Water levels have a fundamental influence on the intertidal zone of coral reefs, where exposure to air limits the upward growth of corals and explains the flattened summits of reefs.  Injury to corals occurs when extreme low water levels and emersion of corals coincides with environmental stressors such as high solar radiation, low air temperature, rainfall and wind stress.  Most stressors are both diurnally and seasonally variable, and therefore the daily and seasonal timing of low water is a key determinant of coral response to emersion and consequent variability in coral cover in intertidal areas.  

The interannual, seasonal, and daily timing of low water was investigated using a high resolution numerical model sea level hindcast (1958-2016) (http://sealevelx.ems.uwa.edu.au). The analysis focused on twenty representative coral reef locations around the Australian coastline between 1992-2016.  From these data, the daily timing of low water was calculated to demonstrate the regionally distinct seasonality of low water events. The relative contribution of tidal and non-tidal variability to the low water extremes were also investigated.  Seasonal patterns and historical coincidence of selected stressors with water levels at each location was investigated using the nearest Bureau of Meteorology daily solar exposure and air temperature data. 

Critical low water events were found to be the result of both tidal and non-tidal factors operating at range of time scales.  The coincidence of low water levels and environmental stressors varied with tidal regime, broad-scale fluctuations in mean sea level and regional climatic variables. 

The study demonstrates the intrinsic temporal and spatial variability in the timing of extreme low water levels, with implications for the assessment of coral reef health.  In an era of escalating and valid concern about anthropogenic impacts on coral reefs, it is important that fundamental baseline processes continue to inform impact assessment.