Andrea, here are a few references with del N-15 values for frewshwater
macrophytes.

Boon, P.I. and S.E. Bunn. 1994. Variations in the stable isotope composition
of aquatic plants and thier implications for food web analysis. Aquatic
Botany 48:99-108.

Keough, J.R., Sierszen, M.E., and C.A. Hagley. 1996. Analysis of a Lake
Superior coastal food web with stable isotope techniques. Limnol. Oceanogr.,
41(1):136-146.

Why are you interested in macrophytes in particular?

Hope this helps, Margaret
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Margaret B. Lansing
U.S. Dept. of Commerce
National Oceanic & Atmospheric Administration
Great Lakes Environmental Research Laboratory
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Ann Arbor, MI  48105
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