Carbonate weathering as a driver of CO2 supersaturation in lakes

Most lakes and reservoirs have surface CO2 concentrations that are supersaturated relative to the atmosphere. The resulting CO2 emissions from lakes represent a substantial contribution to the continental carbon balance. Thus, the drivers of CO2 supersaturation in lakes need to be understood to constrain the sensitivity of the land carbon cycle to external perturbations. Carbon dioxide supersaturation has generally been attributed to the accumulation of inorganic carbon in lakes where respiration exceeds photosynthesis, but this interpretation has faced challenges. 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. © 2015 Macmillan Publishers Limited. All rights reserved