Courtney Drayer
cdrayer@rsmas.miami.edu
The Cadmium reduction method of DON and DIN isotopic analysis and initial
results from Biscayne Bay
Relative abundances of stable nitrogen isotopes can be used to identify various
anthropogenic and natural input sources in terrestrial, freshwater, and marine systems.
Solid sample analysis is completed with well established methodology while
methodology for dissolved nitrogen has been more challenging as a result of the low
natural abundances of inorganic nitrogen in seawater (> 1M). To this end, a method
using bacterial conversion of nitrate to nitrous oxide developed by Sigman et al. (2001)
has proved to be insufficient for the ambient seawater conditions found around South
Florida. In 2005, Mcllvin and Altabet described a new method for nitrogen isotopic
analysis of nitrate and nitrite in seawater with natural abundances. In this technique,
nitrate is converted to nitrite through cadmium reduction and then to nitrous oxide with a
1:1 azide and acetic acid solution. The gas is then analyzed on a stable isotope ratio mass
spectrometer. Additionally, an ammonium analysis can be completed by a step-wise
hypobromite oxidation under basic conditions prior to cadmium reduction (Zhang et al.,
2006). The nitrogen isotopic composition of dissolved organic nitrogen (DON) can also
be determined by preceding the initial cadmium reduction with a persulfate digestion.
Isotopic values for individual nitrogen species are then determined from a weighted
average calculated from nitrite concentrations. The techniques of Mcllvin and Altabet
have been used, successfully, to analyze seawater samples from Biscayne Bay, Florida
(Drayer et al., 2006). The seawater samples are part of a more comprehensive study of
nitrogen isotope systematics in the region Initial results for 15N of DIN range from ~4.5‰ to ~+10‰ and 15N of DON range from ~0.5‰ to ~+18‰. Canal DON 15N
values contained the heaviest measured values in the study at ~+18‰, while nearshore
and offshore DON values were lighter, 4.9 (±1.6‰). A plot of DIN 18O versus 15N
shows a clear sewage component in the canal waters and more complex nitrogen sources
within the Bay. This study will continue for the next two years to increase the spatial and
temporal resolution of the results.