A Novel Method of Assessing Bioerosion by the Sea Urchin Echinometra sp. A on a Fijian Reef

Abstract

Sea urchins are major substratum eroders of coral reefs and the calcium carbonate condition of coral reefs may therefore reflect the abundance of sea urchins. The omnivorous urchins, Echinometra spp. are known as “rock -borers” and are agents of large-scale bioerosion, particularly in the Caribbean, Eastern Pacific and Western Indian Ocean. By burrowing into coral rocks they protect themselves from predators, wave and current action, and to some extent from desiccation at low tide. Bioerosion estimates are known for a few specific reefs only. Although the Echinometra sp. A have not been named yet, they were mistaken for Echinometra mathaei for long in Fiji. This novel study attempted to estimate bioerosion rates of the low but consistent numbers of sea urchins, Echinometra sp. A (green-white-tipped) on the Nukubuco reef, Fiji by calculation of the net carbonate accumulation. Cage experiments reported bioerosion rates (kg CaCO3/m2/urchin/d) of 35-37 x 10-3 at the reef crest and 30-43 x 10-3 at the reef flat. This method of assessing bioerosion showed both strengths and weaknesses. However, the Nukubuco reef balance between reef growth and reef destruction is shifting, with bioerosion becoming the dominant process.