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Rivers discharge a notable amount of dissolved Fe (1.5×109 mol yr−1) to coastal waters but are still not considered important sources of bioavailable Fe to open marine waters. The reason is that the vast majority of particular and dissolved riverine Fe is considered to be lost to the sediment due to aggregation during estuarine mixing. Recently, however, several studies demonstrated relatively high stability of riverine Fe to salinity-induced aggregation, and it has been proposed that organically complexed Fe (Fe-OM) can “survive” the salinity gradient, while Fe (oxy)hydroxides are more prone to aggregation and selectively removed. In this study, we directly identified, by X-ray absorption spectroscopy, the occurrence of these two Fe phases across eight boreal rivers draining into the Baltic Sea and confirmed a significant but variable contribution of Fe-OM in relation to Fe (oxy)hydroxides among river mouths. We further found that Fe-OM was more prevalent at high flow conditions in spring than at low flow conditions during autumn and that Fe-OM was more dominant upstream in a catchment than at the river mouth. The stability of Fe to increasing salinity, as assessed by artificial mixing experiments, correlated well to the relative contribution of Fe-OM, confirming that organic complexes promote Fe transport capacity. This study suggests that boreal rivers may provide significant amounts of potentially bioavailable Fe beyond the estuary, due to organic matter complexes.

While organic matter (OM) interactions in the water column prevent iron (Fe) precipitation and sedimentation, Fe also acts as a precursor of aggregation and a vector of OM to sediments. This study aims to characterize Fe–OM interactions to understand the role of Fe in promoting aggregation and transport of OM. Samples of Fe and OM were collected from water, settling material, and sediment along a gradient starting from the inlet and continuing offshore within a boreal lake. Fe speciation was determined using X-ray absorption spectroscopy (XAS), and the chemical composition of OM was assessed using Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT IR) and Nuclear magnetic resonance spectroscopy (NMR). The results show a decrease in Fe and OM concentrations in the water column with increasing distance from the inlet. Winter sampling revealed a shift in Fe speciation from dominance of organically complexed Fe to an increase in Fe(oxy)hydroxide, accompanied by a loss of aromatic and carboxylate function of OM. Summer sampling revealed no significant changes along the gradient, with Fe(oxy)hydroxide and carbohydrates dominating the water phase. Interestingly, settling particles and surface sediments were dominated by Fe(oxy)hydroxides and aliphatic OM. We propose that phototransformation may be an important process that influences the interaction between Fe and OM and, as a consequence, their fate along the spatial gradient. Our study suggests a photochemically induced loss of carboxylate groups, reflected by an increased carbohydrate-to-carboxylate ratio along the gradient, particularly in winter, and generally lower levels during summer. Loss of carboxylate function promotes the formation of Fe(oxy)hydroxides, which in turn, facilitates the aggregation and sinking of OM, particularly aliphatic components. These insights contribute to a broader understanding of carbon cycling and storage in lakes. Future studies should assess the significance of photochemical processes to OM burial and it how may change given trends in Fe and OM in northern regions.