Bioerosion rates on coral reefs: interactions between macroborers, microborers and grazers (Moorea, French Polynesia)

A two-year experimental study of bioerosion at Moorea Island, French Polynesia, clearly demonstrated the importance of microborers in the initial stages of the establishment of infaunal boring communities. Rates of erosion by micro- and macroborers and by grazers were estimated from measurements of carbonate removal from experimental substrates, using Image Analysis. The studied substrates have been exposed for 2, 6, 12 and 24 months. After 2 months of exposure, the only borers present in the substrates were cyanobacteria and one chlorophyte (Phaeophila sp.) and their bioerosion rate was estimated at 0.6 kg CaCO3 m(-2) yr(-1). In the course of the 2 years of exposure, recruitment of macroborers occurred and their estimated rates of erosion increased during this period from 2.15 to 90 g CaCO3 m(-2) yr(-1). Carbonate removal by grazers was the dominant agent of erosion, responsible for 89% of the total bioerosion: 2.6 kg CaCO3 m(-2) yr(-1), as recorded in substrates exposed for 2 years. The measurable rates of bioerosion by microborers apparently decreased with the time of exposure from 0.6 to 0.2 kg m(-2) yr(-1), but these values are underestimations which need to be corrected by including the intensity of microboring in substrate layers removed by grazing. Bioerosion is dependent on numerous environmental factors such as depth, light availability, and nutrient supply. A good knowledge of bioerosional processes in modern environments could highlight bioerosion significance in the fossil record.