A Multiyear Study of the Variability in Organic Matter Concentration and Composition in a Flashy Temperate Stream

Thursday, 18 December 2014: 5:15 PM
Elizabeth C Minor1, Megan J. Macdonald2 and Prosper Kojo Zigah2, (1)University of Minnesota Duluth, Large Lakes Observatory and Dept of Chemistry and Biochemistry, Duluth, MN, United States, (2)University of Minnesota, Water Resources Science and Large Lakes Observatory, Duluth, MN, United States
River and stream systems are highly dynamic, exhibiting strong temporal and spatial variability in the processing of organic matter. In this study the temporal variability in organic matter concentration and composition in a flashy local stream (Amity Creek, Duluth, MN) was analyzed over the course of 2.5 years in an attempt to better understand the delivery of organic matter from this stream into its receiving body (Lake Superior). Amity Creek is a relatively pristine (2% urban), primarily forested (71%) stream system with a watershed area of 42.6 km2 and an average slope of 9.6°. Sixty-five grab samples of whole water were taken from a sample site 0.3 km from the confluence of this stream with Lake Superior. The samples were analyzed for total, particulate, and dissolved organic carbon (TOC, POC, and DOC) concentrations and characterized by UV-Visible spectrophotometry. Selected samples were also analyzed for isotopic (stable and radiocarbon) signatures. UV-visible proxies (spectral slope, E2/E3 values) show variations with both season and flow. Spectral slopes are lower in spring, higher in the summer and early fall, and more variable in late fall. Higher S values are also seen in low flow periods (which often correspond to mid-to-late summer). E2/E3 values, inversely related to molecular weight, are lower in spring and fall relative to summer and also increase as flow decreases. TOC and DOC concentrations do not appear to vary systematically with season. They are correlated with stream flow in low to mid flow conditions but not at high flow. POC and DOC during high flow (in June 2008) have modern (post-1950s) radiocarbon signatures while DOC sampled during low flow (September 2008) has a pre-bomb radiocarbon signature (Δ14C of -13‰). DIC from the same September sample has a modern signature (Δ14C of 21‰), indicating that the DOC is not from concurrent primary production affected by a reservoir effect but from an older allochthonous source.