More and more acutely, coral reefs appear in jeopardy; indeed, the current increased coral mortality is one of the principal consequence of the accentuated natural and anthropogenic stresses. The maintenance of coral reefs results from the equilibrium between constructive (calcification) and destructive forces (bioerosion) in action. Following the death of corals, bioerosion processes are intense. So far, few works have aimed at understanding the impact of environmental factors, in particular sedimentation, and moreover, of the activity of the boring microflora, on bioerosion processes. Thus, the purpose of the present work was simultaneously to study the role of microboring , macroboring as well as grazing organisms, and the spatio-temporal variability of bioerosion under sedimentation conditions.
An experimental study was conducted over a 4 years period using dead Porites coral experimental blocks which were placed in six sites located along a cross shelf transect (200 km) on the northern Great Barrier Reef. The main boring organisms were identified and bioerosion and accretion rates were quantified using image analysis and scanning electronic microscopy.
This study reveals the sedimentation role on the specific composition of the boring micro-flora and macrofauna communities. Low light dependent species such as Plectonema terebrans and suspending feeders such as bivalves and sponges are selected. The penetration depth of microborers inside the blocks and the settlement of boring spores and larvae on the substrates are thus limited at the inshore sites (layer of mud on blocks’ surfaces).
Rates of microbioerosion increase from inshore sites to the oceanic site (0,13 kg.m^2.a^1 to 1,35 kg.m^2.a^1) and with the time of exposure. Microborers are the principal agent of bioerosion after one year of exposure (47 % to 90 %) but play a secondary role after three years of exposure (20 % to 47 %).
The variability of macrobioerosion rates between sites is small in contrast to the intra-site variability. Rates increase also with the time of exposure (0,02 kg.m^2.a^1 to 0,86 kg.m^2 after 4 years of exposure at Harrier Reef). Macroborers, especially bivalves and sponges, are the main agents of bioerosion in the inshore sites after two, three and four years of exposure.
Grazing rates, alike microbioerosion rates, increase from inshore sites to the oceanic site (0,5 kg.m^2 to 5,6 kg.m^2 after 4 years of exposure) because of the impact of terrigenous inputs on grazing fish populations and their source of food (microborers). Grazing rates as well as micro- and macrobioerosion rates, increase with the time of exposure although not linearly (1 kg.m^2.a^1 to 5 kg.m^2 after 4 years of exposure at Osprey Reef). Grazing fishes are the principal agents of bioerosion in the oceanic sites after two, three and four years of exposure.
Accretion rates are negligible in comparison to total bioerosion rates. Total bioerosion as well as net bioerosion rates vary between sites and increase with the time of exposure. Net bioerosion rates are lower at inshore sites where the internal bioerosion predominates, than at the oceanic sites where the external bioerosion (grazing) prevails. At the oceanic sites, blocks are reduced by more than 30 % of their volume and transformed into carbonate particles.
In conclusion, the impact of terrigenous inputs on bioerosion processes and the crucial role played by microborers in the biodestruction of the dead coral Porites (synergy with others agents and high microbioerosion rates) have been clarified. This suggests that many physical and ecological processes interact in determining rates of bioerosion. This emphasises the need for additional studies of the spatio-temporal variability of bioerosion process, including the role of microborers, in order to assess the influence of the impact of massive coral death on reefs as a whole.