Sea level variability and coral cover on intertidal reefs

Jo Buckee

Sea level fundamentally affects the ecology and biogeography of intertidal zones. Although not typically considered intertidal, corals commonly form shallow reef platforms, where ecologically significant coral disturbance events may occur when emersed corals are exposed to intolerable atmospheric conditions. The major goal of my research is to investigate the role that sea level plays in driving variability in coral cover, the primary metric used to assess the status of coral reef health globally. I do this through detailed field studies at the Houtman Abrolhos Islands (The Abrolhos), Western Australia, and theoretical analysis of historical sea level and atmospheric data for coral reef locations around the Australian coastline. Extensive partial mortality of Acropora corals on shallow leeward reef platforms at the Abrolhos was observed from August to November 2018. The estimated decrease in total live coral cover due to the event was ~40 % (~35 % to 22 %). Emersion mortality was attributed to seasonal mean sea level minima associated with the southward flowing Leeuwin Current, and interannual variability related to the El Niño–Southern Oscillation. The daily and seasonal timing of low tide was also a key determinant of coral disturbance during emersion, due to marked diurnal and seasonal variability in atmospheric stressors. To explore the effect of predictable daily timing of low tide for varying tidal regimes, the coincidence of emersion with selected stressors was investigated for eight coral reef locations around the Australian coastline. Hourly water levels (1992 – 2016) were linked to atmospheric records to identify seasonal patterns and historical occurrence of coral emersion mortality risk. In general, risk was found to be greatest during the Austral spring, when a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors. Following emersion mortality at the Abrolhos, monitoring of benthic cover was carried out over a three-year period. Dense, seasonal macroalgal over-growth of effected branching Acropora colonies resulted in additional coral loss. Overall, coral recovery was found to be rapid (>10 % increase per annum), with live cover returning to pre-disturbance levels (~35 %) 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 cover, reef structure and coral community composition remained stable over time despite emersion disturbance. My results illustrate the fundamental role that sea level variability plays in driving temporal variability in coral cover on reef tops, and highlights the importance of considering emersion history when assessing all intertidal environments, including shallow coral reefs. The study addresses an important knowledge gap in both the field of sea level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.

Sea level variability and coral cover on intertidal reefs

Files and links (1)

Abstract

Details

Metrics