Salinity decreases methane concentrations in Chinese lakes

Lakes are important sources of methane (CH₄), and understanding the influence of environmental factors on CH₄ concentration in lake water is crucial for accurately assessing CH₄ emission from lakes. In this study, we investigated CH₄ concentration in two connected Tibetan Plateau lakes, Lake Keluke (an open freshwater lake) and Lake Tuosu (a closed saline lake), through in-situ continuous measurements taken in different months from 2021 to 2023. The results show substantial spatial and seasonal variations in CH₄ concentrations in the two lakes, while the CH₄ concentrations in Lake Keluke are consistently higher than those in Lake Tuosu for each month. Despite sharing similar environmental conditions due to connected (e.g. pH, water temperature, dissolved oxygen content, and total organic carbon content), the critical difference between the two lakes is their salinity. This implies that salinity is the critical factor contributing to the decrease in CH₄ concentrations in Lake Tuosu, possibly due to the changes in microbial species between freshwater and brackish/saline lakes. Additionally, to further validate the effect of salinity on CH₄ concentrations in lake water, we compared the CH₄ concentrations of 33 lakes (including 5 saline lakes and 28 freshwater lakes) from the Tibetan Plateau, Chinese Loess Plateau, and Yangtze Plain, and found that saline lakes consistently exhibit lower CH₄ concentrations (avg. 0.08 μmol/L), while freshwater lakes generally display higher CH₄ concentrations (avg. 1.25 μmol/L) with considerable fluctuations. Consequently, freshwater and saline lakes exhibit distinct CH₄ emissions, which could be used for more accurate estimation of global CH₄ emission from lakes.