Organic matter sources and composition in four watersheds with mixed land cover

As human-dominated landscapes have expanded, the diversity of organic matter (OM) sources to rivers has increased, and knowledge of the composition and quality of these sources is lacking. It is important to characterize novel forms of OM that can affect retention, transport, and transformation of OM within watersheds. We collected fine particulate OM (FPOM) and dissolved OM (DOM) in four watersheds of Northeast Utah with a mixture of urban, forest, and agricultural land cover. We used the natural abundance of 13C, 15N, and 2H isotopes in mixing models to quantify the proportion of terrestrial, autochthonous, and anthropogenic OM. We also used spectroscopic properties of DOM to characterize OM composition. Percent land cover within a watershed did not account for the anthropogenic influence of wastewater treatment plants (WWTP) or grazing and recreation activities. OM was primarily derived from terrestrial sources, except at sites influenced by wastewater, and autochthonous sources at sites influenced by grazing and recreation activities. Sites influenced by WWTPs had more homogenous DOM that was consistently composed of 35% protein-like DOM, while all other sites had greater variability in DOM composition. Further study is needed to understand landscape drivers of DOM composition in watersheds with WWTPs, and such studies should incorporate samples directly from of WWTP effluent.