Greenhouse gases, organic carbon, major and trace elements in ultra-small water bodies (<100 m2) of permafrost peatlands: Assessment across seasons and permafrost gradient

• Greenhouse gases (GHG), carbon, major and trace solutes in small water bodies (SWB) • Increase in dissolved organic C (OC) concentrations from spring to autumn in SWB. • Peak concentrations of OC and GHG observed at the permafrost thaw front. • Increasing connectivity between subsurface/soil water and SWB over the course of the year. • SWB as key regulators of GHG dynamics and nutrient storage during the “shoulder” seasons. Thawing permafrost releases substantial amounts of greenhouse gases (GHGs), organic carbon (OC), nutrients, and trace metals into surface waters including rivers and, eventually, the Arctic Ocean. However, intermediate components linking soil and surface waters—specifically, small and ultra-small puddles and depressions (<10–100 m 2 ) formed after snowmelt in spring and through permafrost thaw in summer—have largely remained outside the scope of research. In this study, we investigated 206 surface waters from small depressions and puddles (1–100 m 2 ) in spring, summer, and autumn across a permafrost gradient, spanning from sporadic to continuous permafrost zones in the Western Siberian Lowlands (WSL). Our findings showed the highest concentrations of carbon dioxide (CO 2 ) and methane (CH 4 ) in the sporadic/discontinuous permafrost zones during summer and autumn, likely due to intensive mobilization of organic carbon from peat and GHG generation at the active permafrost thawing front. Across all permafrost zones, the highest concentrations of major and trace solutes were observed during summer and autumn, with the lowest concentrations in spring. This trend likely reflects a progressive increase in connectivity between surface and subsurface waters, which are enriched in GHGs, organic matter (OM) and inorganic components due to leaching from peat and soil minerals. For the two main size ranges of studied small water bodies (SWBs) in the WSL — <10 m 2 and 10–100 m 2 — no significant seasonal or spatial differences were observed in CO 2 , CH 4 , DOC, macro- and micronutrient or metal concentrations. When averaged across all seasons and permafrost zones, SWBs showed concentrations of CH 4 and CO 2 approximately 10 times higher and levels of certain macro- (Si and P) and micronutrients (Fe, Co, Rb and Mn) 3 to 7 times higher when compared to thermokarst lakes. Thus, despite their limited surface area, SWBs may serve as crucial regulators of GHG dynamics and nutrient storage, especially during the understudied transitional seasons of spring and autumn.