Gan River Basin And The Wujiang River Dissolved Silicon Biogeochemical Characteristics And Control Factors

Dissolved silica( DSi )is one of the most important nutrient salts in the hydrosphere. It is an essential nutrient for the aquatic plant especially for diatomeae.Riverine DSi contributes to 80 percent of the annual input into the ocean. Yangtze River is the longest river in China and the third longest river in the world.Its nutrient export plays an important role in global and regional environmental issues. At present, there are relatively less studies of DSi biogeochemistry on Yangtze River tributaries. Ganjiang River and Wujiang River are the big branches in the middle and upper reaches of Yangtze River respectively.In this paper, our research objects are Ganjiang Watershed and Wujiang Watershed, starting from small watershed studies, based on the measured data and statistic analysis of the data which recorded at the major hydrological stations. We discuss the biogeochemistry characteristics of Dissolved Silica and its influencing factors in these two rigions. Conclusions are as follows:1. We Analyzed the composition of main ions collected from Gannan Watershed of Ganjiang River and Qianzhong watershed of Wujiang River ant the results shows that cations are dominated by Na+ and Ca2+ , anions are dominated by HCO3- and Cl- in Gannan Watershed and the concentration of dissolved silica(DSi) is relatively high, 5.52mg/L in average. All above represent one typical characteristic: the chemical weathering of silicate rock controlling the chemical components of the river. In Qianzhong Watershed,the situation is different. Cations are dominated by Ca2+, Mg2+ . Na +, K+ are relatively low, anions are dominated by HCO3- and SO42- in Qianzhong Watershed and the average concentration of dissolved silicon(DSi) is 1.94mg/L which is much lower than the DSi concentration in Gannan Watershed. The typical characteristic is the chemical weathering of carbonate rock controlling the chemical components of the river. The main source of DSi is the dissolving of albite in this two watersheds. The weathering is feeble and mainly occurs in the surface. The concentration of DSi in Gannan Watershed is mainly influenced by lithologic effect and the influence of biology is limited. Biological factors may influence the concentration of DSi in Qianzhong Watershed .2. The average value of chemical weathering rate in Ganjiang Watershed is 40.15 t/km2.a from 1966 to 1985;The value in Wujiang Watershed is 86.43 t/km2.a from 1968 to 1983,which is much higher than fomer rate. We use erosion modulus to estimate mechanical erosion rates in Ganjiang Watershed and Wujiang Watershed respectively and the results show that the average value of erosion modulus in Ganjiang Watershed from1966 to1985 is 142.18 t/km2.a; The value in Wujiang Watershed is 461.19 t/km2.a,which is much higher than the fomer value.3. There are similar notable seasonal variations of DSi in Ganjiang Watershed and Wujiang Watershed, DSi and water temperature are significantly correlated , but DSi and DO are negatively correlated. Yearly average DSi concentration of the main stations at Ganjiang River and Wujiang River presents declining trend, which is similar with Datong Station at Yangtze River and the main stations around the world.4. Analyzing Yearly average DSi concentrations, silt discharge, flow, rainfall, water and temperature at Han Linqiao Station of Ganjiang River and Wulong Station of Wujiang River, we found that DSi concentrations and water temperature positively correlate at these two stations and other factors that affecting DSi concentrations do not correlate in these two stations. The decline of the DSi concentration in Ganjiang watershed is likely to be contributed by the water conservancy project in this region. The declining of DSi in Wujiang River closely related to the emission of nitrogen resulting from human industry and agriculture activities. The reasons of DSi concentration decline in Ganjiang watershed and Wujiang Watershed are different from Datong Station of Yangtze River.5. Annual DSi transport is closely related to DSi annual content and annual runoff in Yangtze River and its tributaries of Ganjiang and Wujiang. Annual DSi transport in Ganjiang Watershed is mainly affected by the control of physical weathering(mechanical erosion) whereas the chemical weathering play an insignificant role..Annual DSi transport in Wujiang Watershed is controlled by both chemical weathering and physical weathering, but chemical weathering contributes more. At the same time, annual DSi transport in Wujiang Watershed is likely to be affected by acid rain.