Nonlinear response of methane release to increased trophic state levels coupled with microbial processes in shallow...

Shallow lakes are a crucial source of methane (CH₄), a potent greenhouse gas, to the atmosphere. However, large uncertainties still exist regarding the response of CH₄ emissions to the increasing trophic levels of lakes as well as the underlying mechanisms. Here, we investigate the CH₄ emission flux from lakes with different trophic states in the middle and lower reaches of the Yangtze River basin, China to evaluate the effect of the trophic lake index (TLI) on CH₄ emissions. The mean CH₄ emission fluxes from mesotrophic, eutrophic, middle-eutrophic, and hyper-eutrophic lakes were 0.1, 4.4, 12.0, and 130.4 mg m^-2 h^-1, respectively. Thus, the CH₄ emission flux ranged widely and was positively correlated with the degree of eutrophication. The relative abundance of methanogens with respect to the total population for the mesotrophic, eutrophic, mid-eutrophic, and hyper-eutrophic states was 0.03%, 0.35%, 0.94%, and 1.17%, respectively. The biogeographic-scale pattern of lakes classified as each of these four trophic states indicated that CH₄ emissions could be well-predicted by the NH₄⁺-N concentration in the water column, as both NH₄⁺-N and CH₄ were produced during mineralisation of labile organic matter in lake sediment. In addition, the shift from clear to turbid water, which is an unhealthy evolution for lakes, was associated with a nonlinear increase in the CH₄ emissions from the studied lakes. In particular, the hypereutrophic lakes functioned as CH₄ emission hotspots. Our findings highlight that nutrient levels, as a potential facilitator of CH₄ emissions, should be considered in future research to accurately evaluate the greenhouse gas emissions from shallow lakes.