Thermal stratification leads to significant stratification characteristics of hydrochemistry and aquatic organisms in reservoirs, and thus affects the biogeochemical cycle in the reservoir. This study aims to understand physico-chemical properties and dissolved inorganic carbon change processes and its factors in a karst groundwater-fed reservoir, Dalongdong Reservoir, located in Shanglin County, Guangxi Zhuang Autonomous Region, China. The eight sampling points were placed along the direction of the water flow on June 19-21, 2015. The results show that: ① The reservoir exhibited obvious thermal stratification in the summer. There were significant differences in physical and chemical parameters, such as pH and conductivity (Spc) between the epilimnion and thermocline; ② The dissolved oxygen (DO) and chlorophyll a (Chl-a) content from the surface to the bottom did not show a single decreasing trend, but the maximum value occurred 2.5 m or 5 m below the surface; ③ From the surface to the bottom, dissolved inorganic carbon (DIC) concentrations showed an increasing trend with the average DIC concentration of 2.03 mmol·L-1 in the epilimnion and the average DIC concentration of 4.18 mmol·L-1 at the bottom of the thermocline. The value of stable carbon isotope (δ13CDIC) was more positive in the epilimnion than in the thermocline, where δ13CDIC gradually became partially negative with water depth. Possible reasons of these results include: ① The significant differences in temperature, distribution of aquatic organisms, and strength and direction of metabolisms in different water layers due to thermal stratification; ② The DIC variations in the epilimnion were mainly affected by the carbonate precipitation process and phytoplankton photosynthesis, thereby affecting the DIC stable isotope fractionation. DIC was mainly controlled by biological respiration and the organic matter decomposition process in the thermocline.
Keywords: DIC; karst reservoir; metabolism; thermal stratification; δ13CDIC.