Modelling sub-reef thermodynamics to predict coral bleaching: a case study at Scott Reef, WA

Coral bleaching occurs when corals become stressed, which typically occurs during periods of elevated water temperatures. If water temperatures remain elevated for a sufficient length of time, the corals often die. Coral bleaching affects reefs around the world and the recent increase in the frequency and severity of bleaching episodes has raised considerable concern. A clear understanding of the physics that elevate water temperatures may improve coral bleaching predictions and lead to more effective reef management. Currently, the best method to detect bleaching-like conditions is through a time integration of sea surface temperatures observed by satellite. Unfortunately, these observations only reveal the thermal structure for the top millimetre of water averaged over large areas (presently 2500 km2). The aim of this study is to use environmental physics to predict water temperatures at the reef and sub-reef scales. The study then goes a step further and translates these thermodynamic models into bleaching predictions. Simulations are run using atmospheric and oceanographic data from Scott Reef, a 40 km-wide atoll 300 km off the northwest coast of Australia. Scott Reef presents an ideal test site as it experienced a severe bleaching event in 1998 that was well documented. Averaged coral cover in exposed sites dropped from 54% to less than 10% over the top 30 metres. Additionally, the bathymetry around Scott Reef has been thoroughly surveyed and in 2003 an extensive array of oceanographic instruments was deployed for three months at strategic locations. A one-dimensional turbulence model is used to determine the vertical temperature structure of the water column around Scott Reef. Scott Reef is in a data-sparse region so that all of the heat fluxes have to be estimated from atmospheric conditions recorded at distant weather stations. The model results are verified with the 2003 field data. By driving the model with the appropriate atmospheric conditions, the simulated temperature profiles match the field observations. The model is next used to hindcast the temperature profiles during the 1998 bleaching event. Simulations indicate that anomalously-warm water most likely reached depths of 30 metres, a result that supports the claim that the deep bleaching was due to thermal stress. Field observations confirm that water currents around Scott Reef are predominantly tidal. Additionally, the observations demonstrate that the upper layers of certain regions within Scott Reef cool during strong tides. This finding is characteristic of tidal cooling, a common phenomenon where tides mix cooler, deeper water with warmer surface water. A map of sub-reef regions at Scott Reef that might experience tidal cooling is revealed by the numerical modelling. In a novel approach, stratified waters from the vertical model can be well-mixed in zones identified by a two-dimensional hydrodynamic model. There is a strong correlation between areas where bleached corals survived and locations that are predicted to have access to cooler well-mixed deep waters. The techniques used in this work are applicable to other reef systems. Therefore the results in this thesis are significant as they improve two aspects of coral bleaching prediction. First, the methods can determine if coral at different depths are at risk of bleaching. Second, the methods can distinguish regions within individual reefs that are more susceptible to coral bleaching.