NITROGEN ISOTOPES IN PRESERVABLE HARD PARTS INDICATE TROPHIC LEVEL: A CASE STUDY FROM THE GULF OF CALIFORNIA

Nitrogen isotopes are indicators of trophic level in living organisms. The tissue of organisms high in the food chain record higher δ¹⁵N values than tissues of organisms lower in the food chain. A fractionation of ~3.4‰ generally occurs with each increase in trophic level. The fossil record rarely preserves the tissues commonly used in modern food-web analyses. We assess how differences in δ¹⁵N values within the hard parts of organisms (e.g., shell, otolith, tooth) reflect trophic position. We examined δ¹⁵N values from 17 species of marine invertebrates and vertebrates in the northern Gulf of California, Mexico. Invertebrate hard parts (n=12) include the shells of bivalves, gastropods, and an echinoderm. The vertebrate (n=5) material comes from fish otoliths and a sea lion tooth. Organic matter content in hard parts was typically less than one percent, and 8 to 40 milligrams of shell or tooth enamel was used for analysis. Samples were crushed to a fine powder to allow complete combustion of samples without the need for acidification. Eight of the 17 species fall into the lowest range of δ¹⁵N values, from 5.46 to 9.97‰. These low δ¹⁵N values relative to values from other taxa suggest a low trophic position. These include herbivores, suspension feeders, and a deposit feeder. Surprisingly, two predatory gastropods, Natica sp. and Conus sp., with lower than expected δ¹⁵N values also fall into this group. Three of the 17 species, all suspension feeding bivalves, have intermediate δ¹⁵N values ranging from 13.2 to 14.7‰. Six species scored the highest values, ranging from 15.2 to 18.64‰; these include predatory fish, a predatory gastropod, and the California sea lion. Nitrogen isotopes provide insight into the food web structure of the northern Gulf of California. Our results illustrate that δ¹⁵N values in shells and other hard parts can be useful indicators of trophic level in the fossil record.