From Bog to Fen: Thaw-Gradient and Shoulder-Season Controls on Colloidal Trace Elements in Ultra-Small Arctic Waterbodies

Arctic peatlands host numerous ultra-small thaw ponds that form during the spring “shoulder” season, yet their role in trace element processing remains poorly constrained. Here we investigated major and trace element concentrations and size-partitioning (3 kDa–0.45 µm colloids, complemented by 1 kDa dialysis) along a bog–transition–fen thaw gradient in the Stordalen mire (northern Sweden) to disentangle spatial (hydrological setting) versus seasonal (melt progression) controls. Stable water isotopes indicated persistent connectivity between surface waters and thawing peat and soil reservoirs. Across the thaw gradient, pH increased and electrical conductivity decreased from ombrotrophic bogs to minerotrophic fens, reflecting increasing groundwater influence. Bulk concentrations of hydrolyzing multivalent elements varied among sites: particle-reactive elements (Al, Fe, Y, rare earth elements, Zr, Th, U) were consistently enriched in deeper, groundwater-connected systems, whereas shallow bogs showed greater variability and evaporative influence. Colloidal partitioning revealed a distinct pattern. Monovalent alkalis remained largely in the low-molecular-weight fraction (<10% colloidal), while hydrolyzing multivalent metals occurred predominantly in colloidal form (>60%). Their colloidal dominance increased systematically from bog to fen and correlated positively with pH, indicating strong spatial control linked to Fe-rich nanoparticle formation. In contrast, nutrient-type elements (P, V) displayed limited spatial differentiation but pronounced seasonal variability, with colloidal percentages decreasing with increasing DOC/Fe ratios in the fitted power-law relationships. Rare earth element fractionation further recorded this dual control: early-season positive Eu anomalies and elevated La/Yb ratios suggested mineral and redox-controlled inputs, whereas progressive seasonal attenuation and declining La/Yb suggested increasing importance of complexation with dissolved organic matter. The distribution of multivalent elements between colloidal and low-molecular-weight fractions varied with water sources and flow paths among waterbodies. For nutrient-like elements, this distribution was linked to seasonal changes in the balance between DOC and Fe. Even the smallest puddles and thaw ponds therefore act as dynamic redox interfaces that regulate trace element speciation during the critical early melt period.