Sea level fundamentally affects the biogeography of intertidal zones. Although not typically considered intertidal, corals commonly form shallow reef platforms, where ecologically significant disturbance events may occur when emersed corals are exposed to intolerable atmospheric conditions.
Extensive partial mortality of Acropora corals (~ 4000 ha) at the Houtman Abrolhos Islands occurred in September 2018, when extreme daytime low water levels and coral emersion caused total live coral cover to decline by ~40%. Monitoring of benthic cover was subsequently carried out over a three-year period. Following the initial mortality, dense, seasonal macroalgal over-growth of effected colonies resulted in additional loss of live coral. Overall, however, coral recovery was found to be rapid, with live cover returning to pre-disturbance levels within 18 months. Acropora recovery occurred by radial growth at colony edges, new branches from within the dead centre and re-sheeting of dead in situ branches with live tissue. Apart from Acropora and macroalgal cover, reef structure and coral community composition remained stable over time despite emersion disturbance.
This study highlights the role that sea level plays in driving natural variability in coral cover, the primary metric used to assess the status of coral reef health globally.