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 delta(13)C and delta(15)N values of various organic materials (coral tissues, macrophytes, particulate organic materials, and zooplankton) from Palau (7 degrees N) and Ishigaki (24 degrees 30'N) coral reefs in the western Pacific Ocean. The delta(15)N (4-6 parts per thousand) abundance of coral tissues suggests that zooplankton was not the main source of N for these corals. Low delta(15)N values of other benthic primary producers (2-6 parts per thousand) indicate that their nitrogen originates from nitrogen fixation. These values and the high delta(13)C (-6 to -9 parts per thousand) of macrophytes sampled at the landward part of both coral reefs lead us to conclude that coral reef ecosystems are sinks of CO2 because of primary production accompanied by uptake of new nitrogen.