Research On Optical Properties Of Water Body And Dissolved Organic Matters In Xiaojiang River Backwater Area, Three Gorges Reservoir

The Three Gorges Project has been completed in 2009. Since water level up to 135m in 2003, the water eutrophication of local waters in the Three Gorges Reservoir (TGR) area has become the most important environmental issue, and it has been paid much attention to. This thesis is focused on Xiaojiang River backwater area in TGR. The field sampling and measurement were conducted during May 2007~April 2009. According to the differences of optical indicators of water body in different seasons in the whole two years, the causing factors of seasonal variations of optical properties in the study area were analyzed, and the temporal changes of the optical properties of water body were also discussed. In addition, it discussed the absorption and fluorescence characteristics of color dissolved organic matters (CDOM) and tried to find the composition and sources of CDOM in the water body.(1) After the temporal and spatial analysis of the optical properties of water body in Xiaojiang River backwater area, it found that the spatial variation of Secchi Disc transparency (SD) was not obvious, but the seasonal variation was significant. Mean value of SD in spring was relatively high, and it maintained a lower level in summer. After the autumn, it increased and in winter it reached to the peak at high water level. Visible light attenuation coefficient (Kd(PAR)) was similar to SD, and it had no significant spatial differences, but the seasonal variation trend was opposite to transparency. The euphotic depth (Deu) had the similar spatial and temporal variations with SD.(2) The regression models between of SD and Kd(PAR), Deu in Xiaojiang River backwater area are as follows: SD = 1.0656 [Kd(PAR)] -1.1621 (R2 = 0.8449, N = 235, P <0.005) SD = 0.2907Deu- 0.2073 (R2 = 0.7655, N = 235, P <0.005) Transparency was significantly negative correlated with visible light attenuation coefficient, and it was positive correlated with euphotic depth. The correlations showed that SD can be used to simulate Kd(PAR) and Deu.(3) It can be seen from the correlation analysis between optical indicators and water quality indicators, the causing factors of optical properties of water body in Xiaojiang River backwater area had significant seasonal variation. The dominant factor in spring, summer and autumn was similar, and in general it was mainly affected by the concentration of total particulate matter (in particular the inorganic particulate matters, TIM) in water body. Dissolved organic matter has some impact, but chlorophyll a had little effect. By contrast, chlorophyll a concentrations become the dominant factor in winter, and the influence of suspended materials and dissolved matters on optical properties weakened.(4) The correlation analysis between conventional water quality indicators and optical indicators showed that, the optical properties of water body was mainly influenced by the total particulate matter, dissolved organic matter and chlorophyll a concentration and other factors. Further analysis of water level, water flow and other hydrological indicators in Xiaojiang Hekou suggested that, water flow, water level and rainfall were indirect factors that determined the optical characteristics of Xiaojiang River backwater area.(5) The absorption analysis of CDOM in Xiaojiang River backwater area showed that the Sg values were relatively high. There was some positive correlation between ag(355) and Sg of CDOM, especially in Quma and Gaoyang cross-sections. The correlation was significant, and it was as a result of the complex structure, the ratios of irregular different components and the Sg value from different fitting wavelengths.(6)The fluorescence analysis of CDOM in Xiaojiang River backwater area showed that the humic fluorescence intensity was highest in summer, then in autumn, and it was relatively low in winter and spring. Protein-like fluorescence peak intensity began to increase in early winter and spring, and the summer reached to the peak. Absorption coefficient ag(355) had significant correlations with humic fluorescence peak A, protein fluorescence peak B and peak C of CDOM in Xiaojiang River backwater area, indicating that absorption and fluorescence intensity can be reflected abundance and sources of dissolved organic matters in Xiaojiang River backwater area to some extent. The results of this thesis is valuable for the study of the impact of the early impounding of Three Gorges Reservoir on the tributaries water environment and aquatic ecosystems, and it can provide basic data for investigating the water eutrophication in Xiaojiang River backwater area.