BIOEROSION OF EXPERIMENTAL SUBSTRATES ON HIGH ISLANDS AND ON ATOLL LAGOONS (FRENCH-POLYNESIA) AFTER 2 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 Polynesi a using experimental carbonate substrates (blocks of Porites lutea ske leton). The substrate loss versus accretion was measured after 6 and 2 4 mo of exposure. The results show significant differences between pri stine environments on atolls and environments on high islands subjecte d to different levels of eutrophication and pollution due to human act ivities. Whereas experimental substrates on the atolls maintain a bala nce between accretion and erosion or exhibit net gains from accretion (positive budget), only 1 site on a high island exhibits significant l oss of substrate by net erosion (negative budget). The erosional patte rns set within the first 6 mo of exposure were largely maintained thro ughout the entire duration of the experiment. The intensity of bioeros ion by grazing increases dramatically when reefs are exposed to pollut ion from harbour waters; this is shown at one of the Tahiti sites, whe re the highest average bioerosional loss, up to 25 kg m(-2) yr(-1) (6. 9 kg m(-2) yr(-1) on a single isolated block), of carbonate substrate was recorded.