Eutrophication decreased CO₂ but increased CH₄ emissions from lake: A case study of a shallow Lake Ulansuhai

Eutrophic lakes, especially shallow eutrophic lakes, disproportionately contribute to greenhouse gas (GHG) emissions. To investigate the effects of eutrophication on GHG dynamics, we conducted field measurements every three months from January 2019 to October 2019 in Lake Ulansuhai, a shallow eutrophic lake (mean depth of 0.7 m) located in a semi-arid region in Northern China. We found that Lake Ulansuhai was a predominantly source of atmospheric carbon dioxide (CO₂); however, it converted to a CO₂ sink in July due to eutrophication. It was also a strong source of methane (CH₄) with a mean CO₂ emission of 35.7 ± 12.1 mmol m^-2 d^-1 and CH₄ emission of 5.9 ± 2.9 mmol m^-2 d^-1. The CO₂ concentrations in most sites and CH₄ concentrations in all sites were supersaturated, with the average partial pressure of CO₂ (pCO₂) being 654±34 μatm and the partial pressure of CH₄ (pCH₄) being 157±37 μatm. The partial pressures and emissions of the greenhouse gases exhibited substantial seasonal and spatial variations. The correlation analysis between the trophic level index and the partial pressure of the greenhouse gases indicated that eutrophication could significantly decrease the CO₂ emissions but increase the CH₄ emissions from the lake, resulting in a CH₄ and CO₂ emission ratio of approximately 2 in terms of global warming potential. Eutrophication decreased the pCO₂ in the lake and subsequently increased the pCH₄ due to nutrient input, thereby enhancing primary production. The results indicated that shallow eutrophic lakes in arid regions are strong sources of CH₄ and that eutrophication could alter the greenhouse gas emission patterns.