Cyanobacteria decay alters CH 4 and CO 2 produced hotspots along vertical sediment profiles in eutrophic lakes.

Cyanobacteria-derived organic carbon has been reported to intensify greenhouse gas emissions from lacustrine sediments. However, the specific processes of CH ₄ and CO ₂ production and release from sediments into the atmosphere remain unclear, especially in eutrophic lakes. To investigate the influence of severe cyanobacteria accumulation on the production and migration of sedimentary CH ₄ and CO ₂ , this study examined the different trophic level lakes along the middle and lower reaches of the Yangtze River. The results demonstrated that eutrophication amplified CH ₄ and CO ₂ emissions, notably in Lake Taihu, where fluxes peaked at 929.9 and 7222.5 μmol/m ² ·h, mirroring dissolved gas levels in overlying waters. Increased sedimentary organic carbon raised dissolved CH ₄ and CO ₂ concentrations in pore-water, with isotopic tracking showing cyanobacteria-derived carbon specifically elevated CH ₄ and CO ₂ in surface sediment pore-water more than in deeper layers. Cyanobacteria-derived carbon deposition on surface sediment boosted organic carbon and moisture levels, fostering an anaerobic microenvironment conducive to enhanced biogenic CH ₄ and CO ₂ production in surface sediments. In the microcosm systems with the most severe cyanobacteria accumulation, average CH ₄ and CO ₂ concentrations in surface sediments reached 6.9 and 2.3 mol/L, respectively, surpassing the 4.7 and 1.4 mol/L observed in bottom sediments, indicating upward migration of CH ₄ and CO ₂ hotspots from deeper to surface layers. These findings enhance our understanding of the mechanisms underlying lake sediment carbon emissions induced by eutrophication and provide a more accurate assessment of lake carbon emissions.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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