Mass bleaching and the environment
Some of the important features of mass bleaching are as follows.
- Because mass bleaching is caused by the inability of corals to process all the oxygen from light produced by photosynthesis, it only occurs in corals exposed to strong sunlight.
- Corals become susceptible to bleaching if the water temperature exceeds historical limits. What matters is the amount of heating these limits rather than absolute temperature as this varies geographically.
- Short periods of high temperature may be more stressful than longer periods of a slightly lower temperature but bleaching stress is basically cumulative over periods of weeks to months rather than a sudden all-or-none reaction.
- Bleaching (but probably not mass bleaching) may be caused by stresses other than temperature. These include increased ultra-violet radiation as distinct from solar radiation, lack of water movement around the colonies, low salinity after floods, bacterial infection, sudden chilling, and chemical pollutants. However, it is the combination of high temperature and high solar radiation (particularly its ultra-violet component) that causes maximum bleaching.
- Corals growing on equatorial reefs are more tolerant of heat than those growing at higher latitudes. For example, corals bleach at lower temperatures on the southern Great Barrier Reef than they do in the north, indicating the importance of temperature thresholds.
- Bleaching can occur in high latitude coral reefs including Hawaii, Bermuda and Lord Howe Island.
- There is local geographic variation in the extent of bleaching within the same species.
- Susceptibility to bleaching is affected to a significant extent by the history (experience) of individual coral colonies.
- Some groups of corals are more susceptible than others. The families Acroporidae and Pocilloporidae are commonly the most affected as is the hydrocoral Millepora. The least affected are the corals with massive growth forms, notably the faviids. There are, however, many reported differences in detail among studies. There have been several imaginative ideas about this, one being that least affected species belong to the oldest families, another that the faster growing corals are most susceptible, another that it is mostly a matter of tissue thickness and associated densities of zooxanthellae. However, all corals with zooxanthellae will bleach if conditions become severe enough for long enough.
- Mass bleaching is most common and most severe in shallow communities, however it is not restricted to the shallows. Major occurrences have been recorded to depths of over 25 metres on the Great Barrier Reef and 30 metres on Scott Reef off Western Australia.
- Mass bleaching on the Great Barrier Reef is more common in inshore communities, however even within the same region it is occasionally more severe offshore. It may be particularly prevalent in intertidal pools.
- Corals that are aerially exposed at low tides are the most heat (and light) tolerant of all and often do not bleach when those in nearby deeper water do.
- Bleaching inhibits coral growth, but this is temporary if the stresses are removed and normal levels of zooxanthellae return.
- Bleaching can disrupt the reproductive cycle, an outcome which has the potential to inhibit recovery of some coral species and thus change their relative abundance.
- Lower energy reserves caused by repeated bleaching may have contributed to disease outbreaks that have greatly affected the corals of Florida and the Caribbean. This is now being seen increasingly on the Great Barrier Reef.
- For healthy reefs, there are many accounts of recovery of bleached corals and this may occur in as short a time as several months. Even completely white corals sometimes recover. Recovery of coral communities in which most corals have died normally takes a decade or more in pristine environments depending on larval supply, algal take-over after bleaching, and other local factors.
- Some genetic types of zooxanthellae appear more resistant to high levels of light and/or temperature (up to 1-1.5°C) than others although this also remains controversial.
- Mass bleaching and ocean acidification are likely to have future synergistic effects.
Links to El Niño events
There is no clear link between enhanced greenhouse warming and the frequency or intensity of El Niño events. Analyses of historical records and projections from General Circulation Models are both ambivalent on the subject. The general climatic changes accompanying El Niño development are fairly well understood, although the factors controlling their initiation, intensity and periodicity remain obscure. The 1997-98 El Niño event was the most extreme in recorded history yet it is still possible that this and the two other major events in the past two decades (1981-82 and 2001-02) were exacerbated by other, slower, climatic cycles which are part of the natural variability of the Earth’s climate and not a response to greenhouse warming.
Although any direct causal link between enhanced greenhouse warming and El Niño intensity and frequency is uncertain, it is clear beyond doubt that El Niño cycles and mass bleaching are connected. Mass bleaching is not caused by a direct overall increase in ocean temperature but by short term concentrations of heat in the affected areas. On the Great Barrier Reef these temperature increases are caused by El Niño events which pulse oceanic water from the Western Pacific Warm Pool, perhaps 1-2°C above what was once normal, into coastal regions. If this water is then trapped in the lagoon of the Great Barrier Reef it can warm still further, exacerbating the effects of the original pulse.
Essential points about El Niño, global ocean temperatures, and mass bleaching are:
- Enhanced (anthropogenic) greenhouse warming is occurring as a result of the amount and rate of build-up of CO2.
- The oceans are warming far more slowly than the atmosphere because of thermal inertia. A significant proportion of this warm water is remaining in the surface layers of tropical oceans because it is being accumulated too rapidly to be adequately dispersed and mixed by natural water circulation.
- Thermal energy from enhanced greenhouse warming of the oceans is concentrated into the Western Pacific Warm Pool, the world’s largest mobile body of heat.
- El Niño events themselves are not the result of global warming; they are a natural phenomenon of geological antiquity. However, there is continuing debate over the role of enhanced greenhouse warming in altering the frequency and/or intensity of El Niño events.
- The association between El Niño events and mass bleaching will be a temporary one as far as most reefs are concerned. As the Western Pacific Warm Pool widens and deepens, its central core will increasingly affect more extensive effect outer reef regions in non-El Niño years. At this point the thermal peaks currently delivered by El Niño will become less and less exceptional until they are irrelevant. This process is already beginning for the Great Barrier Reef and similar observations have been made in other parts of the world.
- Although El Niño oscillations will become irrelevant for the Great Barrier Reef, they may continue to play a role in transmitting warm water pulses to other regions of the world for some time to come as the Warm Pool deepens and widens.
- We have already entered the time-frame where mass bleaching is occurring without El Niño enhancement. If the relationship of atmospheric CO2 levels to the amount and/or frequency of bleaching continues on its present trajectory, there will be a point where every year will have a similar impact on corals as the worst El Niño events have had in the past.
Acclimatisation and adaptation
Acclimatisation (where the individual’s tolerance of environmental conditions increases during their lifetime) and adaptation (an evolutionary process involving natural selection through survival of the fittest) are seemingly the only escape routes corals have from the warm world of the future.
Acclimatisation There is evidence on both local and global scales that the same and/or closely related coral species show different tolerances to temperature in different locations. On local scales, good examples are corals that tolerate the very high temperatures found in intertidal pools, in water around natural thermal vents or close to thermal outlets of power stations. Normal maximum water temperatures found in particular geographic areas play a large role in determining tolerance to bleaching. Whole suites of corals can survive 36 °C in the Arabian Gulf, parts of the southern Red Sea and sporadically elsewhere. Like most animals, corals may adapt to tolerate these temperatures by altering biochemical pathways. On local scales this process is likely to be due to acclimatisation whereas across widely separated geographic areas there may be a larger component of genetic selection, especially where local tolerance to extreme conditions is involved.
Adaptation Corals were probably once adapted to higher temperatures in the geological past. However the template of today’s oceans is so different from those of the remote past that most meaningful comparisons are questionable. Studies showing that the genetic history of corals and that of zooxanthellae closely match argue against any medium-term history of adaptation by the changing of symbionts.