Intense methane diffusive emissions in eutrophic urban lakes, Central China

Urban lakes are hotspots of methane (CH₄) emissions. Yet, actual field measurements of CH₄ in these lakes are rather limited and our understanding of CH₄ response to urban lake eutrophication is still incomplete. In this study, we measured dissolved CH₄ concentrations and quantified CH₄ diffusion from four urban lakes in subtropical China during wet and dry seasons. We found that these lakes were constantly CH₄-saturated, contributing the greenhouse gas (GHG) to the atmosphere. Nutrient enrichment significantly increased CH₄ concentrations and diffusive fluxes. Average CH₄ flux rate in the highly-eutrophic lake zones (4.18 ± 7.68 mmol m^-2 d^-1) was significantly higher than those in the mesotrophic (0.19 ± 0.18 mmol m^-2 d^-1) and lightly/moderately-eutrophic zones (0.72 ± 2.22 mmol m^-2 d^-1). Seasonally, CH₄ concentrations and fluxes were significantly higher in the wet season than in the dry season in the mesotrophic and the lightly/moderately-eutrophic lake zones, but an inverse pattern existed in the highly-eutrophic lake zones. CH₄ concentrations and fluxes increased with elevated levels of nitrogen, phosphorus and dissolved organic carbon (DOC). The accumulation of nutrients provided autochthonous substrate for methanogenesis, indicated by a negative correlation between CH₄ and the C:N ratio. Ammonium-nitrogen (NH₄⁺-N) was the best predictor for spatial fluctuation of CH₄ concentrations and diffusive fluxes in the mesotrophic and the lightly/moderately-eutrophic lake zones, while total nitrogen (TN) and total phosphorus (TP) levels showed the highest predictability in the highly-eutrophic lake zones. Based on the findings, we conclude that nutrient enrichment in urban lakes can largely increase CH₄ diffusion, and that urban sewage inflow is a key concern for eutrophication boosting CH₄ production and diffusive emission. Furthermore, our study reveals that small urban lakes may be an important missing source of GHG emissions in the global C accounting, and that the ratio of littoral-to-pelagic zones can be important for predicting lake-scale estimation of CH₄ emission.