Connecting the Age and Fraction of Riverine Labile Organic Carbon to Watershed Geology and Land Use

The cycling of organic carbon (OC) in aquatic systems plays an important role in both the global carbon cycle as well as the quality of water resources. Rivers erode and transport OC and act as the main delivery pathway for terrestrial carbon to the ocean, however some of the OC is metabolized in the river network, providing an input of chemical energy to aquatic ecosystems and influencing the overall carbon cycle of rivers. The traditional view has been that rivers export relatively modern OC and that microbial communities will preferentially metabolize the younger fraction of the bulk OC. However, recent work in both surface water and groundwater systems have indicated that aged OC may be bioavailable and actively metabolized.

Determining the quantity and age of the labile OC in rivers is important to our understanding of the global carbon cycle and to how the carbon cycle will respond to watershed disturbances that mobilize aged OC from soils and underlying rocks and sediments. We selected streams draining watersheds of differing geologies (OC-rich shales and OC-poor crystalline rock) and land use (forested and agricultural) in order to identify how these factors influences the quantity and age/source of the labile OC. To characterize the fraction of labile dissolved organic carbon (DOC) and the age of this labile fraction we performed laboratory incubation experiments on water collected from six streams in the Mohawk and Hudson River watershed in Upstate New York. Carbon-dates of the DOC were taken at the start and end of the incubation experiments and DOC concentrations and UV absorbance were measured over the course of the experiments. To provide additional geochemical context, we also measured stream water particulate OC and dissolved inorganic carbon concentrations and C isotopic signatures along with major dissolved elemental composition.

Based on our incubation experiments, we determine the fraction of labile DOC and the age of the labile component for each of the six streams assessed in this study and link these results to the geology and land use of the watersheds drained by the streams. We then use our results to make general predictions of how the cycling of DOC within rivers may respond to watershed disturbances that change the amount and/or relative contributions of aged vs. modern organic carbon.