Rapid Photomineralization of Dissolved Organic Carbon Flowing from Large Arctic Rivers to the Arctic Ocean

The Arctic Ocean receives substantial freshwater inputs from its watershed, which are delivered into a relatively small, closed ocean basin. Thus, the land-to-ocean flux of terrigenous materials has a significant impact on the biogeochemistry and ecology of the Arctic Ocean nearshore. Recent work has demonstrated that the terrigenous dissolved organic carbon (TDOC) transported by Arctic rivers undergoes rapid degradation within the Arctic basin. However, the relative contribution of the two primary TDOC degradation mechanisms to this process - uptake by bacteria or degradation by sunlight - remains unclear. In this study, we examined the susceptibility of Arctic riverine TDOC to degradation by solar radiation using samples collected during the Arctic Great Rivers Observatory Project, which targets the six largest rivers draining to the Arctic Ocean. These rivers drain both the Eurasian (Ob', Yenisey, Lena, Kolyma) and North American (Yukon, Mackenzie) continents, and together represent greater than two-thirds of the direct freshwater discharge to the Arctic Ocean. We measured TDOC loss rates, and the photoproduction of dissolved inorganic carbon (DIC), following laboratory incubation under simulated full spectrum irradiance. Photomineralization rates were rapid, with between 6% and 9% of the DOC pool lost following solar exposure equivalent to 2.3 days of summer solstice irradiation. DIC photoproduction showed a strong, linear relationship with initial TDOC concentration (r² > 0.9), and was more strongly related to DOC concentration than absorbance characteristics such as specific UV absorbance (SUVA). Unlike previous results for bacterial degradation in select Arctic rivers, there was no clear seasonal, or between-river, variation in the susceptibility of Arctic riverine TDOC to photomineralization. These results indicate that photomineralization may play an important role in the documented loss of TDOC within surface waters of the Arctic Ocean, particularly during the May-June freshet period which occurs concurrently with the Arctic summer solstice, and accounts for almost half of the riverine TDOC delivery to the Arctic Ocean. A better understanding of the balance between solar and bacterial processes for TDOC loss in the Arctic Ocean will help elucidate the eventual fate of this carbon, which may range from its incorporation into bacterial biomass, respiration as CO₂, or photomineralization to products such as CO₂ or CO.