Coral Bleaching and Mortality Thresholds in the SE Gulf: Highest in the World

Bleaching is a stress reaction in corals, during which the symbiosis between corals and the algae (zooxanthellae) living in the coral cells breaks down. As a result, zooxanthellae are expelled, and the coral appears pale or even white (Fig. 6.1; Baker et al. 2008). The link between environmental variables and coral bleaching has been well-established in a variety of studies and synthesized in several places (Phinney et al. 2006; Baker et al. 2008; van Oppen and Lough 2009). Large-scale and region-wide bleaching events, such as occur in the Gulf, have been clearly linked to unusually high temperatures and the accumulation of heat stress in corals. Other drivers, such as UV and water acidity can have compounding effects (Baker et al. 2008) and bleaching can also be caused by these factors alone, or other local drivers such as unusually cold temperatures (Saxby et al. 2003; Lajeunesse et al. 2007). However, it is heat stress that has been demonstrated as the most reliable predictor and defined time-integrated bleaching thresholds exist for various regions of the Indo-Pacific and the Caribbean (Berkelmans 2002a, b; Manzello et al. 2007; Berkelmans 2009).