Carbon biogeochemical processes in a subtropical karst river-reservoir system

Cascade damming has seriously affected the hydrologic regimes and solutes transport of the river systems. However, the effect of damming on carbon cycle and the underlying mechanisms are still unclear. In this study, to reveal the effects of damming on riverine dissolved inorganic carbon (DIC) transport, the water chemistry and stable isotopic signature of dissolved inorganic carbon (delta C-1(3)Dic) were analyzed in the tributaries and four subtropical karst reservoirs on the Pearl River in southern China. From reservoir epilimnion to hypolimnion, DIC concentrations increased from 183.2 to 229.4 mg L-1, 167.6 to 192.0 mg L-1, 179.1 to 181.5 mg L-1, and 182.4 to 183.8 mg L(-1 )while delta C-1(3) mc decreased from -6.6 parts per thousand to -9.7 parts per thousand, -7.8 parts per thousand to -10.4 parts per thousand, -9.7 parts per thousand to -10.0 parts per thousand and - 9.9 parts per thousand to -10.0 parts per thousand in the Chaishitan (CST), Longtan (LT), Yantan (YT), and Dahua (DH) reservoirs, respectively. Our analyses suggest that primary production and organic matter degradation were two dominant processes in carbon evolution in the epilimnion and hypolimnion, respectively. However, there was no obvious spatial heterogeneity of solutes' concentration in the DH reservoir. Our results also suggested that cascade reservoirs had a regional damming effect and could be significantly weakened without damming or in daily regulated reservoirs. Furthermore, delta(1)(3)C(DIC )had a significant positive relationship with the reservoir effect index (Ri), which was affected by hydraulic retention time and water temperature in the water column of reservoirs and is useful to evaluate the thermal stratification strength. This study highlights that dam constructions affected carbon transport and transformation significantly, and the degree of the damming effect on the carbon cycle can be expressed by Ri.