Quantifying endolithic bioerosion rates on remote coral reefs in the Central Indian Ocean

Bioerosion of calcium carbonate is a fundamental process that impacts net coral reef accretion. Besides large grazers, endolithic organisms play a major role in carbonate removal. Here we provide the first rate data for both macro- and microendolithic bioerosion in the remote Chagos Archipelago, Indian Ocean. Based on analysis of experimental blocks using computer tomography, we show similar macrobioerosion rates at 5 m (0.086 & PLUSMN; 0.026 kg m-2 yr-1) compared to at 10 m depth (0.066 & PLUSMN; 0.016 kg m-2 yr-1) after three years of exposure, with a succession from worm to sponge bioeroders over time. Microbioerosion rates analysed with scanning electron microscopy were 2-5 x higher than macrobioerosion rates at 5 m (0.187 & PLUSMN; 0.028 kg m-2 yr-1) and 10 m depth (0.313 & PLUSMN; 0.049 kg m-2 yr-1), but the microborer community was dominated by cyanobacteria in all samples. Total endolithic erosion was small compared to external erosion by parrotfishes, which increased over time, but did not show significant differences between 5 m (0.74 & PLUSMN; 0.11) and 10 m depth (1.12 & PLUSMN; 0.16 kg m-2 yr-1) after three years of exposure. These erosion rates are indicative of the oligotrophic and remote reef setting in the Chagos Archipelago (clear water, low nutrients, high fish biomass). The data will help to improve local carbonate budget estimates and provide a context for wider regional and environmental comparisons.