Connections Between Daily Surface Temperature Contrast and CO2 Flux Over a Tibetan Lake: A Case Study of Ngoring Lake

To get a deep understand of the carbon cycle of Tibetan lakes, the CO2 flux over Ngoring Lake (NL), a large freshwater lake in the source region of the Yellow River, was monitored from 2011 to 2013 using an eddy covariance system. Based on the monthly mean CO2 flux, NL was a significant sink of atmospheric CO2 in 2012, with the greatest negative (downward) CO2 flux of -0.83 (+/- 0.27) g C m(-2) d(-1) occurring in October. While in 2011 and 2013, NL was in quasi-equilibrium with atmospheric CO2. The process controlling the daily variation in the CO2 flux was investigated using statistical analysis. The daily mean of the surface wind speed was significantly correlated with the observed daily CO2 flux. Under the assumption that the vertical gradient in the CO2 concentration near the water surface varied slowly, this correlation confirms the rationality of using the surface wind to express the gas transfer velocity. Following this consideration, because the thermal contrast between NL and the surrounding grassland was also significantly correlated with the daily CO2 flux, a new parameterization scheme of k(600) is suggested in this study. Potential mechanisms for the coupling process between the surface wind and the thermal contrast of the lake are discussed. Based on the surface energy balance analysis, the heat storage rate of the lake played different roles in the CO2 fluxes during the three years, depending on the CO2 saturation of the lake water. The process controlling the monthly mean CO2 flux was also investigated. Plain Language Summary There are more than 1,500 lakes on the Tibetan Plateau. Most of these lakes have not been thoroughly investigated, so little is known about the features of their carbon dynamics. To gain a better understand of the CO2 exchange over these lakes, an observation experiment was conducted over Ngoring Lake (a fresh water lake, with an area of 610.7 km(2)) from 2011 to 2013, and valuable data were obtained. By investigating these observations, we determined that the CO2 flux was negative (downward) during most of 2012, but it was in quasi-equilibrium in 2011 and 2013. Second, the surface temperature difference between the lake and the surrounding grasslands may have largely controlled the daily variation in the gas transfer velocity near the water's surface. Because the surface temperature around a lake can be conveniently and accurately monitored by satellite, this finding may be useful in the estimation of the surface CO2 flux over other remote lakes worldwide.