INITIAL BIOEROSION AND BIOACCRETION ON EXPERIMENTAL SUBSTRATES IN HIGH ISLAND AND ATOLL LAGOONS (FRENCH-POLYNESIA)

In order to estimate the losses of CaCO3 from bioerosion (grazing and boring), experimental coral substrates were laid in the lagoons of hig h 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 bioer osion (grazing and macroboring greater than accretion), three were loc ated off high islands and two in atoll lagoons. The average net rate o f 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 r ate of gain (0.81 kg.m(-2).yr(-1)) was recorded at the eastern site in Takapoto lagoon. Recorded differences in the composition and distribu tion of macroborers, microborers and the algal turf appeared to be rel ated to the presence or absence of grazing sea urchins (Echino-metra m athaei). It is suggested that the high densities of algal turf recorde d at Faaa are a result of elevated nutrients at this site, originating in the sewage discharge from nearby residential and industrial sites.