Reefs in peril
  Reefs in peril
Proximal causes of decline in health of coral reefs hot buttons for peril part of BIOLOGY OF CARIBBEAN CORAL REEFS overfishing/reef collecting on Caribbean coral reefs disease on Caribbean coral reefs SCUBA/snorkeling recreation on Caribbean coral reefs future of Caribbean coral reefs pollution on Caribbean coral reefs eutrophication of Caribbean coral reefs hot button for bleaching part of Biology of Caribbean Coral Reefs

There are several major proximal causes for reef decline. The topic of bleaching is dealt with here, while other topics are accessible via the "hot" buttons.

NOTE this section deals with bleaching from the standpoint of health and survival of coral reefs. The mechanisms of bleaching are explained in another section FILES/NUTRITION OF CORALS

subtitle button for eutrophication section of BIOLOGY OF CARIBBEAN CORAL REEFS Bleaching

photograph of bleached boulder coralphotograph of bleached pillar coralBleaching is caused by loss of symbiotic zooxanthellae or loss of the chlorophyll within the zooxanthellae. In a given locality it may occur in just a single coral species or it may affect all coral species. Even though bleaching may affect only a portion of a coral colony, it still reduces the colony's overall vitality and, thus, its competitiveness.

Bleached boulder coral
Montastrea annularis 0.3X

Bleached cathedral coral
Dendrogyra cylindrus 0.3X

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of bleached corals on a wreck taken from a video

"This old wreck has quite a growth of corals and sponges on it. Oh, look...this brain coral and most of the other brain corals around it are bleached or partially bleached. I wonder if they'll recover." - Aruba 2004

NOTE Diploria sp.


photograph of bleached brain coralBleaching was first seen in the Caribbean area in the late 1970s and was initially thought to be photograph of bleached brain coralcaused by stressful conditions such as storms, low salinity, excess oxidation, sedimentation, pollution, and so on. However, most studies now implicate temperature change as the principal cause, but also including cumulative heat stress combined with increased intensity of UV irradiation. Gleason & Wellington 1993 Nature 365: 836; Winter et al. 1998 Coral Reefs 17: 377.



Bleached brain corals Diploria spp. 0.3X


photo collage of bleached Caribbean coralsIn 1987-88 there was widespread bleaching of corals and and a few related cnidarians throughout the Caribbean region. It occurred in late summer 1987 to spring 1988 and affected a number of species of corals, with staghorn, boulder, and lettuce corals being among the first to bleach, along with fire corals. Williams & Bunkley-Williams 1988 Proc 6th Int Coral Reef Symp Vol. 3: 313.

NOTE fire coral is a type of hydrozoan, not a true coral. Its relatives include hydroids, lace corals, and Portuguese man-of-wars

  illustration showing chronology of coral-bleaching episode during 1987-88The first area to bleach during the episode was the northernmost set of islands, folowed by the windward islands and the coasts of Central and South America:
  illustration showing severity of coral-bleaching episode during 1987-88The severity (% corals affected) of the bleaching episode was highly variable throughout the Caribbean region with no clear pattern being evident:
  illustration showing direction of spread of coral-bleaching episode during 1987-88
Direction of spread is shown by the arrows:
  illustration showing estimated mortality in one area of coral-bleaching episode during 1987-88The extent of mortality is not known precisely. The bleaching was most severe in shallow-to-intermediate depths, but effects in some areas extended to 55m depth. The most immediate effect on coral health was in poor health of the polyps, partly owing to poorly functioning nematocysts. Although a significant portion of a polyp's nutrition would be lost with loss of its symbiotic zooxanthellae, partially bleached corals could maintain its health by translocation of nutrients from unaffected polyps to affected ones.
bleached boulder coral Several studies on bleached colonies of boulder coral Montastrea spp. reveal the following: graph showing effect of bleaching on growth in boulder corals
1. Post-bleaching growth drops to 37% of the pre-bleaching rate (year-long study in Florida). Leder et al. 1991 Coral Reefs 10: 19.
graph showing effect of bleaching on reproduction in boulder corals
2. Not only is growth slowed in bleached Floridean boulder corals after 1987, but reproduction is also inhibited. Investigators credit survival of the the corals after they lose their zooxanthellae to an ability to consume their own tissues, but this may leave too few resources for reproduction. Szmant & Gassman 1990 Coral Reefs 8: 217.

graph showing effect of bleaching on growth in boulder corals

3. Studies in Jamaica on boulder corals during the 1987-88 bleaching event show also that while unbleached colonies grow normally, partially bleached ones survive but do not grow. Goreau & Macfarlane 1990 Coral Reefs 8: 211.

