Photochemical transformation of dissolved organic carbon in lake superior - An in-situ experiment

Document Type

Conference Proceeding

Publication Date



A direct solar irradiation experiment was conducted for Lake Superior water (initial [DOC] = 2.49 mgC/L) and Sturgeon River water (initial [DOC] = 33.95 mgC/L) in Lake Superior on 22-25 August 1999. Water samples were sealed in quartz tubes and suspended in the lake at 0 m, 6 m, and 24 m. Samples were collected after 5.5 h and 15.5 h of exposure to sunlight. The dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations, UV-vis absorbance, and 3D fluorescence were measured on all samples. For irradiated lake water, DIC was photoproduced at a rate of 1.3 to 5.6 μM DIC hr -1 for the first day and 1.3 to 2.1 × 102 nM DIC hr-1 for the second day, normalized to 1 mg DOC/L, and long wavelength absorbance decreased as a function of irradiation time. For both river and lake samples, integral fluorescence also decreased systematically and the fluorescence quantum yield decreased after photoirradiation. In addition, new fluorescence peaks at short wavelengths appeared in irradiated lake samples, suggesting the formation of new chromophores. After irradiation, DOC concentrations in surface samples were lower than those in deep samples. Rate constants for all processes measured decreased during the irradiation time. Rate constants for loss of fluorophores were greater than those for chromophores, which, in turn, were greater than the rate constants for loss of DOC.

Publication Title

Journal of Great Lakes Research