Photon dose affects dissolved organic matter apparent quantum yields in Alaskan Arctic surface waters

Abstract

Photochemical, or light-driven, processing of dissolved organic matter (DOM) in inland waters can completely degrade DOM to carbon dioxide (CO2), a greenhouse gas, through photomineralization or partially degrade DOM through partial photo-oxidation. Evidence suggests that the lability of DOM, quantified as an apparent quantum yield (AQY, fλ), is not constant with increasing light exposure. Thus, the rate of photochemical processing is not constant during light exposure in sunlit surface waters as DOM f λ changes. Findings in this study show that fPM,λ (photo-mineralization) typically decreases as photon dose increases, while fPPO,λ (partial photooxidation) does not show a consistent trend. Differences in DOM chemistry between sites and dates sampled are likely controlling differences in fPM,λ and fPPO,λ trends with increasing photon dose. Daily areal rates of photo-mineralization calculated with fPM,λ after a low photon dose are an overestimate by a factor of 2.08 0.18 compared to rates calculated with fPM,λ from a higher photon dose, while daily areal rates of partial photo-oxidation calculated with fPPO,λ after a low photon dose are an overestimate by a factor of 1.15 0.10 compared to rates calculated with fPPO,λ from a higher photon dose. Because partial photo-oxidation fPPO,λ does not show a consistent trend with increasing photon dose, the impact of low vs. high photon dose experimentally determined fPPO,λ on calculated areal rates is not as large as the impact of fPM,λ. Understanding how photon dose and DOM chemistry control the photochemical degradation of DOM will further constrain current estimates of how much CO2 and partially-oxidized DOM is produced through photochemical processes in these arctic surface waters.