Circumpolar lakes comprise ~ 1.4 million km² of arctic and subarctic landscapes and are vulnerable to change in vegetation, permafrost distribution, and hydrological conditions in response to climate warming. However, the composition and cycling of dissolved organic matter (DOM) is poorly understood for these lakes because most are remote and unstudied. The goal of this study was to assess timescale and source controls on DOM composition in Canvasback Lake, a shallow, sub-Arctic lake in interior Alaska with similar hydrologic and geomorphic characteristics to about a quarter of circumpolar lake ecosystems. Lake dissolved organic carbon (DOC) concentration varied by as much as 16% from the mean (3.34 mg L⁻¹ change) through diel cycles in spring 2016 to fall 2017 and was accompanied by minor changes in DOM composition. At the seasonal scale, DOC concentration increased from spring through fall to very high concentrations under ice in winter. Decreases in both condensed aromatic and polyphenolic compound classes and lignin carbon-normalized yield, plus increased relative abundance of aliphatic compounds, suggests that DOM composition shifts from a pulse of allochthonous DOM in the spring to more autochthonous under-ice. These changes highlight the seasonally-dynamic nature of DOM in circumpolar lakes that are poorly captured by single-visit lake surveys and underscores the need to measure DOM properties and fate consistently across multiple timescales (i.e. seasonally) to better constrain the role of DOM in lake processes. To further assess DOM sources, a suite of endmember leachates were compared to bulk lake DOM, indicating solely allochthonous inputs are not well reflected in lake DOM, highlighting the role of degradation processes or mixing with autochthonous sources. Thus, Canvasback Lake appears less well connected to terrestrial inputs compared to past studies of northern high-latitude lakes and does not behave as previous boreal lake models suggest.