Anthropogenically Induced Shift to Labile Phytoplankton-Derived Carbon Undermines Lake Carbon Burial Efficiency

Inland lakes are vital carbon sinks, yet their climatic benefit hinges on burial efficiency (BE)—the fraction of deposited organic carbon (OC) preserved long-term—beyond mere burial rates. Traditional flux-centric models treat BE as static, overlooking anthropogenic-driven sedimentary feedbacks and overestimating sequestration. Here, we introduce a dynamic BE framework, redefining BE as the ratio of observed to source- and age-predicted burial, isolating in-lake preservation controls. Applying this via compound-specific δ ¹³ C–δ ² H Bayesian mixing, kinetic mineralization modeling, and causal inference to two-century records from eutrophic Lakes Erhai and Chenghai (Southwest China), we reveal a paradox: OC burial rates surged 2.5–3-fold since ~1950, but dynamic BE declined ~20%. This stems from shifts to labile phytoplankton-derived carbon, promoting anoxia, weakening organo-mineral bonds, and favoring bacterial over fungal necromass, enhancing mineralization. Terrestrial inputs show offsetting effects. Proxy-validated extrapolation indicates similar declines may impact ~25% of perturbed lakes globally. Monte Carlo scaling suggests lake carbon burial is overestimated by up to one-fifth, implying unaccounted emissions. Macrophyte restoration offers a high-leverage strategy for bloom-prone systems. These findings advocate for broader adoption of dynamic BE frameworks to improve the accuracy of lake carbon sink projections under global change.