Sunlight‐induced carbon dioxide emissions from inland waters

Abstract The emissions of carbon dioxide (CO 2 ) from inland waters are substantial on a global scale. Yet the fundamental question remains open which proportion of these CO 2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55°N to 69°N in Sweden. The sunlight‐induced production of dissolved inorganic carbon (DIC) averaged 3.8 ± 0.04 g C m −2 yr −1 , which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 ± 1 kt C yr −1 was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO 2 emissions from Swedish lakes. With a median depth of 3.2 m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which corresponds to the absorbed photons. Therefore, the slope of the regression line represents the wavelength‐ and depth‐integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr −1 under overcast and clear sky, respectively. Consequently, these directly sunlight‐induced CO 2 emissions contribute up to about one tenth to the global CO 2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.