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

Rates of bioerosion by grazing and boring were studied in lagoons of 2 high islands (3 sites) and 2 atolls (2 sites each) In French Polynesia using experimental carbonate substrates (blocks of Porites lutea skeleton). The substrate loss versus accretion was measured after 6 and 24 mo of exposure. The results show significant differences between pristine environments on atolls and environments on high islands sub!ected to different levels of eutrophication and pollution due to human activities. Whereas experimental substrates on the atolls maintain a balance between accretion and erosion or exhibit net gains from accretion (positive budget), only 1 site on a high island exhibits significant loss of substrate by net erosion (negative budget). The erosional patterns set within the first 6 rno of exposure were largely maintained throughout the entire duration of the expeiiment. The intensity of bioerosion by grazing increases dramatically when reefs are exposed to pollution from harbour waters; this is shown at one of the Tahiti sites, where the highest average bioerosional loss, up to 25 kg m-2 yr-' (6.9 kg m-' yr-I on a single isolated block), of carbonate substrate was recorded.