Dissolved Organic Matter Controls Seasonal and Spatial Selenium Concentration Variability in Thaw Lakes across a Permafrost Gradient

Little is known about the sources and processing of selenium, an important toxicant and essential micro­nutrient, within boreal and sub-arctic environments. Upon climate warming and perma­frost thaw, the behavior of Se in northern peat­lands becomes an issue of major concern, because a sizable amount of Se can be emitted to the atmosphere from thawing soils and inland water surfaces and exported to downstream waters, thus impacting the Arctic biota. Working toward providing a first-order assessment of spatial and temporal variation of Se concentration in thermo­karst waters of the largest frozen peat­land in the world, we sampled thaw lakes and rivers across a 750-km latitudinal profile. This profile covered sporadic, discontinuous, and continuous perma­frost regions of western Siberia Lowland (WSL), where we measured dissolved (<0.45 μm) Se concentration during spring (June), summer (August), and autumn (September). We found maximum Se concentration in the discontinuous perma­frost zone. Considering all sampled lakes, Se exhibited linear relationship (R² = 0.7 to 0.9, p < 0.05, n ≈ 70) with dissolved organic carbon (DOC) concentration during summer and autumn. Across the perma­frost gradient, the lakes in discontinuous perma­frost regions demonstrated stronger relationship with DOC and UV-absorbance compared to lakes in sporadic/isolated and continuous perma­frost zones. Both seasonal and spatial features of Se distribution in thermo­karst lakes and ponds suggest that Se is mainly released during thawing of frozen peat. Mobilization and immobilization of Se within peat–lake–river watersheds likely occurs as organic and organo-Fe, Al colloids, probably associated with reduced and elemental Se forms. The increase of active layer thickness may enhance leaching of Se in the form of organic complexes with aromatic carbon from the deep horizons of the peat profile. Further, the northward shift of perma­frost boundaries in WSL may sizably increase Se concentration in lakes of continuous perma­frost zone.