The Evolution of Stream Dissolved Organic Matter Composition Following Glacier Retreat in Coastal Watersheds of Southeast Alaska

Climate change is melting glaciers and altering watershed biogeochemistry across the globe, particularly in regions dominated by mountain glaciers, such as southeast Alaska. Glacier dominated watersheds exhibit distinct dissolved organic matter (DOM) characteristics compared to forested and vegetated watersheds. However, there is a paucity of information on how stream DOM composition changes as glaciers retreat and terrestrial ecosystem succession ensues. Importantly, it is unclear over what timescales these transformations occur. Here, we used bulk, isotopic and ultrahigh resolution molecular-level techniques to assess how streamwater DOM composition evolves in response to glacier retreat and subsequent terrestrial ecosystem succession. For this, water samples were collected from eleven streams across a chronosequence spanning a temporal gradient 0 to ~1,400 years since glacier retreat in coastal, southeast Alaska. During the first ~200 years since glacier retreat, stream DOM showed marked and consistent changes in bulk, isotopic, and molecular-level composition. In particular, there was a decreased abundance of ancient, energy-rich (e.g., elevated aliphatic contribution), low aromaticity (e.g., low SUVA 254 and AI mod ) DOM and an increased abundance of soil and vegetation derived aromatic DOM (e.g., more depleted d 13 C, elevated condensed aromatic and polyphenolic contribution) that had a modern radiocarbon age. After ~200 years of ecosystem development, DOM composition was comparable to that observed for other temperate and arctic forested watersheds without permafrost influence. These results underscore the timelines on which glacier retreat may have substantial impacts on watershed biogeochemistry and coastal ecosystems that receive DOM subsidies from these rapidly changing landscapes.