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

Abstract Northern permafrost landscapes store large amounts of organic carbon that can enter lakes as dissolved organic matter (DOM), but links among shoreline terrestrial sources, lake‐water DOM composition, and catchment configuration remain uncertain in low‐precipitation Shield landscapes. We combined shoreline soil‐porewater and lake‐water sampling across a 5°C climate gradient spanning discontinuous and continuous permafrost in the Northwest Territories, Canada, with dissolved organic carbon concentration measurements, ultra‐high‐resolution mass spectrometry (FT‐ICR‐MS), stable carbon isotope composition (δ 13 C‐DOM), radiocarbon (Δ 14 C‐DOM), and whole‐catchment geospatial analyses. Soil porewater had higher dissolved organic matter carbon (DOM‐C) concentrations and more aromatic molecular composition than lake water, supporting its use as a nearshore terrestrial endmember. Lake‐water DOM had lower aromaticity, higher H/C, greater relative intensity of aliphatic formulae, and higher Δ 14 C‐DOM than soil porewater, consistent with reduced expression of the terrestrial molecular signature in lakes. Across lakes, soil‐to‐lake DOM‐C differences, aromaticity, H/C, compound‐class patterns, and Δ 14 C‐DOM were associated with the aquatic‐to‐terrestrial ratio, calculated as total lake and surface‐water area divided by terrestrial watershed area. Water residence time was not correlated with the aquatic‐to‐terrestrial ratio and explained less variation in soil‐to‐lake DOM‐C differences and aromaticity. Molecular composition also varied with permafrost extent, with lower O/C and nominal oxidation state of carbon in continuous‐permafrost sites after accounting for the aquatic‐to‐terrestrial ratio. Together, these results show that shoreline terrestrial DOM, catchment configuration, and regional permafrost‐gradient characteristics jointly structure lake‐water DOM composition. The aquatic‐to‐terrestrial ratio provides a first‐order metric for identifying lakes where DOM composition is likely to reflect stronger terrestrial influence vs. greater aquatic‐network influence.