The breath of the rocks: Lake carbon dioxide emissions from weathering processes at the global scale

Abstract:

Most lakes and reservoirs are known to have surface carbon dioxide (CO₂) concentrations that are supersaturated with respect to the atmosphere, and hence nearly all of them are net emitters of CO₂. Global carbon emissions from lakes account for 0.06 to 0.84 Pg C year^-1, a substantial amount relative to other fluxes of the continental C balance. Therefore, a proper understanding of the land carbon cycle and its sensitivity to external perturbations requires detailed knowledge of drivers of global CO₂ supersaturation in lakes. CO₂ supersaturation has generally been attributed to a widespread imbalance of lake net ecosystem production towards net heterotrophy, but recent findings challenge this interpretation. Here we show that an integrated perspective including lake net ecosystem production together with precipitation and dissolution of carbonate minerals and inputs of dissolved inorganic carbon from the watershed, substantially improves our understanding of the processes leading to CO₂ supersaturation in lakes with alkalinity above 1 meq L^-1. Our results indicate that CO₂ supersaturation is independent of net ecosystem production in many lakes, and that a significant amount of the CO₂ evaded through their surface is directly related to weathering processes in the watershed that supply alkalinity to surface waters. After evaluation of the worldwide distribution of alkalinity across lakes we show that CO₂ emissions related to weathering processes are relevant in tropical and temperate latitudes, but negligible in boreal regions.