PHOTOOXIDATIVE PRODUCTION OF DISSOLVED INORGANIC CARBON IN LAKES OF DIFFERENT HUMIC CONTENT

Photo-oxidation of dissolved organic carbon (DOG) to inorganic carbon (DIG) in five oligotrophic south Swedish lakes of different humic cont ent (3.9-19 mg DOC liter(-1), 0-140 mg Pt liter(-1), Secchi depth 7.6- 1.5 m) was investigated. Sterile-filtered (0.2 mu m) water was incubat ed in UV-transparent quartz tubes and in tubes covered with aluminum f oil. Samples were incubated at different depths (0, 0.20, 0.65, and 2. 00 m) from sunrise to sunset (0400 to 2200 hours) in July. Inorganic C was measured before and after incubation. Total plankton respiration in unfiltered lake-water (production of DIG) was also measured. At the surface, photo-oxidative DIC production was 86-410 mg C m(-3) d(-1), while plankton community respiration was 101-274 mg C m(-3) d(-1). Pho to-oxidation integrated over depth was 44-171 mg C m(-2) d(-1), while respiration was 201-547 mg C m(-2) d(-1) to a depth of 2 m and 398-850 mg C m(-2) d(-1) over the depth of the epilimnion. Depth-integrated p hoto-oxidation was independent of water color or DOC concentration in the lakes. Photooxidation was detected deeper than the penetration of UV-B radiation, indicating that longer wavelengths (UV-A and possibly PAR) are also active. DIC production was linearly related to loss of f luorescence (excitation 355 nm, emission 455 nm) in light-incubated sa mples. Our study shows that photo-oxidation of DOC may be an important process causing the regularly observed supersaturation of DIC in lake s.