Terrestrial inputs and in-lake processes shape dissolved organic matter along a permafrost gradient

Northern permafrost landscapes store vast pools of organic carbon that mobilize into lakes as dissolved organic matter (DOM). However, little is known about how nearshore terrestrial inputs interact with in-lake processes to shape DOM composition and fate. We compared shoreline porewater DOM with lake DOM across a 5 °C climate gradient spanning discontinuous to continuous permafrost in the Canadian Shield using ultra-high-resolution mass spectrometry and natural carbon isotopes (δ¹³C, Δ¹⁴C). We show that terrestrial porewater dominates the initial quantity and molecular makeup of DOM entering lakes. Yet, in-lake photodegradation, microbial processing, and modest algal production shift DOM toward less aromatic, more aliphatic molecules with more ¹⁴C-enriched radiocarbon signatures. Lake DOM-C concentrations averaged 38 % lower than porewater. The extent of these compositional shifts was best explained by the aquatic-to-terrestrial area ratio and associated surface-area-driven photoprocesses, whereas water residence time had weaker explanatory power. Because boreal and tundra lakes contain substantial terrestrial carbon, even modest changes in in-lake processing may amplify their sensitivity to permafrost thaw and landscape change. The aquatic-to-terrestrial area ratio provides a scalable, upstream-integrated metric for identifying lakes most likely to respond to shifts in terrestrial–aquatic coupling. Our study clarifies how shoreline inputs and basin configuration interact to shape DOM fate in permafrost lakes and offers a practical framework to anticipate changes in DOM export, greenhouse-gas release, and carbon burial as Arctic-boreal regions continue to warm.