Abstract

The carbon dioxide flux at the air/water interface in lakes was calculated after the determination of H₂CO₃* (free CO₂) and atmospheric CO₂ using flow injection analysis (FIA) coupled to a conductometric detector. The method is based on the diffusion of CO₂ through a hydrophobic membrane into a flow of deionized water, generating a gradient of conductivity proportional to the concentration of CO₂ in the sample. Using one experimental set-up, the speciation of the inorganic carbon (H₂CO₃* and dissolved inorganic carbon) was accomplished by simply adjusting the sample pH. The determination of CO₂ in the atmosphere was carried out by direct injection of the gaseous samples. The FIA apparatus was taken into the field and CO₂ fluxes were evaluated in several Brazilian lakes. In these lakes, representing different eutrophic stages, the CO₂ flux varied from −242 (invasive) up to 3227 (evasive) µmol CO₂ m⁻² h⁻¹.