Bioerosion of Live Massive Corals and Branching Coral Rubble on Indonesian Coral Reefs

The degree of bioerosion of live massive corals and rubble from branching corals were measured on nine reefs from two regions of Indonesia: the Java Sea and Ambon. Bioerosion in massive corals was measured by collecting live corals, cutting and X-raying slabs, and measuring the cross-sectional area removed from each slab by the various bioeroding organisms. A technique analysing branching coral rubble was developed and similarly used to evaluate the degree of bioerosion on the reefs. This rubble technique has potential advantages over the massive coral technique since it does not require the expense and technical expertise of making and analysing X-rays, nor does it require the destruction of living coral heads. The effectiveness of this rubble technique is evaluated here. Levels of bioerosion in massive coral heads and rubble from branching corals are each compared with environmental variables and health parameters of the nine reefs. Overall, both techniques showed that bioerosion levels were positively correlated with environmental variables indicative of eutrophication. Bioerosion of live massive corals and of branching coral rubble were positively correlated. At the Ambon sites, where the eutrophication levels differ only slightly compared to the Java sites, bioerosion in coral rubble was a more sensitive indicator of eutrophication stress than bioerosion measured from massive coral heads. The rubble technique we outline is a useful rapid reef assessment technique that could be a valuable contribution to the `reef survey toolbox'.