to maintain the samples in situ. The results showed that the decrements of TP in sediment in the non-sterilized systems were 2-6 times higher than those in the systems sterilized by Co60y-ray, which indicated that the biological activities strongly stimulated P release and its intensity could reach to over 4 times higher than that from the physico-chemical effect. The extent of biological effect did not only depend on extremely enrichment of biomass, strong bacterial activity could also play a significant role in P bio-transformation at the interface. The increasing temperature could enhance organism biomass and biological effect. Light was essential for the growth of algae and with the excessive absorption of P by algae, more P was released from the sediment. pH was detected to have no major effect on bacterial biomass and P cycling, but higher pH was propitious to the decomposition of OP and benefited algae to capture CO2.
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