Carbon and nitrogen stable isotopes of primary producers in coral reef ecosystems

To examine the importance of new primary production and nitrogen fixation in coral reef ecosystems, we compared δ 13 C and δ 15 N values of various organic materials (coral tissues, macrophytes, particulate organic materials, and zooplankton) from Palau (7°N) and Ishigaki (24°30′N) coral reefs in the western Pacific Ocean. The δ 15 N (4–6‰) abundance of coral tissues suggests that zooplankton was not the main source of N for these corals. Low δ 15 N values of other benthic primary producers (2–6‰) indicate that their nitrogen originates from nitrogen fixation. These values and the high δ 13 C (−6 to −9‰) of macrophytes sampled at the landward part of both coral reefs lead us to conclude that coral reef ecosystems are sinks of CO 2 because of primary production accompanied by uptake of new nitrogen.