In the northern hemisphere, subarctic peatlands lying on sporadic permafrost are undergoing rapid transformation. This impact the mobility of oxyanions including selenium (Se). Here, we aim to better understand how such environmental changes will influence Se distribution. This study focuses on four ponds with contrasting oxygenation conditions within a peatland area near Kuujjuarapik, Nunavik, Canada. We measured physiochemical parameters, major, and trace element concentrations in the water-column and sediment porewater during summer and winter. The speciation of Se was calculated using thermodynamic models with measured water chemistry as inputs. Net rates of Se and dissolved organic carbon reactions were estimated using an inverse modeling approach with diffusion coefficients and sediment porosity as input, yielding net reaction rates and fluxes across the sediment-water interface.Our result indicated that Se mobility is influenced by both changes in water temperature and DO concentrations. This is illustrated by Se concentration in the water column decreasing from 11 nM in the summer to 1 nM in the winter, concurrent with a change in Se speciation. Such changes in Se distribution are the results of flux of Se from the water column to the sediment, which increase from 33 to 56 nmol cm-3 yr-1 from the summer to the winter.Our results highlight that seasonal dynamics strongly influence selenium mobility , with reducing conditions being necessary to sequester Se, even at low water temperatures. This suggests that warmer summer, favoring anoxic conditions, may enhance Se retention in the ponds, potentially to the detriment of downstream ecosystems