From Source Waters to Rivers: Fe‐Rich Colloids Control Carbon and Metal Partitioning in Western Siberian Hydrological Continua

Colloids are a major, but still insufficiently resolved, transport phase linking terrestrial carbon mobilization to riverine export of metals and nutrients. Identifying where colloids form, how they transform along hydrological continua, and which solutes they preferentially carry is essential for predicting biogeochemical responses to permafrost thaw and changing groundwater connectivity. Here we investigated colloidal partitioning of dissolved organic carbon (DOC), major elements and trace metals along four hydrological continua of the Western Siberian Lowland spanning a 1500 km latitudinal transect and a broad climate and permafrost gradient. During summer baseflow, 29 surface waters and soil solutions were size‐fractionated into low‐molecular‐weight (LMW; < 3 kDa) and colloidal (3 kDa–0.45 μm) pools. Two robust patterns emerged. Major ions and Si remained largely in the < 3 kDa fraction, whereas Fe, P and low‐mobility hydrolyzing elements, including Al, Ti, Zr, Hf, Th, U, Pb and the REEs, were predominantly colloidal, especially in streams and rivers. The strongest reorganization of size partitioning occurred not among rivers of different size, but at the transition from source compartments (soil and fen waters) to the first flowing waters. Along the continua, DOC became progressively enriched in the LMW (< 3 kDa) pool, whereas Fe and many trace metals became increasingly colloidal, indicating partial decoupling between carbon and metal transport. Watershed area exerted only a weak, element‐specific influence. In contrast, pH and colloid composition were first‐order controls: increasing pH enhanced the colloidal fractions of Fe, P, Y, Zr, Th, U and REEs, but increased the low‐molecular‐weight fractions of alkali and alkaline‐earth elements and Mo. Ratio‐based, correlational and PCA analyses consistently identified Fe/Al‐rich organo‐mineral colloids as the dominant carriers of low‐solubility elements, with REEs and P showing stronger affinity for Fe‐rich than for Al‐rich colloids. Similar carrier‐based partitioning reported from humid tropical humic systems suggests that Fe/Al‐rich colloidal transport is not restricted to permafrost landscapes, but may represent a broader mechanism in organic‐rich hydrological systems. In Western Siberia, the permafrost gradient provides a space‐for‐time framework in which future changes in colloidal partitioning are expected to reflect active‐layer deepening and increasing surface–subsurface water exchange.