Bioerosion of experimental substrates on high islands and atoll lagoons (French Polynesia) during 5 years of exposure

Rates of loss of CaCO3 from bioerosion (grazing and boring) and gains from accretion were determined from experimental coral substrates exposed for 5 years, which were laid in the lagoons of high islands and atolls in French Polynesia. Significant differences in net rates of bioerosion occurred between sites with the highest rates of loss being recorded at one of the lagoonal sites. The processes contributing to these losses and gains varied between sites. The results were compared with those obtained at the same sites after 6 and 24 months exposure. Rates of bioerosion increased with increasing exposure as did rates of internal erosion as densities of borers increased and boring sponges began to colonise the experimental substrates. High rates of bioerosion were recorded at both eutrophic and pristine sites suggesting that environmental conditions only influence some of the factors controlling rates of bioerosion. Worldwide, the amount of dead coral substrate available for colonisation by boring organisms is increasing as reefs are being subjected to anthropogenic impacts. We discuss how this may change the balance between reef growth and reef destruction by borers and grazers. In cases where the impact may be short lived as in the case of bleaching, we suggest that the maintenance of water quality is especially important in trying to minimise the rates of increase of bioerosion and increases in density of grazers.