Photochemical oxygen consumption in marine waters: A major sink for colored dissolved organic matter?

Quantification of photochemical O-2 uptake provides a measure of total chem ical photooxidation of dissolved organic matter (DOM). Here we study this p rocess and present estimates suggesting that photooxidation has the potenti al to significantly modify marine DOM pools, complementing or exceeding oxi dation via coupled chemical-biological pathways. We measured apparent quant um yields (AQYs) of photobleaching, O-2 uptake, and H2O2 production in seve ral coastal marine samples and in dilutions of a tropical estuarine water w ith oligotrophic seawater. O-2-loss AQYs varied little among samples or wit h dilution but decreased linearly from 1.2 x 10(-3) at 300 nm to 0.3 x 10(- 3) at 400 nm and dropped about threefold to near-constant values with incre asing absorbed light dose. H2O2 production, about 45% of O-2 uptake, showed similar dependencies, whereas singlet oxygen (O-2((1)Delta(g))) reactions contributed less than 1% of O-2 uptake for typical coastal water. Implicati ons of these findings for photochemical O-2, H2O2, and DOM cycling are disc ussed. Modeling the dose-dependence of O-2 loss and photobleaching at 310 nm requi red three DOM pools. In the simplest case, about 90% is a weakly absorbing, low-AQY pool of DOM admired with two similar-sized pools of more photochem ically reactive DOM. This result suggests that rigorously extrapolating lab oratory data to the environment requires detailed mapping of dose-wavelengt h-photobleaching AQY surfaces. Action spectra and DOM flux estimates for coastal photooxidative chemistry were derived. Site-specific potential rates are comparable to available in situ data. Globally, the DOM photolysis capacity appears to be larger than estimated coastal DOM inputs, especially in tropical and temperate areas, i ncluding areas with maximal DOM inputs.