UNUSUAL STABLE CARBON AND NITROGEN ISOTOPE VALUES IN FRESHWATER FILAMENTOUS ALGAE

Previous studies show that elevated δ¹⁵N values in stream water indicate anthropogenic contamination and that primary producers make excellent monitors of this pollution. Aquatic primary producers typically derive their carbon from dissolved inorganic carbon (DIC) with an isotopic fractionation of -20‰, and therefore are typically 20‰ lighter than DIC. Samples of filamentous algae from eight sites in the highly urbanized Casperkill Creek (Poughkeepsie, NY) were collected and 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. δ¹³C-DIC was about -10‰, therefore the algae was expected to be around -30‰; however, values as low as -42‰ were found. 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 large amounts of methane. Since biogenic methanogenesis favors light carbon, usually around -60‰, this process could account for the unusually low δ¹³C values. This indicates that these landfills are supplying methane to the creek and perhaps other pollutants as well. 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 &muM). Ammonium is known to be an isotopically light N species as well as the favored N species for plants. Moreover, plants prefer the light isotopes of NH⁴. Therefore, when [NH⁴] is high, plants use the light NH⁴ molecules as their N source and as [NH⁴] decreases the plants 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.