Carbon biogeochemical cycle is enhanced by damming in a karst river

Highlights: • After damming, carbon species developed obvious stratification in water profile. • Carbon species showed larger variations in reservoir than in corresponding river. • After damming, biological activity became key factor and accelerated carbon cycle. Currently, there is a lack of systematic knowledge concerning carbon (C) biogeochemical cycles in impounded rivers. In this study, we investigated different C species and related environmental factors from July 2007 to June 2008 and from May 2011 to May 2012 in the impounded Wujiang River, SW China to understand the influence of dam construction on the riverine C cycle. The results showed that average concentrations of dissolved CO₂, dissolved inorganic C (DIC), dissolved organic C, and particulate organic C (POC) were 81.73 μmol/L, 2283.55 μmol/L, 158.11 μmol/L, and 37.54 μmol/L, respectively. Meanwhile, δ¹³C_DIC ranged from − 10.07‰ to − 4.92‰ with an average of − 8.33‰, while δ¹³C_POC ranged from − 35.30‰ to − 22.28‰ with an average of − 29.20‰. Thermal and chemical stratifications developed seasonally and exerted a significant influence on the C cycle of the released water. The C species and related δ¹³C showed remarkable heterogeneity in time and space. Evidence from δ¹³C demonstrated that the C system in this river was primarily influenced by carbonate weathering, whereas in the reservoir, it was primarily controlled by algal activity. The coefficients of variance for different C species in the reservoir and released water were higher than those in the river. Our study indicated that biological activity became a key controlling factor for the C biogeochemical cycle and accelerated it after damming, especially in the warm seasons. The results of this study have important implications for understanding the C cycle in elongated and deep reservoirs.