Significant fraction of CO2 emissions from boreal lakes derived from hydrologic inorganic carbon inputs

Annual CO2 emissions from lakes and other inland waters into the atmosphere are estimated to almost entirely compensate the total annual carbon uptake by oceans(1-3). CO2 supersaturation in lakes, which results in CO2 emissions, is frequently attributed to CO2 produced within the lake4-8. However, lateral inorganic carbon flux through watersheds can also be sizeable(9-11). Here we calculated lake surface water CO2 concentrations and emissions using lake pH, alkalinity and temperature from a compilation of data from 5,118 boreal lakes(12). Autumn surface water CO2 concentrations and CO2 emissions from the 5,118 lakes co-varied with lake internal autumn CO2 production. However, using a mass balance approach we found that CO2 emission in the majority of lakes was sustained by inorganic carbon loading from the catchment rather than by internal CO2 production. Small lakes with high dissolved organic carbon and phosphorus concentrations, shorter retention times and longer ice-free seasons had the highest CO2 concentrations. CO2 emissions from these small lakes was twice that of comparable lakes in colder regions, and similar to emissions from subtropical and tropical lakes. We conclude that changes in land use and climate that increase dissolved inorganic carbon may cause emission levels from boreal lakes to approach those of lakes in warmer regions.