In-situ measurement on air–water flux of CH4, CO2 and their carbon stable isotope in lakes of northeast Tibetan Plateau

Abstract

Inland waters are important sources of greenhouse gases (GHGs) to the atmosphere that may partially offset the terrestrial carbon sink. However, GHG emissions from high altitude saline lakes on the Tibetan Plateau are currently poorly constrained. In this study, we measured methane (CH4) and carbon dioxide (CO2) concentrations and their carbon stable isotope (δ13C), and calculated diffusive water-to-air fluxes from two saline high-altitude lakes, Qinghai Lake and Hala Lake, located in northeast Tibetan Plateau in October 2020. The two lakes were mostly supersaturated in CH4 (89.8%–4623.9% sat) and acted as sources of CH4 to the atmosphere. Conversely, more than 96% investigated area of Qinghai Lake and all investigated area of Hala Lake was a sink of CO2. The average diffusive fluxes of CH4 and CO2 in the surface waters of Qinghai Lake were 34.51 μmol m−2 d−1 and −1.29 mmol m−2 d−1, while diffusive fluxes of CH4 and CO2 in Hala Lake averaged 5.48 μmol m−2 d−1 and −5.24 mmol m−2 d−1, respectively. Salinity (Sal), fluorescent dissolved organic matter (fDOM), temperature (Temp) and dissolved oxygen (DO) are key factors for diffusive CH4 and CO2 flux in Qinghai Lake and Hala Lake. CH4 diffusive fluxes across water–air interface from Qinghai Lake and Hala Lake were found to be significantly lower than other freshwater lakes, likely due to their saline nature. A simple isotope mixing model (Miller–Tans plots) revealed a distinctly different isotopic source values of CO2 and CH4 in Qinghai Lake and Hala Lake. The CH4 production pathway of Qinghai Lake was mainly by the acetate fermentation, while that of Hala Lake was mainly by the CO2 reduction. The results show that the continuous measurement can help capture the spatial variability of GHGs fluxes in saline lakes.