The Seasonal Variation Of Dissolved Organic Carbon In The Typical Tributaries Of The Three Gorges Reservoir And The Influence On The Carbon Flux Of The Mainstream

Dissolved organic carbon (DOC) and other water environmental parameters, such as turbidity, temperature, chlorophyll-a,p(CO2) and pH, were monthly investigated in the mainstream of Three Gorges Reservoir (TGR) and its tributaries (Zhuyi River, Meixi River and Caotang River) in 2013, based on what the seasonal differences of the DOC concentrations and its sources were analyzed. Hydrodynamic parameters (flow velocity and flow direction) on the interfaces of the mainstream and the tributaries were monthly investigated by ADCP, then the inflow and outflow water fluxes were calculated. Through analyzing the distribution patterns of DOC concentrations and flow velocities on the interfaces, as well as the DOC concentrations of the mainstream and the tributaries, the DOC concentrations of the inflow and outflow water masses were determined. Based on that, the rate of DOC transport between the mainstream and the tributaries were calculated, and its influence on the DOC concentration of the mainstream was discussed. The results showed that:(1) The DOC concentrations of the mainstream ranged from 53.88 μmol/L to 128.82 μmol/L, and that of the tributary embayments ranged from 47.35 μmol/L to 205.49 μmol/L. The DOC concentrations in the TGR were high in summer and autumn, and low in spring and autumn. The DOC concentrations of the tributary embayments were almost above 100 μmol/L in summer and autumn, however, that were usually less than 100 μmol/L in winter and spring. Overall, the DOC concentrations of the tributary embayments were higher than that of the mainstream in spring, summer and autumn.(2) The inflow and outflow water masses coexisted in the tributary estuaries. Water exchange between mainstream and tributaries was significant, which in Zhuyihe River, Mei River and Caotang River ranged from 261.51 m3/s to 709.24 m3/s,392.22 m3/s to 672.35 m3/s and 657.04 m3/s to 1113.07 m3/s respectively. In most time of the year, the DOC of tributary was transported to the mainstream. However, the DOC of mainstream was transported to the tributary from October to December. In the all, the net transport of the DOC between the tributary and the mainstream was from the tributary to the mainstream. In 2013, the net flux of the DOC transpoted from Zhuyi River, Meixi River and Caotang River to the mainstream was 25.4 × 106 mol/yr、106.1 ×106 mol/yr、118.0×106 mol/yr respectly.(3) Compared with the DOC transport between the mainstream and the tributaries of the TGR, the infulence of the tributary upstream water on the DOC concentration of the tributary embayments is too weak to consider. By the contribution of carbon biosequestration in surface water, the DOC concentrations in tributary embayments were high than that of mainstream in spring, summer and autumn. In 2013, the flux of DOC transpoted from the tributaries to the mainstream was 4.1 × 109 mol/yr, accounted for 9.6 percent of the DOC flux of the mainstream. After impoundment, DOC concentrations grown up along the Yangtze River mainstream in the TGR (from Chongqing City to the TGD). It could be related to the DOC transport from tributaries to the mainstream.