Coral Reefs
CORAL BLEACHING George W. Stoyle
Coral bleaching is characterised by loss of colour due to the loss of the host organism’s symbiotic algae, usually as a result of elevated sea surface temperatures and increased solar radiation. The process of bleaching is caused by the host organism rejecting its symbiotic zooxanthellae due to a breakdown of the enzyme system responsible for detoxifying oxygen produced during photosynthesis. As a defensive response coral polyps dispose of the algae in order to prevent oxygen toxicity. Without nourishment provided by zooxanthellae (providing 50-95% of metabolic function) corals are reliant on direct feeding methods using nematocysts to catch live prey. If environmental conditions do not stabilise zooxanthellae will not return and coral organisms will begin to experience alterations in reproduction, tissue degeneration and loss, reduced growth rates and eventually the colony will die. In the event of sustained temperature increases mass bleaching incidents can occur with high levels of coral mortality. The loss of live coral cover reduces biodiversity affecting tourism and fisheries as well as threatening the vital goods and services provided to over 500 million people globally. As a consequence the livelihoods of many communities may be lost.
The frequency, severity and scale of coral bleaching is predicted to escalate as sea surface temperatures increase as a result of global warming. Mortality could exceed 95% in some regions with the potential for species extinctions. A sustained temperature increase of 1-2°C would cause warming that falls within the lethal limits of most reef-building coral species. However, although all corals have the potential to bleach as a response to varying stressors, some species are less susceptible than others. In some parts of the world corals have adapted to withstand very high or low water temperatures without bleaching, indicating high levels of resistance. Bleaching is also an adaptive mechanism allowing the coral to be repopulated with a different type of zooxanthellae following the event.
This may confer greater resilience and stress resistance. Different strains of zooxanthellae exist both between and within different species of coral hosts, and different strains of algae show varied physiological responses to both temperature and irradiance exposure. The coral/algal association may have the scope to adapt within a coral’s lifetime. Resilience and resistance of corals to bleaching is a process little understood, but could hold one of the keys to preventing or minimising future mass bleaching events. It is therefore critical to obtain real-time information whenever bleaching events are occurring. By revisiting sites known to be susceptible to bleaching and comparing similar sites throughout the region, it will be possible to build up a synoptic overview allowing identification of resistant corals and associated environmental factors potentially contributing to resistance and subsequent resilience.
IDENTIFICATION
The obvious charateristic of a bleach coral is whiteness, but there are other conditions that look similar to bleaching, such as parrotfish grazing scars, various coral diseases and new growth. The main thing to look for is a change in color of the tissue attached to the skeleton, but NOT the loss of actual tissue. This can remain in-tact and relatively healthy, but look translucent. Polyps also remain alive and in some cases it should be possible to see these during the day and certainly during the night.