Large-scale spatial variability in bioerosion of experimental coral substrates on the Great Barrier Reef (Australia): importance of microborers

To study the effects of terrigenous inputs on rates of bioerosion by grazers and micro- and macroborers, two inshore sites, one mid-shelf site, two outer barrier sites, and one site located in the Coral Sea were selected along a cross-shelf transect (200 km from onshore to offshore) on the northern Great Barrier Reef. At each site, two grids were firmly attached to the substrate on which replicate experimental blocks, cut from live colonies of Porites sp., were laid in order to study the intra-site bioerosion variability as well as the variation between sites. After I year of exposure, rates of bioerosion were estimated using petrographic sections, scanning electron microscopy, and image analysis. Sites differed significantly. The lowest rates were found at the inshore sites (0.46 +/- 0.14 kg CaCO3 m(-2) year(-1)), while the highest rates were found at the outer barrier sites (3.6 +/- 0.52). The pattern of bioerosion exhibited at these six sites varied also, mainly due to different levels of bioerosion by grazers and microborers. Rates of macrobioerosion were low. We suggest that this was due to varying levels of terrigenous inputs, hydrographic patterns, and interactions between agents of bioerosion.