September 13, 2010

Coral Bleaching

Filed under: Adaptation, Animals, Plants

Global warming causes coral bleaching – and there is absolutely no doubt about it, right? Tens of thousands of websites found searching for “Global warming and coral bleaching” seem to agree that when the ocean warms, the oxygen content reduces, and the corals become “bleached.” The heat affects the tiny algae which live symbiotically inside the corals and supply them with food. The heat stress damages the algae and in consequence leads to coral death. The argument for the global warming/coral bleaching connection is bolstered by the massive El Niño event in 1997 and 1998 that led to unusually warm tropical waters throughout the world’s lower latitudes and coral bleaching in many locations. But, as with so many other topics covered in World Climate Report, the idea that corals are in peril because of global warming turns out to be considerably more complicated than is commonly presented to the public at large.

Three recent articles give us reason to question the alarmists’ claims that coral reefs are in deep trouble due to the buildup of greenhouse gases.

The first piece was published in Marine Environmental Research by M.J.C. Crabbe of the United Kingdom’s University of Bedfordshire (we cannot make this up – this marine scientist has the last name of Crabbe). Crabbe notes “Coral reefs throughout the world are under severe challenge from a variety of environmental factors including overfishing, destructive fishing practices, coral bleaching, ocean acidification, sea-level rise, algal blooms, agricultural run-off, coastal and resort development, marine pollution, increasing coral diseases, invasive species, and hurricane/cyclone damage.” We agree – coral reefs are facing no end of challenges in our modern world!

Crabbe studied reefs in Jamaica, and he notes that “The Jamaican reefs are subject to a number of both acute and chronic stressors, the last including overfishing and continuing coastal development, including the much-publicised development on land adjacent to Pear Tree Bottom reef and the resurfacing of the North Jamaican coastal highway.” Again, there is a lot more to the story of reefs than just global warming.

He studied various reefs from 2000 to 2008, and this period included a mass bleaching event in 2005. Crabbe concluded “Despite the multiple influences on the reef sites over the study period, the size classes of the corals studied showed resilience to change.” We suspected this all along – the coral reefs have been around for 100’s of millions of years! He states “What is apparent from this study is that despite the chronic and acute disturbances between 2002 and 2008, demographic studies indicate good levels of coral resilience on the fringing reefs around Discovery Bay in Jamaica.” Crabbe warns that “Unfortunately, previously successful efforts to engage the local fisherman in controlling catches around Discovery Bay have not been maintained, and it may be that the development of a Discovery Bay Marine Park is the only solution.” We get the message – don’t blame global warming, blame the local fishermen!

Next up comes from two scientists from the University of Exeter’s Marine Spatial Ecology Lab who focused on coral in other Caribbean reefs; Mumby and Harborne noted that “Because the Bahamas was severely disturbed by the 1998 coral bleaching event, and later by hurricane Frances in the summer of 2004, coral cover was low at the beginning of the study, averaging only 7% at reserve and non-reserve sites.” Corals have been around for eons, they have survived periods much hotter than anything experienced today, they have survived massive El Niño events, and as seen in their study area, the corals can be severely damaged by hurricanes. Delicate corals would have never made it – robust corals would win in the world of natural selection.

In the Caribbean, macroalgae compete vigorously with coral, and macroalgae are controlled to a large extent by herbivorous parrotfishes living within the reef. The parrotfish are more common in reserves than in non-protected areas, and sure enough “The proportional increase in coral cover after 2.5 years was fairly high at reserve sites (mean of 19% per site) and significantly greater than that in non-reserve sites which, on average, exhibited no net recovery.” They conclude “Reducing herbivore exploitation as part of an ecosystem-based management strategy for coral reefs appears to be justified.” An important implication of the research is that the long-term impact of and recovery from coral bleaching events may be largely controlled by herbivore fish – rather than just global warming.

Finally, we looked at a recent article from the scientific journal entitled Coral Reefs written by ten scientists from French Polynesia, France, Florida, and California. Apparently, doing work on the reefs of Moorea (an island in French Polynesia) attracts a crowd? Adjeroud et al. studied the Tiahura Outer Reef Sector (TORS) in Moorea from 1991-2006 (sign us up for this duty) and they noted that “Coral assemblages in Moorea, French Polynesia, have been impacted by multiple disturbances (one cyclone and four bleaching events between 1991 and 2006).” Their conclusions include the statement “In addition, our results reveal that corals can recover rapidly following a dramatic decline. Such decadal-scale recovery of coral cover has been documented at some locations, but our results are novel in demonstrating rapid recovery against a backdrop of ongoing, high frequency, and large-scale disturbances.”

Enough said!


Adjeroud, M., F. Michonneau, P.J. Edmunds, Y. Chancerelle, T. Lison de Loma, L. Penin, L. Thibaut, J. Vidal-Dupiol, B. Salvat, and R. Galzin. 2009. Recurrent disturbances, recovery trajectories, and resilience of coral assemblages on a South Central Pacific reef. Coral Reefs, 28, 775–780.

Crabbe, M.J.C. 2009. Scleractinian coral population size structures and growth rates indicate coral resilience on the fringing reefs of North Jamaica. Marine Environmental Research, 67, 189–198.

Mumby, P.J. and A.R. Harborne. 2010. Marine reserves enhance the recovery of corals on Caribbean reefs. PLoS ONE, 5, 10.1371/journal.pone.0008657.

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