Photo‐oxidative production of dissolved inorganic carbon in lakes of different humic content

Abstract

Photo‐oxidation of dissolved organic carbon (DOC) to inorganic carbon (DIC) in five oligotrophic south Swedish lakes of different humic content (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 µm) water was incubated in UV‐transparent quartz tubes and in tubes covered with aluminum foil. 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 DIC) 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. Photo‐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 398–860 mg C m⁻² d⁻¹ over the depth of the epilimnion. Depth‐integrated photo‐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 fluorescence (excitation 355 nm, emission 455 nm) in light‐incubated samples. Our study shows that photo‐oxidation of DOC may be an important process causing the regularly observed supersaturation of DIC in lakes.