Mike
Some stuff for you on bleaching in croal reefs
1. Assessments as to late 2000 are that 27% of the world's reefs have been effectively lost, with the largest single cause being the massive climate-related coral bleaching event of 1998. This destroyed about 16% of the world's coral reefs in only 9 months, during the largest El Niño and La Niña climate mode ever recorded. While there is a good chance that many of the 16% of damaged reefs will, with time, recover, it is estimated that half of these decimated reefs will never recover to full health.
2.A new species of bacteria, Vibrio corallyticus, was consistently found in the tissues of the bleached Pocillopora at a level that already fulfills the first of Koch's postulates. The virulence is even more amazing. At 23° C, there are no visible signs of disease. At 25° C, bleaching occurs. At 27° C, there is rapid tissue lysis. A virulence factor is being produced by this bacteria that correlates extremely well with the temperatures commonly cited as causing coral bleaching. Furthermore, Rosenberg describes the bleaching as spreading; a characteristic seen all too often by both field observers and aquarists ( from Eric)
More
http://www.newscientist.com/article.ns?id=dn3606
3.Three hypotheses have been advanced to explain the cellular mechanism of bleaching, and all are based on extreme sea temperatures as one of the causative factors. High temperature and irradiance stressors have been implicated in the disruption of enzyme systems in zooxanthellae that offer protection against oxygen toxicity. Photosynthesis pathways in zooxanthallae are impaired at temperatures above 30 degrees C, this effect could activate the disassociation of coral / algal symbiosis. Low- or high-temperature shocks results in zooxanthellae low as a result of cell adhesion dysfunction. This involves the detachment of cnidarian endodermal cells with their zooxanthellae and the eventual expulsion of both cell types.
It has been hypothesized that bleaching is an adaptive mechanism which allows the coral to be repopulated with a different type of zooxanthellae, possibly conferring greater stress resistance. Different strains of zooxanthellae exist both between and within different species of coral hosts, and the 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. Such adaptations could be either genetic or phenotypic.
Coral species live within a relatively narrow temperature margin, and anomalously low and high sea temperatures can induce coral bleaching. Bleaching events occur during sudden temperature drops accompanying intense upwelling episodes, (-3 degrees C to –5 degrees C for 5-10 days), seasonal cold-air outbreaks. Bleaching is much more frequently reported from elevated se water temperature. A small positive anomaly of 1-2 degrees C for 5-10 weeks during the summer season will usually induce bleaching.
4. Corals tolerate a narrow temperature range between 25 degrees Celsius and 29 degrees Celsius depending on location. Corals bleach in response to prolonged temperature change and not due to rapidly fluctuating temperatures. Lab experiments show that corals bleach when water reaches a constant 32 degrees Celsius (Brown and Ogden, 1993).
Experiments have shown that an increase in UV light causes coral bleaching. UV light experienced by corals can increase in calm waters. The amount of mycosporine-like amino acids in a coral's tissues helps to determine how much UV it can withstand without bleaching (Gleason, 1993).
5. In the past decade, large numbers of coral reefs have died suddenly, leaving once thriving marine communities barren. This phenomenon, known as coral bleaching, occurs throughout the tropical oceans of the world. In 1998 a particularly severe bleaching event struck the corals of the Rangiroa Atoll in French Polynesia. Not only did large numbers (more than 99 percent of the pre-bleaching cover) of fast-growing Pocillopora corals die, but colonies of slow-growing and resilient Porites coral also died (more than 40 percent of the pre-bleaching cover).
Extreme temperatures and increased solar irradiation are the main causes of coral bleaching. During the 1998 bleaching episode in Rangiroa, a record-breaking El Niño raised the sea surface temperature almost 1°C higher than the normal highs for that period of the year, and held it there for three months. Such sustained high temperatures had never before occured in the 50 years temperatures have been recorded in the region. (Although a 1°C increase over three months seems trivial, it is the equivalent of a summer-long heat wave.) This suggests that the coral were killed by the high temperature
6.
http://www.cordio.org/bleaching.asp
7.
http://www.reef.crc.org.au/discover/coralreefs/bleaching/coralbleaching.html
8.
http://earthwatch.unep.net/emergingissues/oceans/coralbleaching.php
So there is no question that temp is related to coral belaching in the oceans, if not directly them indirectly. If you look at ocean temp plots and coral bleaching plots date/date there is a director correlation. It may be that such temps induce bactrerial infections, being the real cause.
