Inter- and intra- specific variation in bleaching susceptibility among scleractinian corals

Coral bleaching is the disassociation (either expulsion or degradation) of zooxanthellae and the coral host, and it is a general stress response of corals. Anomalous sea surface temperatures have caused widespread incidence of bleaching referred to as mass bleaching; however, chronic, longer-term stress from global climate change is also potentially increasing coral vulnerability to subsequent stress. Sustained and ongoing increases in sea surface temperatures are expected to result in greater incidence of mass bleaching of scleractinian corals, assuming that corals are incapable of acclimating or adapting at required rates. Acclimation is a short-term relief from stress, such as producing heat shock proteins; whereas, adaptation works on populations, hence would use natural selection to produce corals that are more tolerant to stress. A significant limitation in assessing the fate of corals subject to changing environmental conditions is a precise quantitative metric for measuring incidence and severity of coral bleaching. In the field, bleaching is often reported based on the conspicuous "paling" of individual coral colonies, species, or assemblages, but there is not currently a clear and unambiguous definition that can be used to say exactly when individual corals (or populations) are bleached. The purpose of this thesis was to compare among alternative methods used to quantify the incidence and severity of coral bleaching, both at the level of individual colonies and local populations or species, to establish a rigorous quantitative definition for coral bleaching. Bleaching, therefore, can be defined as a loss of greater than half of the zooxanthellae population density, concurrent with rapid changes in physiological quenching efforts, and often displayed as a colour change of 2-3 shades. This metric was then used to explore taxonomic, spatial (geographical), and temporal variation in bleaching susceptibility among scleractinian corals. Chapter 2 of the thesis focused on measures of zooxanthellae density, specifically testing for intraspecific variation in zooxanthellae densities of the common reef coral, Acropora millepora, in the Palm Islands, inshore Great Barrier Reef. Various methods are available to quantify zooxanthellae densities; however, a direct comparison of these techniques has yet to be done. Here, we compare destimates of zooxanthellae densities obtained using conventional airbrushing coupled with post-tissue-blasting surface area determination, versus a technique whereby zooxanthellae densities are quantified from a known area (0.25 cm²) of tissue after corals have been fixed and decalcified. Estimates of zooxanthellae densities obtained using the two different methods were significantly correlated (R＝0.40, n＝81, p