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Abstract Dissolved organic matter (DOM) impacts the structure and function of aquatic ecosystems. DOM absorbs light in the UV and visible (UV–Vis) wavelengths, thus impacting light attenuation. Because absorption by DOM depends on its composition, UV–Vis absorbance is used to constrain DOM composition, source, and amount. Ferric iron, Fe(III), also absorbs in the UV–Vis; when Fe(III) is present, DOM-attributed absorbance is overestimated. Here, we explore how differing behavior of DOM and Fe(III) at the catchment scale impacts UV–Vis absorbance and evaluate how system-specific variability impacts the effectiveness of existing Fe(III) correction factors in a temperate watershed. We sampled five sites in the Connecticut River mainstem bi-weekly for ~ 1.5 years, and seven sites in the Connecticut River watershed once during the summer 2019. We utilized size fractionation to isolate the impact of DOM and Fe(III) on absorbance and show that variable contributions of Fe(III) to absorbance at 254 nm (a 254 ) and 412 nm (a 412 ) by size fraction complicates correction for Fe(III). We demonstrate that the overestimation of DOM-attributed absorbance by Fe(III) is correlated to the Fe(III):dissolved organic carbon concentration ratio; thus, overestimation can be high even when Fe(III) is low. a 254 overestimation is highly variable even within a single system, but can be as high as 53%. Finally, we illustrate that UV-Vis overestimation might impart bias to seasonal, discharge, and land-use trends in DOM quality. Together, these findings argue that Fe(III) should be measured in tandem with UV–Vis absorbance for estimates of CDOM composition or amount. 

Riverine dissolved iron (Fe) affects water color, nutrients, and marine carbon cycling. Fe size and coupling with dissolved organic matter (DOM), in part, modulates the biogeochemical roles of riverine Fe. We used size fractionation to operationally define dissolved Fe (< 0.22 μm) into soluble (< 0.02 μm) and colloidal (0.02–0.22 μm) fractions in order to characterize the downstream drivers, concentrations, and fluxes of Fe across season and hydrologic regime at the freshwater Connecticut River mainstem, which we sampled bi‐weekly for 2 yrs. Drivers of colloidal and soluble Fe concentrations were markedly different. The response of colloidal Fe concentration to changes in discharge was modulated by water temperature; colloidal Fe decreased with increasing discharge at temperatures < 10.5°C, but increased with increasing discharge at temperatures > 10.5°C. Conversely, soluble Fe concentrations were only positively correlated to discharge at high temperatures (> 20°C). Soluble Fe was strongly positively correlated to a humic‐like DOM fluorescence component, suggesting coupling with DOM subsets, potentially through complexation. While average colloidal Fe fluxes varied twofold seasonally, soluble Fe fluxes varied ninefold; therefore, soluble Fe variability was more important to the overall dissolved Fe variability than colloidal Fe, despite lower concentrations. Seasonal Fe fluxes were decoupled from discharge: dissolved and soluble Fe fluxes were greatest in the fall, whereas discharge was greatest in the spring. Fluxes of soluble Fe, which may be more bioavailable and more likely to be exported to the ocean, were lowest in the summer when downstream biological demand is high, having implications for primary productivity and iron uptake.