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

• Divergent effects of eutrophication on CO2 and CH4 emissions were found. • Eutrophication decreased pCO2 and CO2 flux via increasing primary production. • Eutrophication increased pCH4 and CH4 emissions due to the supply of substance. • Global warming potential was dominated by CH4 in the eutrophic lakes. 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 (CO 2 ); however, it converted to a CO 2 sink in July due to eutrophication. It was also a strong source of methane (CH 4 ) with a mean CO 2 emission of 35.7 ± 12.1 mmol m −2 d −1 and CH 4 emission of 5.9 ± 2.9 mmol m −2 d −1 . The CO 2 concentrations in most sites and CH 4 concentrations in all sites were supersaturated, with the average partial pressure of CO 2 (pCO 2 ) being 654±34 μatm and the partial pressure of CH 4 (pCH 4 ) 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 CO 2 emissions but increase the CH 4 emissions from the lake, resulting in a CH 4 and CO 2 emission ratio of approximately 2 in terms of global warming potential. Eutrophication decreased the pCO 2 in the lake and subsequently increased the pCH 4 due to nutrient input, thereby enhancing primary production. The results indicated that shallow eutrophic lakes in arid regions are strong sources of CH 4 and that eutrophication could alter the greenhouse gas emission patterns.