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

Abstract

Rates and apparent quantum yields of photominer-alization (AQYDOC) and photomethanification (AQYCH4) of chromophoric dissolved organic matter (CDOM) in Sague-nay 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. AQYDOC determined under oxygenated, suboxic, and air-saturated conditions increased, decreased, and remained largely constant with photobleaching, respectively; AQYDOC obtained under air saturation with short-term irradiations could thus be applied to longer exposures. AQYDOC 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 tobe2.31 × 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 photo-chemical DOC loss was diverted to CH#inf#4#/inf#, giving a photochemical CH#inf#4#/inf# production rate of 4.36×10#sup#-6#/sup# mol m#sup#-2#/sup# yr#sup#-1#/sup# in the Saguenay River and, by extrapolation, of (1.9- 8.1)×10#sup#8#/sup# mol yr#sup#-1#/sup# in the global ocean. AQY#inf#CH4#/inf# changed little with photobleaching under air saturation but increased exponentially under suboxic conditions. Spectrally, AQY#inf#CH4#/inf# 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 CH#inf#4#/inf# photoproduction under air saturation while the opposite held true under O#inf#2#/inf#-deficiency. An addition of micromolar levels of dissolved dimethyl sulfide (DMS) substantially increased CH#inf#4#/inf# photoproduction, particularly under O#inf#2#/inf#-deficiency; DMS at nanomolar ambient concentrations in surface oceans is, however, unlikely a significant CH#inf#4#/inf# precursor. Results from this study suggest that CDOM-based CH#inf#4#/inf# photoproduction only marginally contributes to the CH#inf#4#/inf# supersaturation in modern surface oceans and to both the modern and Archean atmospheric CH#inf#4#/inf# budgets, but that the photochemical term can be comparable to microbial CH#inf#4#/inf# 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 CH#inf#4#/inf# photoproduction.