The Influence of Seasonal Variability of Eutrophication Indicators on Carbon Dioxide and Methane Diffusive Emissions in the Largest Shallow Urban Lake in China

Eutrophication is prevalent in urban lakes; however, a knowledge gap exists regarding eutrophication influences on carbon dynamics in these ecosystems. In the present study, we investigated the carbon dioxide (CO₂) and methane (CH₄) concentration and diffusion fluxes in Lake Tangxun (the largest shallow Chinese urban lake) in the autumn and winter of 2022 and spring and summer of 2023. We found that Lake Tangxun served as a source of GHGs, with average emission rates of 5.52 ± 12.16 mmol CO₂ m⁻² d⁻¹ and 0.83 ± 2.81 mmol CH₄ m⁻² d⁻¹, respectively. The partial pressure of dissolved CO₂ (<i>p</i>CO₂) (averaging 1321.39 ± 1614.63 μatm) and dissolved CH₄ (<i>d</i>CH₄) (averaging 4.29 ± 13.71 μmol L⁻¹) exceeded saturation levels. Seasonal variability was observed in the <i>p</i>CO₂ and <i>d</i>CH₄ as well as CH₄ fluxes, while the CO₂ flux remained constant. The mean <i>p</i>CO₂ and <i>d</i>CH₄, as well as carbon emissions, were generally higher in summer and spring. <i>p</i>CO₂ and <i>d</i>CH₄ levels were significantly related to total nitrogen (TN), total phosphorus (TP), and ammonium-nitrogen (N-NH₄⁺), and N-NH₄⁺ was a main influencing factor of <i>p</i>CO₂ and <i>d</i>CH₄ in urban eutrophic lakes. The positive relationships of <i>p</i>CO₂, <i>d</i>CH₄ and trophic state index highlighted that eutrophication could elevate CO₂ and CH₄ emissions from the lake. This study highlights the fact that eutrophication can significantly increase carbon emissions in shallow urban lakes and that urban lakes are substantial contributors to the global carbon budget.