Eutrophication-driven autochthonous organic carbon production decreases carbon burial efficiency in impoundments

Impoundments are critical sites of organic carbon (OC) sequestration, with ponds emerging as hotspots due to their exceptionally high burial rates. However, the mechanisms sustaining these rates-whether enhanced OC inputs or suppressed mineralization-remain unclear. Here, we combine sediment traps, stable isotope sourcing, incubation experiments, and reactivity continuum modeling to compare OC dynamics in eutrophic ponds versus non-eutrophic reservoirs. Despite similar sedimentary OC content, ponds exhibited a 2.4-fold higher deposition flux, driven by greater sediment accumulation. Stable isotopes revealed autochthonous phytoplankton dominance (69 %) in ponds, contrasting with terrestrial-dominated reservoirs (61 %). Incubation data showed ponds mineralized OC 2.1 times faster than reservoirs, with higher CH₄/CO₂ production ratios (0.64 vs. 0.47), reflecting the lability of algal-derived OC. Modeling confirmed ponds bury more absolute OC but with lower efficiency (OCBE) due to rapid degradation. We demonstrate that ponds' high OC burial stems from elevated inputs-particularly phytoplankton production-rather than reduced mineralization losses. However, this eutrophication-driven production paradoxically reduces OCBE by supplying labile carbon while amplifying CH₄ emissions that may offset burial benefits. These results highlight how eutrophication transforms small impoundments into high-throughput, low-efficiency carbon processors, with important implications for managing both carbon sequestration and greenhouse gas emissions in freshwater systems.