Cyanobacteria decay alters CH₄ and CO₂ 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 CH₄ and CO₂ 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 CH₄ and CO₂, this study examined the different trophic level lakes along the middle and lower reaches of the Yangtze River. The results demonstrated that eutrophication amplified CH₄ and CO₂ emissions, notably in Lake Taihu, where fluxes peaked at 929.9 and 7222.5 μmol/m²·h, mirroring dissolved gas levels in overlying waters. Increased sedimentary organic carbon raised dissolved CH₄ and CO₂ concentrations in pore-water, with isotopic tracking showing cyanobacteria-derived carbon specifically elevated CH₄ and CO₂ 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 CH₄ and CO₂ production in surface sediments. In the microcosm systems with the most severe cyanobacteria accumulation, average CH₄ and CO₂ 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 CH₄ and CO₂ 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.