Spatial and temporal variability in stream dissolved organic carbon quantity and quality in an Adirondack forested catchment

Abstract

The dissolved organic carbon (DOC) quantity and quality in streams regulate many ecosystem processes at the watershed scale. There is, however, a dearth of information on the spatial variability of in-stream DOC quality in small catchments. Our study used direct DOC concentration measurements and fluorescence spectroscopy indices to determine how stream DOC quantity and quality changed over space and time in a forested catchment of the US Northeast, and provided insight into how these systems might respond to changes in land use and/or climate in the coming years. Land cover (e.g., wetlands, lakes) exerted a dominant role over changes in flow and/or air temperature at regulating DOC concentrations in the watershed. Wetland areas acted as large sources of humic-rich DOC, while lakes were DOC sinks, especially for humic-rich DOC. DOC quality indices were generally significantly (p<0.05) correlated to DOC concentrations regardless of location and time of year, with high DOC concentrations (>7mgCL−1) primarily tied to the mobilization of terrestrial highly degraded humic rich DOC, likely to be less bioavailable (and less fresh) than the DOC exported at times when DOC concentrations are low (<7mgCL−1). Overall, results pointed to a two-phase DOC export whereby mobile and bioavailable fractions of DOC in surface water (protein-like DOC) are produced throughout the watershed (including the wetland), while recalcitrant humic-like DOC fractions are predominantly generated by the wetland, before preferentially sedimenting out in the lake. As the climate continues to change, we will likely find not only an increase in the amount of DOC exported at the watershed scale, but also a shift in the quality of this DOC toward less bioavailable DOC fractions rich in humic substances.