Carbonate weathering as a driver of CO₂ supersaturation in lakes

Abstract

Most lakes and reservoirs have surface CO₂ concentrations that are supersaturated relative to the atmosphere¹. The resulting CO₂ emissions from lakes represent a substantial contribution to the continental carbon balance^2,3,4. Thus, the drivers of CO₂ supersaturation in lakes need to be understood to constrain the sensitivity of the land carbon cycle to external perturbations^4,5,6. Carbon dioxide supersaturation has generally been attributed to the accumulation of inorganic carbon in lakes where respiration exceeds photosynthesis^7,8, but this interpretation has faced challenges^9,10,11. Here we report analyses of water chemistry data from a survey of Spanish reservoirs that represent a range of lithologies, using simple metabolic models. We find that, above an alkalinity threshold of 1 mequiv. l⁻¹, CO₂ supersaturation in lakes is directly related to carbonate weathering in the watershed. We then evaluate the global distribution of alkalinity in lakes and find that 57% of the surface area occupied by lakes and reservoirs—particularly in tropical and temperate latitudes—has alkalinity exceeding 1 mequiv. l⁻¹. We conclude that lake inputs of dissolved inorganic carbon from carbonate weathering should be considered for the CO₂ supersaturation of lakes at both regional and global scales.