Catchment productivity controls CO2 emissions from lakes.

Most lakes are oversaturated with CO₂ and are net CO₂ sources to the atmosphere, yet their contribution to the global carbon cycle is poorly constrained. Their CO₂ excess is widely attributed to in-lake oxidation of terrestrially produced dissolved organic carbon. Here we use data collected over 26 years to show that the CO₂ in 20 lakes is primarily delivered directly through inflowing streams rather than being produced in situ by degradation of terrestrial carbon. This implies that high CO₂ concentrations and atmospheric emissions are not necessarily symptoms of heterotrophic lake ecosystems. Instead, the annual mean CO₂ concentration increased with lake productivity and was proportional to the estimated net primary productivity of the catchment. Overall, about 1.6% of net primary productivity (range 1.2-2.2%) was lost to the atmosphere. Extrapolating globally, this is equivalent to CO₂ losses of ∼0.9 Pg C yr⁻¹ (range 0.7-1.3), consistent with existing estimates. These data and our catchment productivity hypothesis re-enforce the high connectivity found between lakes, their catchment and the global C cycle. They indicate that future concentrations of CO₂ in lakes, and losses to the atmosphere, will be highly sensitive to altered catchment management and concomitant effects of climate change that modify catchment productivity. [ABSTRACT FROM AUTHOR]