Abstract. Rates and apparent quantum yields of photomineralization (AQY_DOC) and photomethanification (AQY_CH4) of chromophoric dissolved organic matter (CDOM) in Saguenay River surface water were determined at three widely differing dissolved oxygen concentrations ([O₂]) (suboxic, air saturation, and oxygenated) using simulated-solar radiation. Photomineralization increased linearly with CDOM absorbance photobleaching for all three O₂ treatments. Whereas the rate of photochemical dissolved organic carbon (DOC) loss increased with increasing [O₂], the ratio of fractional DOC loss to fractional absorbance loss showed an inverse trend. CDOM photodegradation led to a higher degree of mineralization under suboxic conditions than under oxic conditions. AQY_DOC determined under oxygenated, suboxic, and air-saturated conditions increased, decreased, and remained largely constant with photobleaching, respectively; AQY_DOC obtained under air saturation with short-term irradiations could thus be applied to longer exposures. AQY_DOC decreased successively from ultraviolet B (UVB) to ultraviolet A (UVA) to visible (VIS), which, alongside the solar irradiance spectrum, points to VIS and UVA being the primary drivers for photomineralization in the water column. The photomineralization rate in the Saguenay River was estimated to be 2.31 × 10⁸ mol C yr⁻¹, accounting for only 1 % of the annual DOC input into this system.

Photoproduction of CH₄ occurred under both suboxic and oxic conditions and increased with decreasing [O₂], with the rate under suboxic conditions ~ 7–8 times that under oxic conditions. Photoproduction of CH₄ under oxic conditions increased linearly with photomineralization and photobleaching. Under air saturation, 0.00057 % of the photochemical DOC loss was diverted to CH₄, giving a photochemical CH₄ production rate of 4.36 × 10⁻⁶ mol m⁻² yr⁻¹ in the Saguenay River and, by extrapolation, of (1.9–8.1) × 10⁸ mol yr⁻¹ in the global ocean. AQY_CH4 changed little with photobleaching under air saturation but increased exponentially under suboxic conditions. Spectrally, AQY_CH4 decreased sequentially from UVB to UVA to VIS, with UVB being more efficient under suboxic conditions than under oxic conditions. On a depth-integrated basis, VIS prevailed over UVB in controlling CH₄ photoproduction under air saturation while the opposite held true under O₂-deficiency. An addition of micromolar levels of dissolved dimethyl sulfide (DMS) substantially increased CH₄ photoproduction, particularly under O₂-deficiency; DMS at nanomolar ambient concentrations in surface oceans is, however, unlikely a significant CH₄ precursor. Results from this study suggest that CDOM-based CH₄ photoproduction only marginally contributes to the CH₄ supersaturation in modern surface oceans and to both the modern and Archean atmospheric CH₄ budgets, but that the photochemical term can be comparable to microbial CH₄ oxidation in modern oxic oceans. Our results also suggest that anoxic microniches in particulate organic matter and phytoplankton cells containing elevated concentrations of precursors of the methyl radical such as DMS may provide potential hotspots for CH₄ photoproduction.