Rapid external erosion of coral substrate in subtropical Hong Kong waters

Highlights: • Coral substrate erosion was studied by deploying coral blocks in the field • Erosion and accretion were quantified using microCT • High erosion was found in more than half of the study sites • External erosion was the main contributor to the overall erosion • Sea urchin grazing was likely the cause of the high external erosion Erosion of coral substrate plays a crucial role in reef calcium carbonate budget, but little is known about erosion in subtropical corals. In a 2-year study of coral substrate erosion, we deployed Porites skeletal blocks at nine sites across subtropical Hong Kong waters. External erosion varied from 0.05 to 3.07 kg m⁻² yr⁻¹ and accounted for 23.4–99.2% of the total erosion. More than half of the study sites had substantial external erosion (> 1 kg m⁻² yr⁻¹), and the values were positively correlated with density of the sea urchin Diadema setosum. Excluding urchins from access to the skeletal blocks using cages reduced external erosion by more than 90%. Overall, our study revealed that external erosion caused by urchin grazing contributed predominantly to the total coral skeletal loss in Hong Kong waters. Control of sea urchin population is needed to reduce coral erosion in places with high urchin density. (149 words).