Residual water bodies formed after thermokarst lake drainage in permafrost regions act as hot spots of nutrient and carbon cycling and greenhouse gas emissions

Drained thermokarst (thaw) lakes of permafrost regions represent potentially important but poorly constrained hot spots of greenhouse gas (GHG), carbon, and nutrient cycling. To elucidate the biogeochemistry of residual water bodies (RWB) formed after thermokarst lake drainage in permafrost peatlands, we measured dissolved CO₂ and CH₄ concentrations, CO₂ emissions, and dissolved (<0.45 μm) carbon, major, and trace element concentrations in the continuous and discontinuous permafrost zones of the Western Siberian Lowlands. Residual water bodies located within drained lake basins were examined across early and late successional stages. Carbon dioxide emissions from RWB surfaces ranged from 0.1 to 2.0 g C-CO₂ m^-2 d^-1 and did not exhibit systematic variation with successional stage or permafrost zone. In contrast, dissolved CO₂ (200-1200 μmol L^-1) and CH₄ (1-30 μmol L^-1) concentrations followed a consistent pattern of "Early stage > Late stage > Lake." Partial mismatch between dissolved CO₂ concentrations and CO₂ fluxes arises because concentrations integrate longer-term biogeochemical processes, whereas fluxes respond to short-term physical controls on gas exchange. The isotopic composition of dissolved inorganic carbon (δ¹³C-DIC; -14 to -28‰) indicated dominant DIC production from terrestrial organic matter (plants and peat), with additional contributions from in-lake biogeochemical processing and gas exchange. Labile and highly soluble components of lake water-including DIC, major ions (Na, Mg, Ca, Cl), nutrients (P, K, Si), redox-sensitive elements (Fe, Mn), and several trace elements (Co, Ni, Sr, Rb, Mo, As)-showed similar stage-dependent decreases in concentration. This pattern is attributed to intensive biogeochemical cycling driven by vegetation establishment and nutrient uptake on drained lake bottoms. In contrast, low-solubility lithogenic elements and several trace metals (e.g., Cr, V, Cu, Zn, Pb) showed no consistent successional trend, and in some cases increased from early to late stages, suggesting inputs from mineral sources via suprapermafrost inflow. Overall, residual water bodies formed after thermokarst lake drainage differ markedly from mature lakes in their carbon, GHG, and solute composition. Their biogeochemistry is primarily regulated by terrestrial vegetation succession and peat soil inputs, highlighting drained thermokarst lakes as critical yet underrepresented hot spots in Pan-Arctic carbon and nutrient cycling under ongoing climate warming.