Blinded by the Light: Seasonal Sensitivity of Arctic River Carbon to Photo- and Biodegradation

While climate change is affecting every ecosystem on Earth, it is particularly pervasive in the Arctic. High-latitude ecosystems store at least 2.5 trillion tons of organic carbon trapped in frozen permafrost, which is vulnerable to thaw and degradation as a result of global climate change. As the Arctic warms, some of this organic matter will be released to the atmosphere as greenhouse gas emissions, while a large proportion will be exported to surface-water networks and exposed to sunlight as dissolved organic carbon (DOC). The bioavailability and photo-lability of this DOC will affect aquatic food webs and ecosystem carbon balance. Here, we quantified the vulnerability of DOC in Arctic surface waters to photo- and biodegradation with 28-day in-situ light and dark incubations. In 2021 and 2022, we collected water samples from 72 locations across the North Slope of Alaska four times across the flow season (June-September). Our sampling locations spanned three river networks with diverse catchment sizes, ecosystem types, and disturbance histories, allowing us to test longstanding hypotheses about controls on Arctic DOC dynamics. Across all sampling events and sites, biodegradation rates were low (i.e., <10% over 28 days). Conversely, photo-degradation showed a strong seasonal decline from >25% loss during snowmelt in June to <10% loss in the late season. Differences in DOC degradation rates between landscape positions (alpine, upper foothills, lower foothills, coastal plain) and stream network locations (zero order stream, tributary, or main stem) were much weaker than the seasonal shift from photic to biotic control. Ultimately, these findings highlight that the abiotic and biotic processes controlling DOC degradation are seasonally dynamic. Thus, constraining predictions of springtime photo-exposure in the future could improve our understanding of DOC transport and persistence in Arctic waterways.