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

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.