Seasonal organic matter dynamics in thermokarst lakes of a degrading palsa-rich sporadic permafrost zone (Nunavik, Canada)

• Enhanced methanotrophic activity and sediment processing in summer in the emerging lake. • Lake morphology strongly influenced DOM dynamics in the water column during summer. • Mature lake had highest aromatic DOM and chromophoric DOM in summer, with more recalcitrant soil. • Winter induced shifts toward condensed DOM structures and stimulated Fe redissolution. As Arctic and Subarctic regions warm rapidly, thawing permafrost is releasing large amounts of dissolved organic matter (DOM) into forming and expanding thermokarst lakes. Lake functioning is strongly shaped by DOM quality and nutrient inputs, as well as site-specific environmental conditions. However, the effects of seasonality, lake morphometry, and ontogeny on biogeochemical mechanisms remain poorly characterized. We investigated three thermokarst lakes with contrasting ontogeny and morphometry in a degrading palsa-rich sporadic permafrost region. Sampling of water column, sediment porewater, lake sediments, and surrounding soils was conducted during summer and winter to analyze terminal electron acceptors (TEAs), DOM via UV–Visible and fluorescence spectroscopy, and sediments using δ 13 C signatures and Fourier-transform infrared spectroscopy. Results reveal how a thermokarst lake’s biogeochemistry is controlled by key factors: oxygen availability, substrate quality, and lake morphometry. Winter anoxia mobilized dissolved Fe and terrestrial DOM from sediments, while preserving protein-like FDOM and accumulating CO 2 in the water column. Summer oxic conditions favor photochemical breakdown of large, recalcitrant structures, yielding smaller aromatic DOM. Lake ontogeny alters the quality of landscape inputs: the young lake showed signs of elevated sediment microbial processing and rapid oxygen depletion, driven by the thawing palsa; whereas the mature lake relies on photochemically produced aromatic DOM from older, recalcitrant soils. In conclusion, lake morphometry further modulates these processes by shaping mixing, stratification, and light exposure, emphasizing the importance of interacting physical and biogeochemical controls in structuring DOM cycling in thermokarst lakes.