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 disso lved organic matter (DOM). Photochemical production of dissolved inorg anic carbon (DIC) was measured in sterile lake water from Sweden and B razil after 6 hours of sun exposure. Tubes were exposed to four solar radiation regimes: Full-radiation, Full-radiation minus UV-B, Full-rad iation 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, res ulting in high UV-B attenuation coefficients (K-d = 5-466 m(-1)). Unde r 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 h ave larger effects (39% and 44%, respectively). Photooxidation was pro portional to DOG-content and DIG-production was positively related to decrease in DOC and to loss of absorbance at 250 nm. There was no sign ificant difference in DOC and radiation normalized DIG-production betw een 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 tem perate freshwaters do not seem to differ with respect to sensitivity t o photooxidation.