Lake eutrophication increases unprotected organic carbon pools in sediments

Reactive iron oxide-bound organic carbon (Fe–OC) is a key pathway for long-term carbon burial, but its effectiveness under increasingly widespread eutrophication remains uncertain. Here we show that Fe–OC accounts for 16% of total organic carbon and buries at ~14 Mt C yr −1 in lakes globally, equivalent to ~27% of the marine Fe–OC sink despite lakes covering only ~1.25% of marine area. However, bloom-prone lakes exhibit lower Fe–OC content and contribution to sedimentary organic carbon than non–bloom-prone lakes. Field observations and incubation experiments further reveal that bloom-induced redox fluctuations and low-aromaticity algal organic matter inputs jointly accelerate Fe–OC turnover and reduce organic carbon loading on Fe minerals, causing Fe–OC losses of up to ~68%. These findings suggest that eutrophication weakens the long-term carbon burial capacity of lakes by increasing unprotected organic carbon pools in sediments, necessitating reevaluation of their contributions to the global carbon budget.