The 3 studies reinforce the idea that bleached corals, with less photosynthetic ability, grow and reproduce less than unbleached ones. But whether the lessened photosynthetic ability is totally responsible is less clear...perhaps what causes the bleaching also has other negative effects on the corals.

  photograph of bleached brain coral Diploria sp.Although bleaching occurred in virtually all parts of the Caribbean in 1987, Belize was spared until autumn 1995 when higher than normal sea temperatures (+1oC) were associated with extensive bleaching of corals. About 60% of the most common corals surveyed were affected, as compared with less than 10% of the same corals in spring of the following year. UV light did not seem to be involved and, in fact, researchers actually note a negative correlation between bleaching and UV irradiance levels. Hayes & Goreau 1998 Rev Biol Trop 46: 173; McField 1999 Bull Mar Sci 64: 155.

photograph of bleached boulder coral Montastrea sp.During 6wk in the summer of 1997, seawater temperatures above 30oC in the Florida Keys were associated with extensive bleaching of corals, including dominant species of boulder corals. Warner et al. 1999 Proc Nat Acad Sci 96: 8007.






Bleached boulder coral
Montastrea sp. 0.4X



photograph of ring-bleached boulder coral Montastrea sp.Several coral species in Bonaire in 1992 exhibited ring-bleaching (5cm-diameter riings) when seawater temperature dropped by 3-4oC from a normal 27-28oC level. Among species showing most severe bleaching were boulder and lettuce corals. Kobluk & Lysenko 1994 Bull Mar Sci 54: 142.



Ring-bleached boulder
coral Montastrea sp. 0.3X


photographs of bleached and healed mound coral Montastrea annularis and brain coral Diploria labyrinthiformisCan a coral colony recover from bleaching? Yes, as long as the conditions that caused the initial bleaching are gone or at least ameliorated. A “rule of thumb”, if there is such a thing, is that if colour has not returned by 1yr, then the coral is likely dying or already dead. Having said this, a detailed study on recovery in mound corals Montastrea annularis in the Florida Keys following the major 1987 bleaching event shows that even if the coral regains its original zooxanthellae density and colour, other metabolic processes such as regeneration of tissue and skeletal biomass, and renewal of energy stores may take quite a bit longer. Fitt et al. 1993 Coral Reefs 12: 57; photographs courtesy the authors.




Bleached and recovered mound (Montastrea) and
brain (Diploria) corals on Carysfort Reef, Florida. In
both instances recovery was well underway by June
1988, about 8mo after the major bleaching event

  photographs showing onset of white-plague disease in bleached boulder corals Montastraea annularisStudies on boulder corals in the US Virgin Islands show that bleaching may lead to greater susceptibility to disease. For example, in September 2005, higher than normal temperatures caused 90% bleaching of boulder corals Montastraea annularis. The corals partly recovered by end of October 2005 but, by November/December 2005, white-plague disease appeared (see photo series) and, by springtime 2006, up to 50% of the corals were dead. Miller et al. 2006 Coral Reefs 25: 418.

Mass bleaching of coral reefs is commonly associated with exposure to higher than normal temperatures, but for the first time low temperature has been implicated in mass bleaching of acroporid corals (Great Barrier Reef, Australia). Through a combination of low tide, wind, low humidity, and a sudden cold snap in late July 2003, air-exposed staghorn coral Acropora aspera in Heron Island at the southern end of the Reef bleached massively a few days later (see photographs). The authors do not speculate whether it was the cold temperature itself (12oC, about 16oC lower than normal) or the shock of rapid change, or perhaps both, that caused the bleaching. Hoegh-Guldberg & Fine 2004 Coral Reefs 23: 444.

NOTE low temperatures are known from laboratory experiments to cause bleaching, but this observation is the first of its kind for reef corals. The authors do not report on whether the corals recovered or not

photograph of cold-caused bleaching of staghorn corals Acropora aspera in Heron Island, Australia photograph of cold-caused bleaching of staghorn corals Acropora aspera in Heron Island, Australia
Post-bleaching staghorn corals Acropora aspera in Heron Island, Australia, Aug 2003 Close view of branches of A. aspera a few days after cold-temperature exposure

photograph of bleached gorgonians Pseudoplexaura sp.Although most attention to bleaching has focussed on scleractinian corals, other common “bleachers” on Caribbean reefs are gorgonians. An example of mass bleaching of gorgonians followed a period of elevated summer/autumn sea-surface temperatures in La Parguera, Puerto Rico. About 20% of over 1800 colonies of 24 taxa underwent bleaching, with higher proportions (>50%) being found for representatives of genera Muricea, Plexaurella, Pseudoplexaura, photograph of bleached gorgonian Muricea muricataPterogorgia, and Briareum. Prada et al. 2010 Coral Reefs 29: 41; photographs courtesy the authors.

Pseudoplexaura sp.



Bleached Muricea muricata