Initial bioerosion and bioaccretion on experimental substrates in high island and atoll lagoons (French Polynesia)

In order to estimate the !osses of CaC03 from bioerosion (grazing and boring), experimental coral substrates were laid in the lagoons of high islands and atolls in French Polynesia. After six months of exposure, the sites could be divided into those exhibiting net bioerosion and those exhibiting net accretion. Of the five sites exhibiting net bioerosion (grazing and macroboring greater than accretion), three were located off high islands and two in atolllagoons. The average net rate of bioerosion was greatest at Faaa in Papeete Bay, Tahiti ( 10.45 kg.m-2.yr-1 ). Of the two sites exhibiting net accretion, the greatest rate of gain (0.81 kg.m-2.yr-1 ) was recorded at the eastern site in Takapoto lagoon. Recorded differences in the composition and distribution of macroborers, microborers and the algal turf appeared to be related to the presence or absence of grazing sea urchins (Echinametra mathaei). It is suggested that the high densities of algal turf recorded at Faaa are a result of elevated nutrients at this site, originating in the sewage discharge from nearby residential and industrial sites.