| Summary: | Northern high-latitude freshwater systems are an essential link between terrestrial and marine carbon cycles, transporting organic matter from ecosystems on land to the Arctic Ocean. However, polar regions are warming faster than the global average due to anthropogenic climate change, potentially releasing hundreds of tons of carbon currently stored in peatland and permafrost soils. Arctic ecosystem dynamics are also changing under continued warming, and changing land cover and hydrology will alter the amount and composition of organic matter exported by surface waters. Dissolved organic matter (DOM) is a crucial component of the carbon cycle, with the global DOM pool rivaling the amount of carbon stored in the atmosphere. The composition of DOM is fundamentally linked to its reactivity and, as such, partially determines whether mobilized DOM will be mineralized and released as greenhouse gases or stored on long time scales in marine systems. Understanding DOM dynamics in Arctic freshwater systems is thus crucial to understanding the impacts of climate change. This dissertation investigates DOM sources, molecular composition, and processing pathways across the pan-Arctic watershed and uses a suite of investigative tools, including optical measurements, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and spatial analysis, to investigate how a changing Arctic landscape may impact DOM dynamics. The research focused on four distinct areas: refining dissolved organic carbon (DOC) fluxes from the pan-Arctic watershed, characterizing the influences of peatland cover and permafrost on exported DOM, evaluating how DOM composition and microbial community influence the processing of DOM, and characterizing the composition and reactivity of DOM from non-vascular plant sources. DOM composition varied across ecosystems, reflecting the chemodiversity inherent to this carbon cycle component. In the Onega River, a relatively small and understudied high-latitude river, DOM exhibited a uniquely terrestrial ...
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