U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigation Report 99-4018B

Isotopic Tracing of Nitrogen Sources and Cycling in the Mississippi River Basin

By Carol Kendall, William A. Battaglin, Gilbert Cabana, Cecily C. Chang, Steven R. Silva, Stephen D. Porter, Donald A. Goolsby, Donald H. Campbell, Richard P. Hooper, and Christopher J. Schmitt

ABSTRACT

As part of five pilot studies, several different isotopic approaches for tracing nutrient sources, land uses, and nitrogen cycling processes that are contributing nitrogen to the Gulf of Mexico are being evaluated. Analysis of nitrate for both ¹⁵N and ¹⁸O enhances our ability to distinguish between variations in ¹⁵N due to mixing of sources and variations due to denitrification. Analysis of organic matter for ¹³C, ¹⁵N, and ³⁴S provides information on the terrestrial versus aquatic origin of the material. The ¹⁵N of fish collected at large-river are evaluated as integrators of local variability in nitrogen sources. Preliminary data from these fish suggest that source signatures are being substantially overprinted by other processes at these sites.