Controls on the apparent quantum yield for photomineralization of dissolved organic matter in arctic freshwaters

The oxidation of dissolved organic matter (DOM) to carbon dioxide by sunlight (photomineralization) can account for up to 30% of carbon dioxide (CO₂) emitted from inland surface waters in the Arctic. Water-column photomineralization rates depend on the magnitude and shape of the photomineralization apparent quantum yield spectrum (φ_PM,λ), which few studies have quantified directly. Here, we used a light-emitting diode (LED)-based approach to directly quantify φ_PM,λ of CO₂ from photomineralization in arctic surface waters exposed to increasing amounts of narrow-banded light at ultraviolet and visible wavelengths. Waters with the highest aromatic DOM and dissolved iron had the highest φ_PM,λ at all wavelengths. The magnitude of φ_PM,λ at all wavelengths decreased by up to 92% with increasing cumulative light absorbed by chromophoric dissolved organic matter (CDOM) in a given water, consistent with the rapid depletion of a photo-labile DOM fraction. Together, the results suggest that the extent of light absorption by CDOM, aromatic carbon content, and iron concentration control the magnitude and shape of φ_PM,λ, which in turn strongly influences rates of photomineralization and CO₂ production in inland surface waters. Experiments to quantify φ_PM,λ should consider that greater cumulative light absorbed leads to underestimates of φ_PM,λ and photomineralization rates.