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This study investigated the variation of bioerosional processes in relation to disturbances of reefal communities due to eutrophication. La Saline fringing reef (Reunion Island) is subjected to nutrient inputs from the adjacent land. Bioerosion by grazers, microborers, and macroborers was measured using experimental substrata exposed for 1 year in three sites characterized by different levels of nutrient input and benthic community response. The relationship between bioerosion and epilithic algal cover of hard substrata and the interactions between the various agents of bioerosion were analyzed with parametric statistics. Significant variations in bioerosion were found among sites, ranging from 1.63 to 3.52 kg CaCO 3 m -2 year -1 for grazing rates, from 6.73 to 32.25 g m -2 year -1 for macroboring rates, and from 43.78 to 67.56 g m -2 year -1 for microboring rates. One of the major factors controlling these variations appeared to be changes in the epilithic algal cover on substrata in response to changes in reefal water chemistry. In low nutrient areas, where dead corals were colonized mainly by algal turfs, erosion by microorganisms was low (43.78 g m -2 year -1 ) due to intense grazing (3.52 kg m -2 year -1 ). In reef zones receiving high nutrient inputs, the development of encrusting calcareous algae and macroalgae was associated with the lowest grazing (1.63 kg m -2 year -1 ) and macroboring (6.73 g m -2 year -1 ) rates recorded among sites. In contrast, high microboring rates (57.54 and 67.56 g m -2 year -1 ) were found in enriched areas in association with high macroalgal cover.

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.

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