The role of inorganic nutrients and herbivory in controlling microbioerosion of carbonate substratum

The effect of herbivore abundance and nutrients on microborer communities and their rates of bioerosion of Strombus gigas shells was studied using herbivore-exclusion cages and inorganic fertilizers at Glovers Reef, Belize. Microborers colonizing shells in each treatment were identified and their colonization rates were calculated from scanning electron microscopy of the boring casts. In all treatments the dominant microborer was the green alga Phaeophila sp. Cyanobacteria were most abundant within fertilized and fungi in unfertilized treatments. The highest microbioerosion rates and most distinctive microborer community were found on the treatment with both reduced herbivores and fertilization. All fertilized treatments had significantly higher bioerosion rates than unfertilized treatments. Treatments with macrograzer access had microbioerosion rates less than half the fertilized cages. Bioerosion rates in unfertilized treatments were lowest and not different with and without macrograzers. Consequently, increased nutrient concentrations on reefs have the potential to increase rates of microbioerosion and macrograzers can modify the composition and density of the microborer community