Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to aerial stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival, growth and functionality in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline to identify seasonal patterns and historical occurrence of coral emersion mortality risk.
Local tidal characteristics were found to dictate emersion mortality risk on a seasonal and regional basis. In general, risk was greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and at night in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels.