Impact of chemically enhanced diffusion on dissolved inorganic carbon stable isotopes in a fertilized lake

At high pH the chemical reaction of CO 2 with OH − can significantly increase the mass transfer of CO 2 between air and water. The reaction of CO 2 with OH − strongly fractionates carbon isotopes in comparison to simple diffusion. These processes, chemically enhanced diffusion (CED) and chemically enhanced fractionation (CEF), greatly influence the carbon budgets and carbon isotope ratios for water bodies with high pH. Using floating chambers, we estimated mass transfer coefficients for CO 2 and a nonreactive gas, CH 4 , in an experimentally eutrophied lake. The mass transfer coefficient estimated from CH 4 flux did not vary greatly between measurements ( k 600 = 1.83 ± 0.33 cm h −1 ; mean ±1 SD) and agreed well with other independent estimates of mass transfer. The mass transfer coefficient of CO 2 , however, was chemically enhanced by 3.5‐ to 7.5‐fold. This enhancement was related to pH and temperature but was slightly higher than predictions from an existing model. We determined the role of CEF by modifying a model of CED to include both carbon isotopes ( 12 C and 13 C). A whole‐lake addition of inorganic 13 C to Peter Lake created dynamics in δ 13 C‐dissolved inorganic carbon (DIC) and provided a test of the new model. The value of δ 13 C‐DIC decreased from approximately −9‰ to −21‰, a result that was well predicted by the model including CEF but could not be duplicated when CEF was omitted. Thus CED and CEF influenced the mass balance of air‐water CO 2 exchange and had isotopic consequences for DIC. Although CEF is considered inconsequential for mean oceanic conditions, this model could be applied to marine systems for inorganic carbon modeling in areas where pH is elevated or physical mass transfer is limited because of low turbulence.