Size and optical properties of dissolved organic matter in large boreal rivers during mixing: Implications for carbon transport and source discrimination

Study region: The lower Athabasca River (LAR), Northern Alberta, CanadaStudy focus: The functionality of dissolved organic matter (DOM) in natural waters largely depends upon its size and composition. Identifying the sources, associated properties and transport behavior of DOM is vital to understand its effects on downstream ecosystems. River mixing has great potential to alter DOM quality and transport, but the extent of these impacts is largely unknown in large boreal rivers. New hydrological insights for theregion: Inputs of DOM from tributaries served as a major source and shifted DOM quantity and quality in the LAR towards higher concentrations of dissolved organic carbon (DOC), and greater degrees of humification and aromaticity. Seasonal variations in DOM quality were observed during spring freshet, including elevated molecular mass (i.e., size), and proportions of protein-like components (i.e., tryptophan-like). The conservative and delayed mixing of DOM was apparent at three large confluences, and contrasting mixing patterns for different tributaries suggested that these patterns were governed by both hydrological conditions and river geomorphology.Results suggest that DOM may be generally useful as a conservative tracer at large confluences in boreal rivers, thus highlighting its potential importance as a tracer for source discrimination in mixing zones.