CO₂ storage and alkalinity trends in lakes

Greater transfer of CO₂ from the atmosphere to lakes, as a first step towards its storage in lake sediments, can be driven by an increase in lake-water alkalinity. Higher alkalinity values in fresh-water lakes can be attained through biological productivity, carbonate-mineral dissolution, reduction reactions in water and sediments, or input of chemical bases. Alkalinity increase in lakes owing to the proton consumption in biogeological reduction of iron, manganese, nitrogen and sulfur species is at least in part balanced by alkalinity reducing processes, such as oxidation and carbonate deposition. At present, lakes varying in composition from very dilute to highly saline are at. or near, an equilibrium with the atmospheric CO₂. On a global scale, transfer of about 1% of atmospheric CO₂ to fresh-water lakes would raise their dissolved carbonate concentration by several tens of milligrams per liter. The present near-annual increment of atmospheric CO₂ (2 × 10¹⁴ mole C yr^-1) could be taken up by a mean net primary productivity of about 850 mg C m^-2 yr^-1, a value that is typical of eutrophic and tropical lakes. A comparable CO₂ transfer to lakes may be attained by increases in alkalinity in the range 10^-4-10^-3 moll^-1 and/or in pH in the range from 7.0 to 8.5.