Cyanobacteria decay alters CH and CO 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 and CO 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 and CO, 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 and CO emissions, notably in Lake Taihu, where fluxes peaked at 929.9 and 7222.5 μmol/m·h, mirroring dissolved gas levels in overlying waters. Increased sedimentary organic carbon raised dissolved CH and CO concentrations in pore-water, with isotopic tracking showing cyanobacteria-derived carbon specifically elevated CH and CO 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 and CO production in surface sediments. In the microcosm systems with the most severe cyanobacteria accumulation, average CH and CO 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 and CO 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.