Analysis of δ13C, δ15N, and δ34S in organic matter from the biominerals of modern and fossil Mercenaria spp.

The response of primary producers to changes in environmental and sea-level conditions is an important topic in the study of estuarine ecosystems. Currently, sea levels are rising, and North American estuaries are in declining ecological health (USEPA, 2002 National Coastal Condition Report. USEPA, Washington, DC). This investigation examines the base of the estuarine food web by evaluating the diets of modern and fossil Mercenaria spp. These clams are infaunal, primary consumers that feed on suspended organic matter. Proteinaceous materials secreted within the biominerals of shell-building organisms are derived from the same diet sources as soft tissue and, therefore, are also capable of recording diet information. We have developed an empirical relationship between the isotopic composition of shell organic matter and soft tissue, which allows food web analyses in fossils to proceed in the same manner as modern food web studies. The Δtissue-shell for δ13C, δ15N, and δ34S is 0.1, 0.7, and 1.8‰. Isotopic analyses of modern Mercenaria spp. shells from coastal Virginia (δ13C: −12.8 to −15.9‰, δ15N: 11.3 to 13.7‰), and the Gulf of Mexico coast of Florida (δ13C: −19.4 to −21.0‰, δ15N: 7.2–9.9‰) demonstrate that diets are derived from phytoplankton and local salt marsh plants. Shell organic-matter δ13C varies from −13.1 to −27.5‰ and δ15N varies from 2.4 to 9.8‰ in Quaternary Mercenaria spp. from coastal Virginia, North and South Carolina, and Georgia. These data demonstrate that past diet sources have changed in space and time. The fact that past sea-level changes have been accompanied by shifts in local primary production is evidence that significant changes might also occur in association with presently rising sea levels.