| 其他摘要 | Phosphorus is a main limiting agent for the primary production in the aquatic ecosystems, and is also an essential nutrient for lake eutrophication. Nowadays, lake eutrophication is a universal environmental problem which has imposed as a threat to water resource, human health and social sustainable development. Internal phosphorus loading by sediment release plays a big role in the trophic status of the lake water, and affects the lake restoration. Therefore, it is necessary to investigate the composition, form distribution and transformation of phosphorus in the sediments for a comprehensive understanding of phosphorus biogeochemical cycles and for effective control and management of lake eutrophication. However, most of those previous studies mainly focused on inorganic phosphorus (Pi) fractions and their bioavailbility. Little information is available on the geochemical characteristics of organic phosphorus (Po) and its role for lake eutrophication, despite constituting a large proportion of lake sediments.
The middle and lower reaches of Yangtze and Southwestern China Plateau are the major two areas not only of freshwater lakes, but also of eutrophication lakes in China. The objectives of this study are to investigate the forms, composition and vertical distributions of Po in the sediments of Meiliang Lake, Yuantouzhu Lake, Gonghu Lake, East Taihu Lake, Hongfeng Lake, Baihua Lake and Erhai Lake (those lakes representing different features, such as, drainage area, hydrodynamic condition, entironment, pollution degree) by applying conventional chemical sequential fractionation and solution 31P nuclear magnetic resonance (NMR) spectroscopy, the role of Po for lake eutrophication was also elucidated. Meanwhile, the relationship between phosphorus and organic matter was discussed using molecular fluorescence spectroscopy and high performance size exclusion chromatography. The conclusions have been reached as follows:
1. The recovery of Po ranged from 94.3% to 101.1%, with the average of 98.5±2.2% by using an improved soil Po fractionation scheme. This suggests that total Po in sediments was satisfactorily extracted with this procedure, due to the longer shaking time for labile Po and Po associated with humic substances and an additional step for residual Po determination, thus the results were relatively accurate for Po concentrations in lake sediments. The proportion of residual Po to total Po reached 29.9–57.2% in these sediments, which indicated that the procedure was feasible to fractionate the sediment Po, and laid a foundation for further investigation of Po forms in lake sediments.
2. Different distribution patterns of P fractions observed in those two different regions may be related to different lake types, different decomposition processes of organic matter and the transformation between Pi and Po fractions in different lake environments. The concentrations of Po fractions in the heavily polluted sediments were higher than those in moderately and no polluted sediments. It is suggested that Po should be paid more attention beside Pi for lake eutrophication.
3. The difference in the molecular weight distributions of organic matter in the various extracts from the sediments of Meiliang Lake, Yuantouzhu Lake and Gonghu Lake may suggest the composition of organic matter and Po differed. However, the similarity of the values of organic C/N and C/P and their three-dimensional fluorescence spectroscopy in these three sediments may elucidate their characteristics and relationships in Taihu Lake (shallow lake).
4. The results showed that the sediment extracts were dominated by orthophosphate (Ortho-P) and orthophosphate monoesters (Monoester-P), with smaller amounts of orthophosphate diesters (Diester-P), pyrophosphate (Pyro-P) and phosphonates by solution 31P NMR spectroscopy analysis. The relative distribution of Ortho-P for hypertrophic and shallow lake sediments was higher than that for eutrophic and mesotrophic and deep lake sediments, while the relative distribution of Monoester-P was the opposite indicated that various P compounds were involved in the P recycling in those lakes to different extents. In addition, the abundance of inositol phosphate in Monoester-P supported the high concentration of residual Po in sediment Po fractionation studies mentioned in earlier chapter.
5. The different profiles of TP、Pi、Ortho-P、Monoester-P、TOC and TN in Meiliang Lake and Erhai Lake sediments suggested that the mechanisms of decomposition and transformation of organic matter and phosphorus varied in different lake environments. This may explain the role of internal loading for lake eutrophication in these two lakes. Furthermore, the occurrence of DNA-P detected in the surface sediments is consistent with the distribution of DNA reported previously. It is suggested that DNA-P in sediments mostly originated from DNA of bacteria and microorganisms. |
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