Photoproduction of dissolved inorganic carbon in temperate and tropical lakes - dependence on wavelength band and dissolved organic carbon concentration

We have evaluated photoeffects of UV-B, W-A and PAR radiation on dissolved organic matter (DOM). Photochemical production of dissolved inorganic carbon (DIC) was measured in sterile lake water from Sweden and Brazil after 6 hours of sun exposure. Tubes were exposed to four solar radiation regimes: Full-radiation, Full-radiation minus UV-B, Full-radiation minus W-B and W-A (PAR) and darkness. In both areas, lakes with most DOC (varying between 3 and 40 mg C l(-1)) were highly humic, resulting in high UV-B attenuation coefficients (K-d = 5-466 m(-1)). Under Full-radiation, photooxidative DIG-production varied from 0.09 to 1.7 mg C l(-1) per 6 h, without W-B from 0.07 to 1.4 mg C l(-1) and with PAR only from 0.02 to 0.7 mg C l(-1). UV-B radiation explains a minor part (17%) of the photoooxidative DIG-production, while W-A and PAR have larger effects (39% and 44%, respectively). Photooxidation was proportional to DOG-content and DIG-production was positively related to decrease in DOC and to loss of absorbance at 250 nm. There was no significant difference in DOC and radiation normalized DIG-production between Swedish and Brazilian lakes. The UV-B dose during incubations was approximately 3 times higher in Brazil compared to Sweden, while UV-A and PAR doses were similar. We conclude that DOC from tropical and temperate freshwaters do not seem to differ with respect to sensitivity to photooxidation.