[Concentration and Carbon Isotope Composition of DOC and DIC in the Miyun Reservoir Watershed in Summer]

This study aims to complement existing research on the carbon cycle of water in reservoirs, for the effective control of nutrient input in drinking water. The content and isotopic compositions of water dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) from the Miyun Reservoir watershed in Beijing were studied, and water from the Jingmi diversion canal was also studied as a baseline reference. The results showed that the content of DOC in the Miyun Reservoir watershed in summer varied from 1.07 to 5.19 mg·L^-1, with an average value of 2.61 mg·L^-1. DOC in tributaries was lower than that of most rivers in China and globally, while DOC in reservoirs, particularly in Miyun Reservoir, was high. The high water level might be the main reason for the high DOC in the Miyun Reservoir. In summer, the δ¹³C_DOC in the Miyun Reservoir watershed ranged from -27.4‰ to -24.3‰, with an average value of -25.8‰. Results from the isotopic analysis showed that the water in the Miyun Reservoir watershed was less affected by human sources, with the exception of some points. In summer, DOC in tributaries and in the Miyun Reservoir was mainly derived from soil organic matter and terrestrial C3 plants. In addition, endogenous substances also contributed to DOC in the watershed. DOC concentration increased along the Chao and Bai Rivers, and DOC from soil erosion and human sources was higher in the Chao River. The δ¹³C_DIC in the study area varied from -12.6‰ to 5.75‰, with a mean value of -9.44‰. The weathering of carbonate rocks accounted for the major river DIC, and DIC had been clearly assimilated by the phytoplankton by photosynthesis. DOC and DIC in the Miyun Reservoir were significantly higher than in the Jingmi diversion canal (P<0.01), and the δ¹³C_DIC in the Miyun Reservoir showed a clear positive deviation. This indicated that there were differences in carbon concentration and components in the two kinds of water, which might affect the denitrification capacity of the water in the Miyun Reservoir after mixing. In general, the dissolved carbon in the water of the Miyun Reservoir watershed is less affected by human sources. The conversion of DIC to DOC may also be a potential source of DOC in the study area.