Anthropogenic dissolved organic matter accumulation fuels greenhouse gas diffusive emissions in urban lakes along trophic state levels

Lakes are an important reservoir of dissolved organic matter (DOM) and also emit large amounts of greenhouse gases (GHGs), such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Yet, the linkages between quantity and quality of DOM and GHGs emissions are still not well understood. Hence, we investigated how DOM optical properties mediated dissolved GHGs levels and diffusive fluxes along a trophic state level in subtropical urban lakes, China. Our study showed that anthropogenic protein-like DOM (tryptophan- and tyrosine-like components) dominated the DOM pools, comprising 62% of total fluorescence DOM (FDOM). Partial least squares path modeling (PLS-PM) showed that the direct contribution of relative abundance of DOM to CO2 and CH4 diffusion in the highly-eutrophic state was higher than in the mesotrophic and lightly/moderately-eutrophic states. In particular, protein-like DOM primarily accounted for CO2 variations (44%-49%), while humic- and protein-like DOM contributed comparably to CH4 variations (38%-40% vs. 36%-40%). Furthermore, in the lightly/moderately- and highly-eutrophic states, nutrients contributed to DOM, CO2 and CH4 in both direct and indirect ways, while phytoplankton (chlorophyll a, Chl-a) negatively and directly affected carbon emissions. However, only nitrogen content directly contributed to N2O production and diffusion in the mesotrophic and lightly/moderately-eutrophic states. We concluded that protein-like DOM enrichment had the potential to trigger CO2 and CH4 diffusion and this process was largely mediated by nutrient levels. N2O diffusion was found to be mainly influenced by nitrogen loadings and forms. Our findings highlight the urgency for effective strategies to reduce anthropogenic DOM inputs and control nutrient levels to reduce GHGs diffusion in highly human-impacted aquatic systems.