| Summary: | Nitrogen isotope fractionation in natural systems has become an established method for the description and study of biogeochemical cycling on a large scale. Deviations in naturally occurring dissolved and particulate 15N and 14N levels on relatively small geographic scales have been well documented. However, relatively little work has been done to investigate how these local variations are reflected at the ecosystem scale. Using samples collected at sea and analyzed onshore using isotope ratio mass spectroscopy (EA-IRMS), this project tracked the variations in 14N and 15N in a small-scale ecosystem - in this case, part of the Northeast U.S. continental shelf - in order to quantify the approximate geographic scale of nitrogen cycling. We found that local “baseline” variations will indeed propagate up the food web, from raw nutrients through phytoplankton into zooplankton. Mapping of isotopic gradients provides a baseline from which to interpret nitrogen isotopic measurements of higher trophic levels in the region.
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