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Stable nitrogen isotopes have customarily been used to dehneate trophic position with only scant regard to source variability in isotopic composition. A cornpilabon of literature data indicates, however, that marine invertebrates are enriched in ISN relative to those inhabiting freshwaters. Estuarine mussels were also found to have intermediate 6I5N values which reflected their location along a freshwatermarine gradient. The use of invertebrate 6I5N as a measure of continental-marine coupling in complex coastal environments could therefore provide additional support for conclusions based on traditional 6% analysis.