Identifying the Sources of Dissolved Organic Matter in Streams Using Elemental Analysis Isotopic Ratio Mass Spectroscopy (EA-IRMS) Across a Land Use Gradient.

Differences in land use are known to influence the dynamics of dissolved organic matter (DOM) in stream ecosystems. Previous research has linked changes in land use to changes in both DOM quantity and quality, both of which can affect ecosystem processes within streams. However, the actual source of the carbon and nitrogen entering these lotic ecosystems has yet to be described. We used EA-IRMS (Elemental Analysis – Isotopic Mass Spectroscopy) to determine the δ13C and δ15N values of bulk stream water DOM to understand how the source of C and N varies with land use. Approximately 30 streams in New Hampshire and Maine that vary in land use and ultimately drain into The Great Bay, a nitrogen impaired estuary, were sampled. We also measured dissolved organic carbon (DOC) and nutrient concentrations, characterized DOM optical properties using fluorescence and absorbance spectroscopy, and measured DOM bioavailability via DOC biodegradation assays. Regression-based analyses were then used to relate DOM isotopic ratios, optical metrics, and land-use parameters (e.g. % agriculture, population density) to one another and with rates of DOC decomposition.