Dynamics of organic matter (OM) sources in natural aquatic systems have been studied for decades, but urban studies have revealed less studied OM sources such as stormwater, lawn clippings, and wastewater effluent. Increasing temperatures associated with global climate change may also increase in-stream primary production and alter the composition of OM in surface waters. Traditionally, the OM pool in freshwater systems has been simply defined as a homogenous pool of varying size classes, including fine particulate (FPOM), and dissolved OM (DOM). Our goal was to identify and quantity the composition of FPOM and DOM as derived from autochthonous, terrestrial, or potential anthropogenic sources. We hypothesized anthropogenic changes in land-use would increase the proportion of autochthonous sources of OM in rivers of northeast Utah. We sampled OM at 33 sites in 4 watersheds that encompass a range of land-uses. Stable isotopes of carbon, nitrogen, and deuterium were collected for all size classes of OM, and dissolved DOM was analyzed with a spectrofluorometer. Stable isotopes were used to estimate the proportion of autochthonous and terrestrial sources of OM using the Stable Isotopes Analysis in R (SIAR) model. Fluorescence indices and a PARAFAC model were created from DOM excitation emission matrices (EEMs).
FPOM appeared to be a mixture of autochthonous and terrestrial sources but overlap in endmember isotope values made quantifying the proportion of each source difficult. Higher deuterium values (-120‰ to -80‰) were associated with sites receiving wastewater effluent, while sites with agriculture, forest, and other urban land-uses had lower deuterium isotope values (-200‰ to -110‰). All DOM samples were resolved into a 5-componant PARAFAC model. Two components were protein-like, 2 were humic-like, and 1 appeared to trace wastewater effluent. The percent of protein-like DOM tended to be higher in urban versus non-urban sites (mean 35%, S.D. 12% versus mean 25%, S.D. 15%). We conclude that deuterium isotopes may be used as a tracer or wastewater effluent and DOM is composed of more labile, protein-like DOM with increased wastewater input. A greater understanding of the sources of OM can inform management and policy decisions aimed at mitigating the effects of OM pollution and global climate change. For example, by identifying the primary sources OM within a region, management decisions such as tradeoffs between decreasing the effects of cattle grazing versus building or improving waste water treatment facilities could be addressed.