Abstract
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.
Highlights
About 0.4 Gt of riverine organic carbon is transported to the world’s oceans each year, of which 0.15–0.17 Gt is particulate organic carbon (POC) (Hedges et al, 1997; Ludwig et al, 1996; Schlesinger and Melack, 1981)
Based on the analyses of δ13CPOC, δ15NTN, and C / N ratios in the suspended and surface sediments in the Wujiang River, this study identified source and flux of POC in the Wujiang River and examined the impacts of reservoirs and climate
The results indicated that POC in suspended particulate matter (SPM) was mainly derived from phytoplankton and C3-dominated soil with increased phytoplankton input in sites affected by reservoirs
Summary
About 0.4 Gt of riverine organic carbon is transported to the world’s oceans each year, of which 0.15–0.17 Gt is particulate organic carbon (POC) (Hedges et al, 1997; Ludwig et al, 1996; Schlesinger and Melack, 1981). Because different sources of POC are characterized by distinguished δ13CPOC, δ15NTN, and C / N ratios, these indicators have been widely used to constrain sources and transformation of riverine POC. Kendall et al (2001) used C / N ratios and δ15NTN as source criteria in 54 % of the samples in four large river systems across the USA, whereas Wu et al (2007) use δ13CPOC and δ15NTN to estimate the contribution of POC sources in the Changjiang River
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