Eutrophication Drives Extreme Seasonal CO₂ Flux in Lake Ecosystems

Lakes process a disproportionately large fraction of carbon relative to their size and spatial extent, representing an important component of the global carbon cycle. Alterations of ecosystem function via eutrophication change the balance of greenhouse gas flux in these systems. Without eutrophication, lakes are net sources of CO₂ to the atmosphere, but in eutrophic lakes this function may be amplified or reversed due to cycling of abundant autochthonous carbon. Using a combination of high-frequency and discrete sensor measurements, we calculated continuous CO₂ flux during the ice-free season in 15 eutrophic lakes. We found net CO₂ influx over our sampling period in 5 lakes (− 47 to − 1865 mmol m⁻²) and net efflux in 10 lakes (328 to 11,755 mmol m⁻²). Across sites, predictive models indicated that the highest efflux rates were driven by nitrogen enrichment, and influx was best predicted by chlorophyll a concentration. Regardless of whether CO₂ flux was positive or negative, stable isotope analyses indicated that the dissolved inorganic carbon pool was not derived from heterotrophic degradation of terrestrial organic carbon, but from degradation of autochthonous organic carbon, mineral dissolution, and atmospheric uptake. Optical characterization of dissolved organic matter revealed an autochthonous organic matter pool. CO₂ influx was correlated with autochthony, while efflux was correlated with total nitrogen and watershed wetland cover. Our findings suggest that CO₂ uptake by primary producers during blooms can contribute to continuous CO₂ influx for days to months. Conversely, eutrophic lakes in our study that were net sources of CO₂ to the atmosphere showed among the highest rates reported in the literature. These findings suggest that anthropogenic eutrophication has substantially altered biogeochemical processing of carbon on Earth.