[Source and spatio-temporal variation characteristics of dissolved inorganic carbon in Wanfenghu Reservoir, China]

The flux and form of dissolved inorganic carbon (DIC), an important part of carbon budget, play a key role in the biogeochemistry of aquatic ecosystem. By analyzing physicochemical parameters and water DIC and δ¹³C_DIC characteristics in Wanfenghu Reservoir, we examined the behavior and source of DIC. In the epilimnion, water pH in the entire reservoir was conservative, being weakly alkaline. Nitrate (NO₃^--N) had the maximum coefficient of variation and a high spatio-temporal variation. Due to the dilution effect, the lowest values of electrical conductivity (EC), partial pressure of carbon dioxide (pCO₂) and DIC appeared during the summer high flow phase. On the water column in summer, redox potential (Eh) and NO₃^--N did not change with water depth, while other indicators changed significantly, with greatest variation in the thermocline. Water temperature (T), pH and Eh all decreased with increasing water depth in both seasons, while pCO₂ showed an opposite trend. Water EC, total alkalinity (TA), and DIC decreased with increa-sing water depth in summer, but with a smaller gradient of change in winter. The DIC in water was negatively correlated with water pH and Eh, while positively correlated with EC and pCO₂ in both seasons. 2) The concentration of DIC was 2.66-4.9 mmol·L^-1 in summer and 3.38-4.52 mmol·L^-1 in winter. During the period of thermal stratification, the variation gradients of DIC and δ¹³C_DIC in the thermocline were most significant. DIC was positively correlated with δ¹³C_DIC of epilimnion in summer. DIC was negatively correlated with δ¹³C_DIC in epilimnion in winter and on water column in both summer and winter. However, the variation of DIC and δ¹³C_DIC with water depth was not obvious in winter. 3) In summer, δ¹³C_DIC was -7.71‰- -1.38‰, indicating that the dissolution of carbonate minerals was dominant. In winter, δ¹³C_DIC was -16.93‰- -9.44‰, signifi-cantly lower than that in summer but with a wider range, indicating biological input of CO₂ and mineralization of organic matter were the main sources. The δ¹³C_DIC varied significantly in different seasons and water depths because of differences in carbon sources and changes in the relative contribution proportion of carbon sources.