High rates of nitrogen fixation on coral skeletons after predation by the crown of thorns starfish Acanthaster planci

At One Tree Reef, Great Barrier Reef, Australia, between 1983 and 1985, corals killed by the crown of thorns seastar Acanthaster planci L. gave rise to skeletons which were colonised rapidly by blue-green and other algae. For the next 3 to 9 mo these coral skeletons showed over three times more nitrogen fixation (acetylene reduction) than control substratum rates (9 to 32 nmol vs 3 to 10 nmol C₂H₂ cm^-2 h^-1, over all seasons). These values convert to relatively high annual fixation rates of 37 to 127 kg N ha^-1 yr^-1 but, at the low densities of A. planci on One Tree Reef (ca. 0.65 ha^-1), this has little impact on the total nitrogen fixation rate and, as a result, on the level of organic nitrogen in the system. However, it is suggested that on reefs subjected to high aggregations of a. planci such an effect would enhance the level of organic nitrogen and lead to greater primary and secondary production throughout the reef system.