Relationship between eutrophication and greenhouse gases emission in shallow freshwater lakes

In shallow lakes, there are complex relationships between lake eutrophication and greenhouse gas emissions that deserve to be studied, which are important for solving lake eutrophication, slowing down climate warming, and reducing carbon emissions. In order to explore the relationship and mechanism between eutrophication and greenhouse gases (GHGs), the net GHGs emission flux and transformation of carbon, and nitrogen in 45 shallow freshwater lakes were investigated from May to September 2022. Eutrophication facilitated potential denitrification rate (D_t) without increasing nitrous oxide (N₂O) production based on the significantly positive relationship between eutrophication and D_t. This should be attributed to the shift from incomplete (N₂O producing process) to complete denitrification (N₂ producing process). Compared to NarG mediating nitrate (NO₃^-) to nitrite (NO₂^-), fewer eutrophication indicators showed a positive relationship with NosZ mediating N₂O to N₂, suggesting that more stringent conditions are required for complete denitrification, which was achieved in the lakes we investigated. Optimal reduction in net carbon dioxide (CO₂) emissions occurs at high levels of primary productivity, as indicated by the V-shaped relationship between chlorophyll a (Chl a) and CO₂ emissions. However, in hyper-eutrophic lakes, there is an upward trend in CO₂ production. The possible explanations should include CO₂ production and fixation as well as methane (CH₄) oxidation. The bell-shaped relationship between the net flux of CH₄ emission and Chl a could be explained that CH₄ was heavily oxidized due to sufficient oxygen caused by algal bloom. This fact gave evidence for the increase of the net flux of CO₂ emission in high primary productivity lakes. Therefore, the relationship and mechanism between net GHGs emission flux and eutrophication remained complex and various.