Net erosion and accretion of experimental blocks of Porites sp. skeleton deployed for 10.6 to 20.3 years at Lizard Island, northern Great Barrier Reef

Rates of carbonate erosion and accretion of coral skeletons are critical for understanding carbonate budgets that affect reef growth but require long-term studies. This study examines internal and external rates of accretion and erosion rates using experimental blocks (n = 32) of coral skeleton (Porites sp.) deployed between 1980 and 2000 at 6 reef zones around Lizard Island. This is the longest study of its kind in the GBR (10.6 to 20.3 years). Volume analysis of trace bioerosion holes from computerised tomography scans are paired with counts of organisms that were obtained by dissolving a portion of each coral block. Surface grazers contribute the most to overall bioerosion except at Reef flat and Deep leeward zones where net external and internal erosion contribute similarly to net rates. Reef zones appear to affect net external erosion rates with lower rates on the Reef flat and higher rates at Lagoonal zones. Net external and internal rates are equivalent at the Reef flat zone likely from limited grazer access at low tide. This unique study interprets bioeroder communities and net rates of bioerosion with important applications for understanding long-term rates.