Fluxes in CO2 and CH4 and influencing factors at the sediment-water interface in a eutrophic saline lake

Although saline aquatic ecosystems are significant emitters of greenhouse gases (GHGs), dynamic changes in GHGs at the sediment-water interface remain unclear. The present investigation carried out a total of four sampling campaigns in Daihai Lake, which is a eutrophic saline lake situated in a semi-arid area of northern China. The aim of this study was to investigate the spatio-temporal dynamics of carbon dioxide (CO2) and methane (CH4) fluxes at the sediment-water interface and the influencing factors. The mean concentrations of porewater CO2 and CH4 were 44.98 ± 117.99 μmol L-1 and 124.36 ± 97.00 μmol L-1, far exceeding those in water column of 11.14 ± 2.16 μmol L-1 and 0.33 ± 0.23 μmol L-1, respectively. The CO2 and CH4 fluxes at the sediment-water interface (FS-WCO2 and FS-WCH4) exhibited significant spatial and temporal variations, with mean values of 9.24 ± 13.84 μmol m-2 d-1 and 3.53 ± 4.36 μmol m-2 d-1, respectively, indicating that sediment is the source of CO2 and CH4 in the water column. However, CO2 and CH4 fluxes were much lower than those measured at the water-air interface in a companion study (17.54 ± 14.54 mmol m-2d-1 and 0.50 ± 0.50 mmol m-2d-1, respectively), indicating that the diffusive flux of gases at the sediment-water interface was not the primary source of CO2 and CH4 emissions to the atmosphere. Regression and correlation analyses revealed that salinity (Sal) and nutrients were the most influential factors on porewater gas concentrations, and that gas fluxes increased with increasing gas concentrations and porosity. The microbial activity of sediment is greatly affected by nutrients and Sal. Additionally, Sal has the ability to regulate biogeochemical processes, thereby regulating GHG emissions. The present investigation addresses the research gap concerning GHG emissions from sediments of eutrophic saline lakes. The study suggests that controlling the eutrophication and salinization of lakes could be a viable strategy for reducing carbon emissions from lakes. However, further investigations are required to establish more conclusive results.