We explored the use of stable nitrogen (N) isotope analysis to assess trophic position of consumers in two marine ecosystems: the kelp forests of southern California and a coral atoll in the tropical Pacific. The δ15N values of consumers in both ecosystems increased from known herbivores (invertebrates and fish) to higher-level consumers (predatory invertebrates and fish). In the absence of data on trophic enrichment in 15N for our study species, we used the oft-cited value of +3.4‰ increase in δ15N value per trophic level and estimates of the δ15N producer baseline value to estimate trophic position. The trophic position of consumers computed using N isotopes compared favorably to published observations of diet. Nitrogen isotope analysis revealed that some of our higher-level fish consumers from rocky reefs (i.e., some rockfish) were feeding largely on invertebrates rather than on fish, as is often assumed. Our analysis also suggests that higher-level consumers on coral reefs may consume more herbivorous prey (i.e., both fishes and invertebrates) than previously reported. Our data support the use of nitrogen isotope values to assess trophic position and, thus, their utility as one metric with which to explore the effects of short- and longer-term natural and human-induced changes on kelp forest and coral reef food webs.
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