"While other threats to coral reefs have been known for some time[…], increased incidences of coral reef diseases (1) and coral bleaching have been a more recent concern. Although diseases of reef-building corals have been known since the early 1970s, there are emerging concerns that their impacts on reef communities are increasing. New diseases, apparently unprecedented disease outbreaks which sometimes lead to mass mortalities, and the occurrence of coral diseases in locations where they were previously unknown all continue to be.
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A key question is whether diseases are actually having increased impacts on reef systems, or whether the apparent increase is an artefact of more intensive observation and reporting. As scientific observation of coral reefs has unquestionably increased in the past decade, an increased number of observations of coral disease would be expected even if the actual frequency of occurrence of disease were constant.
There are, however, reasons to believe that the frequency and severity of coral diseases are increasing, and that they are having significant negative impacts on reefs. New observations of coral disease have been made even in areas with relatively good scientific baselines. A new disease variant dubbed "White Plague Type II", for example, was first observed in the Florida Keys in 1995 and has subsequently caused substantial coral mortality.
While it is unlikely that coral pathogens have arisen de novo (such as through mutations), it is quite possible that they have been transported beyond their natural ranges. For example, a fungus, Aspergillus sydowii, – which is believed to originate on land – has significantly infected sea fans throughout the Caribbean: it may have entered the marine environment through sediments from land runoff.
Many reefs are being placed under increasing anthropogenic stress, which may both render corals more susceptible to pathogens and itself be a cause of some diseases. It has been speculated that there has been a global increase in the occurrence of coral diseases in response to increasing anthropogenic stress from sedimentation, eutrophication, and other forms of pollution: evaluation of this impression requires better understanding of the causes of coral diseases. This would include determining whether all of the conditions described as "disease" actually represent abnormal physiological responses against the background of natural variability.
Coral bleaching is a generalized reaction to environmental perturbations of many kinds . Like coral disease, it is a natural disturbance to reef communities. If there is no extensive mortality, natural recovery can take place in a matter of months. However, if there is mass mortality, natural recovery may only occur on decadal time scales.
As with coral disease, concerns have emerged about increases in coral bleaching due to land-based activities.
The possible effect of global warming on coral bleaching is another scientific concern. Corals on most reefs live near their upper limits of thermal tolerance, making them potentially vulnerable to sea-surface warming. Significant increases in sea surface temperature over the last 50 years have been observed in some tropical areas. Corals have considerable ability to acclimatize to elevated water temperatures, but it is not known whether they will be able to adapt to the projected rate of temperature increase.
It is worth noting, therefore, that any anthropogenic component of global warming could negatively affect reefs by increasing the rate, as well as the magnitude, of ocean warming. Until recently, the scientific consensus was that, although mass bleaching occurs in response to local episodes of high water temperature, available evidence did not support the occurrence of widespread coral bleaching in response to global warming.
A new consensus is emerging, however, that global climate change may indeed threaten the long-term viability of coral reefs on a global basis. The most geographically widespread, and probably most severe, bleaching ever recorded occurred during the 1997-98 El Niño Southern Oscillation (ENSO) event, although not all of the bleaching can be attributed to ENSO-induced elevation of water temperatures.
[Marine scientists in 1999] reported the extensive coral bleaching and mortality that took place in 1998 in the Indian Ocean, where water temperatures were often 3 to 5 o C above normal in this ENSO year. Mortalities of up to 90% were observed in many shallow areas of Sri Lanka, Maldives, India, Kenya, Tanzania, and Seychelles, while mortalities of 50% were common in other parts of the Indian Ocean and in waters below 20 metres.
As these authors point out, the socio-economic impacts of such losses are very significant, with potential reductions in fish stocks, negative impacts on tourism and future problems with coastal erosion.
While the economic loss resulting from reef damage is quite difficult to determine worldwide, [a 1996 study] estimated that the societal costs of a number of activities which result in reef damage are up to 50 times the private benefits obtained from them (using a 10% discount rate over a 25 year term). Intervention in this case would be reef management – including, inter alia, restriction on access to reefs – and the costs would be those of implement-ing the required management measures and the lost individual benefits.
The 1997-98 ENSO event may fall within the bounds of natural variability rather than be an indication of anthropogenically induced climate change. The extremity of the associated bleaching event, however, is indicated by the bleaching-induced death of some coral colonies on the order of 1000 years old.
Since a possible consequence of global warming is an increased frequency of extreme climatic events such as the 1997-98 ENSO, this would presumably cause more frequent coral bleaching, altering the balance between disturbance and recovery. The problem will be exacerbated to the extent that anthropogenic stresses compromise the ability of reefs to recover from bleaching events. Contamination and other stresses interfere with natural recovery from bleaching and other natural disturbances, and could lead to reef degradation even in the absence of an increase in such disturbances."
(1) "Disease" is defined as "Any impairment (interruption, cessation, proliferation, or other disorder) of vital body functions, systems, or organs." Thus, abnormal conditions caused by physiological stress, poor nutrition, genetic mutation, or other factors are considered diseases as well as those conditions caused by pathogens.
Source: GESAMP71:47-48
GESAMP (IMO/FAO/UNESCO-IOC/WMO/WHO/IAEA/ UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). 2001. Protecting the oceans from land-based activities - Land-based sources and activities affecting the quality and uses of the marine, coastal and associated freshwater environment. Rep. Stud. GESAMP No. 71, 162 pp. ISBN 82-7701-011-7.