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 for