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Rates and apparent quantum yields of photomineralization (AQYDOC)and photomethanification (AQYCH4) of chromophoric dissolvedorganic matter (CDOM) in Saguenay River surface water were determined atthree widely differing dissolved oxygen concentrations ([O2]) (suboxic,air saturation, and oxygenated) using simulated-solar radiation.Photomineralization increased linearly with CDOM absorbance photobleachingfor all three O2 treatments. Whereas the rate of photochemical dissolvedorganic carbon (DOC) loss increased with increasing [O2], the ratio offractional DOC loss to fractional absorbance loss showed an inverse trend.CDOM photodegradation led to a higher degree of mineralization under suboxicconditions than under oxic conditions. AQYDOC determined underoxygenated, suboxic, and air-saturated conditions increased, decreased, andremained largely constant with photobleaching, respectively;AQYDOC obtained under air saturation with short-term irradiationscould thus be applied to longer exposures. AQYDOC decreasedsuccessively from ultraviolet B (UVB) to ultraviolet A (UVA) to visible(VIS), which, alongside the solar irradiance spectrum, points to VIS and UVAbeing the primary drivers for photomineralization in the water column. Thephotomineralization rate in the Saguenay River was estimated to be2.31 × 108 mol C yr−1, accounting for only 1 % ofthe annual DOC input into this system. Photoproduction of CH4 occurred under both suboxic and oxic conditionsand increased with decreasing [O2], with the rate under suboxicconditions ~ 7–8 times that under oxic conditions. Photoproduction ofCH4 under oxic conditions increased linearly with photomineralizationand photobleaching. Under air saturation, 0.00057 % of the photochemicalDOC loss was diverted to CH4, giving a photochemical CH4 productionrate of 4.36 × 10&minus;6 mol m−2 yr−1 in the SaguenayRiver and, by extrapolation, of(1.9–8.1) × 108 mol yr−1 in the global ocean.AQYCH4 changed little with photobleaching under air saturation butincreased exponentially under suboxic conditions. Spectrally,AQYCH4 decreased sequentially from UVB to UVA to VIS, with UVBbeing more efficient under suboxic conditions than under oxic conditions. Ona depth-integrated basis, VIS prevailed over UVB in controlling CH4photoproduction under air saturation while the opposite held true underO2-deficiency. An addition of micromolar levels of dissolved dimethylsulfide (DMS) substantially increased CH4 photoproduction, particularlyunder O2-deficiency; DMS at nanomolar ambient concentrations in surfaceoceans is, however, unlikely a significant CH4 precursor. Results fromthis study suggest that CDOM-based CH4 photoproduction only marginallycontributes to the CH4 supersaturation in modern surface oceans and toboth the modern and Archean atmospheric CH4 budgets, but that thephotochemical term can be comparable to microbial CH4 oxidation inmodern oxic oceans. Our results also suggest that anoxic microniches inparticulate organic matter and phytoplankton cells containing elevatedconcentrations of precursors of the methyl radical such as DMS may providepotential hotspots for CH4 photoproduction

Photomineralization and photomethanification of dissolved organic matter in Saguenay River surface water