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Recent Coral Mortality Associated with Bleaching: Is It Caused by High Temperature or Disease? ... or Both?
Volume 5, Number 10: 6 March 2002

Near the conclusion of the 1997-98 El Niņo (October 1998), Harvell et al. (2001) documented various aspects of the health of Briareum asbestinum corals at Carysfort, Sand Key and Western Dry Rocks in the Florida Keys, USA, finding that (1) between 75 and 90% of the B. asbestinum colonies at the three locations were bleached and that (2) necroses were present on a very high percentage of the bleached colonies.  At their return ten weeks later (January 1999), the contemporaneous bleaching and epizootic episodes had both ended, after having produced a mean mortality across all sites of approximately 68%.  Of the bleached colonies that survived, most had regained their pigments, while signs of prior necroses - which previously had been visible on bleached colonies only - were, in the words of the authors, "virtually undetectable."

Without the careful detective work of the marine scientists - which led to the discovery that a certain cyanobacterium was associated with all diseased coral tissues - the Briareum epizootic may well have been considered just another example of bleaching-related mortality.  But as a result of their key discovery, as well as a number of other things gleaned from their analyses of ancillary evidence, they felt confident in declaring the Briareum mortality to be a consequence of the disease agent.  "By any measure," they wrote, "this was a significant epizootic that would have gone undetected or been attributed to bleaching stress in the absence of our evaluation of the role of an infectious disease."  In fact, they actually went so far as to suggest that "the high levels of coral mortality throughout the world's tropical oceans, previously attributed to bleaching, may be facilitated or accelerated by opportunistic diseases infecting stressed hosts."

Harvell et al.'s findings are not unusual, nor are their conclusions unrealistic; for there is ample evidence of many disease agents having stepped up their attacks upon corals in recent years.  As examples, they cite white band disease, which has almost eradicated Acropora palmate and A. cervicornis from the Caribbean (McClanahan and Muthiga, 1998), yellow blotch disease, which has been raising havoc with Montastrea faveolata in the Caribbean (Santavy et al., 1999), white plague, which is negatively impacting several scleractinian species of coral in the Florida Keys (Richardson et al., 1998), and Aspergillus sydowii, which has decimated many Caribbean sea fan corals (Nagelkerken et al., 1997).  Thus, it is easy to understand why Harvell et al. say that "projections for damage due to bleaching have not taken into account the likelihood that some of the impacts ascribed to bleaching are actually caused by stress-facilitated disease outbreaks."

At the same time, however, Harvell et al. readily admit that "it seems a reasonable hypothesis that bleaching and temperature-related stress will predispose corals and other tropical reef organisms to opportunistic infections."  Indeed, there are many such disease agents that have always lurked in the background, waiting for just the right set of conditions to step forward and play the role of ecosystem spoiler; and they seem to have raised the level of their attacks upon corals to a whole new level of intensity in recent years.  Consequently, making the link between temperature-induced bleaching and the resulting stress that likely lowers coral resistance to disease is not unreasonable, especially in light of the climate-alarmist claim that the past two decades have experienced a degree of warming that is "unprecedented" in the past millennium.

In this regard, however, we note that (1) it was probably warmer than it is currently during the first few centuries of the past millennium (see Medieval Warm Period in our Subject Index), that (2) it was certainly warmer than it is currently during the several-thousand-year period of the mid-Holocene (see Earth's Climatic History: The Last 10,000 Years in our Subject Index), and that (3) it was also clearly warmer than it is now during most of all four of the prior ten-thousand-year interglacials (see our Journal Review Nearly Half a Million Years of Climate and CO2).  Yet in spite of these several lengthy periods of greater-than-present warmth, paleoecological studies of life and death assemblages of corals in the most coral-disease-affected part of the world indicate that the current coral distress there is unprecedented over this entire period of time (see our Journal Reviews The Distant Past is a Key to the Recent Past and Getting the Baseline Right, as well as our Editorial of 12 September 2001: Health Status of the World's Coral Reefs).

These many observations suggest to us that something other than global warming must be predisposing corals to succumb to opportunistic temperature-related infections that have always been present but have never been able to do the severe and widespread damage they have done recently until recently.  And we can only conclude that that something else is man and his host of direct deleterious influences on coral reef ecosystems, as described in our Editorial of 19 September 2001: Human-Induced Destruction of Coral Reefs and Other Coastal Ecosystems.

The ever-present potential for disease cannot account for what we have witnessed over the past two decades in the way of coral devastation, for that potential has always existed.  The purported warmth of the past two decades cannot account for it either, for the even greater periodic warmth of the more distant past was clearly tolerated by the several coral species now being decimated.  Hence, the only viable explanation that can reasonably be advanced is that the proximal activities of man have risen to such a high level of intensity, and have so weakened the resiliency of the planet's corals, that the combined net effect of the other two phenomena, i.e., the high-temperature-enhanced spread of coral diseases, is now able to do what it never could do before, i.e., produce the unprecedented damage of the past two decades.

Clearly, if we are truly serious about saving the planet's coral reef ecosystems, we will concentrate on the pivotal elements that we definitely have the capacity to control and which can, without question, make a real difference in the quest to save the planet's corals: our numerous local activities that directly influence the local physical and chemical characteristics of individual coral reef environments.

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

Harvell, D., Kim, K., Quirolo, C., Weir, J. and Smith, G.  2001.  Coral bleaching and disease: Contributors to 1998 mass mortality in Briareum asbestinum (Octocorallia, Gorgonacea).  Hydrobiologia 460: 97-104.

McClanahan, T.R. and Muthiga, N.A.  1998.  An ecological shift in a remote coral atoll of Belize over 25 years.  Environmental Conservation 25: 122-130.

Nagelkerken, I., Buchan, K., Smith, G.W., Bonair, K., Bush, P., Garzon-Ferreira, J., Botero, L., Gayle, P., Harvell, C.D., Heberer, C., Kim, K., Petrovic, C., Pors, L. and Yoshioka, P.  1997.  Widespread disease in Caribbean sea fans: II. Patterns of infection and tissue loss.  Marine Ecology Progress Series 160: 255-263.

Richardson, L.L., Goldberg, W.M., Kuta, K.G., Aronson, R.B., Smith, G.W., Ritchie, K.B., Halas, J.C., Feingold, J.S. and Miller, S.L.  1998.  Florida's mystery coral-killer identified.  Nature 392: 557-558.

Santavy, D.L., Peters, E.C., Quirolo, C., Porter, J.W. and Bianchi, C.N.  1999.  Yellow-blotch disease outbreak on reefs of the San Blas Islands, Panama.  Coral Reefs 18: 97pp.