Stable Carbon and Nitrogen Isotopes of Periphyton in a Highly Urbanized Stream

Previous studies show that elevated δ¹⁵N values in stream water indicate anthropogenic contamination and that aquatic primary producers make excellent monitors of this pollution. Additionally, aquatic primary producers typically derive their carbon from dissolved inorganic carbon (DIC) with an isotopic fractionation of about -20‰. Samples of filamentous algae from eight sites in the highly urbanized Casperkill Creek (Poughkeepsie, NY) were collected in July 2007 and May 2008 and were analyzed for δ¹⁵N and δ¹³C in an attempt to understand the biogeochemistry of this small stream and assess potential pollution sources. Algal δ¹³C values in the Casperkill were lighter than expected, with values as low as -42‰ measured. Two of the three sites with low δ¹³C values are downstream from capped landfills, and the other from a stagnant pond, both of which may produce methane. Since methane usually contains light carbon, oxidation of this carbon source could account for the unusually low δ¹³C values. δ¹⁵N values were also not as expected. The reach of the stream with the highest nitrogen concentrations, the reach assumed to receive the highest pollution load, had the most negative δ¹⁵N values, which is contrary to what was expected. However, this stream has elevated ammonium concentrations (up to 73 µM). Ammonium is known to be an isotopically light N species as well as the favored N species for algae. Moreover, algae prefer the light isotopes of NH₄. Thus, when NH4 is high, algae use the light NH₄ molecules as their N source and as NH₄ decreases the algae use the remaining heavier NH₄. Therefore, as NH₄ (and nitrogen pollution) decreases, δ¹⁵N values increase (R² = 0.88) – contrary to the generally accepted response. This indicates that understanding nitrogen species cycling within the aquatic environment is crucial to the use of δ¹⁵N values as a pollution indicator.