Enhancement of dissolved organic matter bioavailability by sunlight and its role in the carbon cycle of Lakes Superior and Michigan

The degree to which sunlight and microbes interact in the cycling of dissolved organic matter (DOM) of natural waters is unclear. During 1999 and 2000, the effects of sunlight on the lability of DOM in two of the Laurentian Great Lakes were examined. Bacteria-free (< 0.2 μm) water samples collected in Lake Superior and Lake Michigan were exposed to ambient sunlight for several days and subsequently inoculated (1:10) with < 1.0 μm lake water containing natural bacteria and assayed for bacterial growth rates (³H-leucine incorporation into protein) in the dark. Exposure of water collected from offshore surface layers to sunlight caused a net reduction of DOM bioavailability (bacterial growth ∼20 to 30% of dark control), whereas both exposure of deep water and river water to sunlight resulted in a net enhancement of DOM bioavailability (bacterial growth ∼150 to 260% of dark control). Irradiated water from nearshore exhibited an intermediate response in terms of changes in DOM bioavailability (bacterial growth ∼75 to 115% of dark control). Despite the reduction of DOM bioavailability in surface waters by sunlight exposure in the short-term (relative to dark control), subsequent incubation of these waters in the dark increased bacterial production rates relative to in situ rates, suggesting that the net effect of exposure to sunlight, even in surface waters, was to increase DOM bioavailability. Rapid and sustained photochemical alterations of DOM bioavailability occurring in the euphotic zone of natural waters should accelerate the turnover of surface water, older deep water, and refractory terrigenous carbon.