The three steps of the carbonate biogenic dissolution process by microborers in coral reefs (New Caledonia)

Biogenic dissolution of carbonates by microborers is one of the main destructive forces in coral reefs and is predicted to be enhanced by eutrophication and ocean acidification by 2100. The chlorophyte Ostreobium sp., the main agent of this process, has been reported to be one of the most responsive of all microboring species to those environmental factors. However, very little is known about its recruitment, how it develops over successions of microboring communities, and how that influences rates of biogenic dissolution. Thus, an experiment with dead coral blocks exposed to colonization by microborers was carried out on a reef in New Caledonia over a year period. Each month, a few blocks were collected to study microboring communities and the associated rates of biogenic dissolution. Our results showed a drastic shift in community species composition between the 4th and 5th months of exposure, i.e., pioneer communities dominated by large chlorophytes such as Phaeophila sp. were replaced by mature communities dominated by Ostreobium sp. Prior the 4th month of exposure, large chlorophytes were responsible for low rates of biogenic dissolution while during the community shift, rates increased exponentially (x10). After 6 months of exposure, rates slowed down and reached a plateau with a mean of 0.93 kg of CaCO3 dissolved per m(2) of reef after 12 months of exposure. Here, we show that (a) Ostreobium sp. settled down in new dead substrates as soon as the 3rd month of exposure but dominated communities only after 5 months of exposure and (b) microbioerosion dynamics comprise three distinct steps which fully depend on community development stage and grazing pressure.