Articles | Volume 13, issue 12
Research article
23 Jun 2016
Research article |  | 23 Jun 2016

Source and flux of POC in a karstic area in the Changjiang River watershed: impacts of reservoirs and extreme drought

Hongbing Ji, Cai Li, Huaijian Ding, and Yang Gao

Abstract. Isotopes of carbon (C) and nitrogen (N) along with C / N ratios of particulate organic carbon (POC) were used to identify source and transformation of organic carbon in the suspended and surface sediments in a typical karstic watershed (the Wujiang River, an important tributary of the Changjiang River). Isotope data for suspended sediments indicate that POC was mainly derived from phytoplankton and C3-dominated soil with an increased contribution of phytoplankton in sites directly affected by the reservoir. In contrast, the POC in surface sediments was mainly derived from C3- and C4-dominated soil with little reservoir influence. The positive correlations of carbon and nitrogen isotopes between suspended and surface sediments indicated that these two carbon pools are tightly coupled. Our conservative estimation suggests that 1.17  ×  1010 g of POC is transported to the Three Gorges Reservoir during the study period in 2013. POC yield in the Wujiang River (0.13 t km−2 yr−1) is much lower than those of large rivers with a high abundance of carbonate minerals. Based on the distribution pattern of POC yield, it is inferred that carbonate minerals (lithology) do not contribute significantly to the riverine POC. The cascade of reservoirs and extreme drought had a significant influence on the POC flux in the Wujiang River.

Short summary
The mineral composition, C / N ratios as well as 13C and 15N, of POC was firstly analyzed in suspended and surface sediments in the Wujiang River after the Three Gorges Dam began impounding sediment in 2004. A comparison of POC yield was made between karstic rivers and non-karstic rivers to evaluate the influence of carbonate distribution on POC transport. Considering the cascade reservoir and climate in the Wujiang River, the impacts of reservoirs and extreme drought were estimated in this study.
Final-revised paper