Turnover of dissolved organic carbon fuels nocturnal CO_(2) emissions from a headwater catchment reservoir, Southeastern China: Effects of ecosystem metabolism on source partitioning of CO2 emissions

Dam reservoirs in headwater catchments, as critical zones for their proximity to terrestrial sources, play important roles in dissolved organic carbon(DOC) cycling. However, the effects of ecosystem metabolism(EM) on DOC cycling are not well known. Here, in-situ diurnal and monthly observations were conducted to measure EM(including gross primary production(GPP), ecosystem respiration(ER) and heterotrophic respiration(HR)), DOC turnover and CO_(2)emissions in a headwater catchment reservoir in Southeastern China in 2020. Our study showed the nocturnal CO_(2)emission rate was about twice as high as in daytime, and was strongly driven by EM. The values for DOC turnover velocity ranged from 0.10 to 1.59 m/day,and the average DOC turnover rate was 0.13 day-1, with the average removal efficiency of 12%. The contribution of respired DOC to daily CO_(2)emissions ranged from 17% to 61%. The accumulated efficiencies were estimated to be 13% for the selected 15 reservoirs throughout the Changjiang River network, corresponding to about 0.34 Tg C/year of the respired DOC.The modified CO_(2)flux was 0.75 Tg C/year, and respired DOC accounted for about 45% of total emitted CO_(2)from the 15 larger reservoirs. Our research emphasizes the necessity of incorporating the effects of EM into studies of reservoir DOC removal and COemissions.