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We studied the photoproduction of biologically labile compounds from 2 chemically defined fractions of dissolved organic matter (DOM) and whole (unfractionated) DOM from an acidic freshwater wetland (Okefenokee Swamp, Georgia, USA). Water samples were collected in summer (July 1995) and winter (February 1996), and for the winter sample, DOM was separated into humic substances and 'non-humic' compounds by fractionation on an XAD-8 resin. Following exposure in quartz flasks to either natural or s~rnulated sunlight, bacteria were found to utilize as much as 300% more organic matter in irradiated whole DOM and chemical fractions compared to non-irradiated counterparts. Previous studies have shown that humic substances, which have strong absorption in the UV region of sunlight, are important sources of biologically labile photoproducts. We now demonstrate that compounds outside this operational definition are also sources for labile photoproducts, and that production rates on an absorbance-normalized basis can be equivalent for the humic and non-humic components of DOM. Hurnic substances were nonetheless calculated to be the primary source of biologically lablle photoproducts in these samples, based on the fact that they absorbed most (>85%) of the sunlight. Exposure of DOM fractions to sunlight for the equivalent of 7 h of noontime sun (at 34" N latitude) resulted in the bacterial utilization (over a subsequent 4 d period) of up to 6 % of the humic substances pool (compared to 1.5 % for non-irradiated controls), and 11 % of the non-humic compounds (compared to 3 % for non-irradiated controls).

Photochemical formation of labile organic matter from two components of dissolved organic carbon in a freshwater wetland