Sources, fates, and export of organic carbon in the Colorado River Basin

Rivers transform, mineralize, and export organic carbon and are a substantial portion of regional and global organic carbon (OC) budgets. Quantifying the drivers of OC transformation and mineralization are needed to understand how rivers export OC to downstream ecosystems. A substantial flux of OC in rivers is dissolved organic carbon (DOC). DOC sources and fates in large rivers, which drive riverine fluxes, are poorly understood. Furthermore, reservoirs decrease OC export by trapping particulate organic carbon (POC), but the effect of reservoirs on DOC export is less clear. Tailwater ecosystems directly below dams are fundamentally different than reaches upstream of reservoirs, but how these ecosystems alter OC export is less understood. I conducted three studies with the overall objective to quantify source specific fluxes of organic carbon in the Colorado River Basin.;I quantified autochthonous DOC in a tailwater ecosystem as a focus of my first chapter. Few studies have focused on source specific fluxes of DOC, especially autochthonous DOC. I found that although the flux of autochthonous DOC was 7% of the total DOC flux, it was a 43% of gross primary production. Therefore, autochthonous DOC can contribute to daily DOC budgets and a large fraction of primary production in tailwater ecosystems.;In chapter 2, I quantified sources and fates of DOC in the Grand Canyon reach of the Colorado River. Because sources and fates of DOC are empirically difficult to measure in large rivers, I developed a model to estimate source specific fluxes of DOC, including influx and uptake rates of DOC. Using a transport model along with a Bayesian approach, I was able to quantify fluxes of algal and terrestrial-derived DOC. The influx of algal-derived DOC was 2X greater than the influx of terrestrial-derived DOC. Terrestrial-derived DOC was piped and exported downstream. Algal-derived DOC uptake rates were similar to DOC uptake rates empirically measured in streams 1000-fold smaller than the Colorado River, Grand Canyon. The modeling approach I developed can be used to quantify sources and fates of DOC in rivers where otherwise empirical measurements are not possible.;In chapter 3, I estimated OC fluxes along with DOC composition and quality to quantify how OC export changed from free-flowing rivers to reservoirs to tailwater ecosystems. I found that reservoir-tailwater ecosystems alter the sources of OC from more terrestrial to more algal-like. The OC transformation from terrestrial to more algal-like may lead to underestimation of the quantity of terrestrial OC that was buried in reservoirs because total fluxes of OC did not represent transformation processes of OC in river-reservoir-tailwater ecosystems.