[Coral-List] Coral bleaching and ocean acidification (Thomas Goreau)

Katharina Fabricius k.fabricius at aims.gov.au
Thu Feb 12 16:54:54 EST 2009

Tom and Coral Listers, 

We agree that coral bleaching is a great threat for the future of coral reefs. However, to be concerned about the maintenance of coral calcification rates is not a fashion fad or bandwagon, it is equally critical for the future of coral reefs and many other marine ecosystems. It is therefore important to identify the causes for the recent decline in calcification rates. Our study has shown that coral calcification has recently started to decline on the Great Barrier Reef (De'ath et al., 2009), and other studies are now emerging showing very similar rates of decline in other parts of the world (Tanzil et al., 2009). In our Report, we suggested that increasing temperature stress and declining saturation state of seawater aragonite are diminishing the ability of corals to deposit calcium carbonate. Furthermore, we argue that the nonlinear and late onset of the decline may reflect synergistic effects between more frequent stress from temperature stress and declining carbonate saturation state of seawater. 

We agree with Tom Goreau that higher sea surface temperatures (SST) are an important factor that contributes to the decline in calcification. Indeed, our study showed that unusually warm SSTs are related to unexpectedly low calcification rates in some but not all corals. We have referenced Suzuki et al. (2003) who showed that calcification is reduced for up to 2 years in corals after temperature stress induced bleaching. Bleaching is the most visible symptom of the many physiological symptoms of temperature stress in corals. For example, the histology of coral tissues show changes before corals start to bleach, as the study by Ainsworth et al has shown (2008). It is not fully understood which of these temperature stress responses are the proximal cause(s) for the observed reduction in calcification rates. For these reasons, we have used the inclusive term ‘temperature stress’ rather than ‘coral bleaching’ in our study when we refer to the damage caused by warming SST. 

Furthermore, the focus of our study were sub-lethal physiological effects rather than coral mortality resulting from temperature stress. Nobody in their right mind denies that coral mortality due to heat stress is a severe risk for the future of coral reefs, but this was not part of our study. We only sampled corals that had survived the two severe GBR mass bleaching events of 1998 and 2002. Also note that massive Porites tend to be more thermally tolerant than other species. 

Unlike Tom Goreau, we cannot exclude declining saturation state of seawater aragonite (?arag) as a contributing factor, and we do not agree that tropical oceans are unaffected by this change in ocean chemistry. For example, Gledhill et al. (2008) recorded a significant decline in ?arag in the Greater Caribbean Region between 1996 and 2006, at a rate of approximately 0.012 ± 0.001 ?arag yr-1 (from 4.05 to 3.95 ?arag), and this closely agrees with model predictions (eg Hoegh-Guldberg et al., 2007). Numerous laboratory experiments have shown that such decline in ?arag leads to declining coral calcification, and several reviews exist to summarise this expanding body of work.

We therefore disagree with Tom Goreau that the observed declines are due solely to coral bleaching events and that the consequences of ocean acidification in tropical waters are inconsequential for coral reefs.

Katharina Fabricius, Janice Lough and Glenn De’ath

Ainsworth, T. D., Hoegh-Guldberg, O., Heron, S. F., Skirving, W. J. and Leggat, W. (2008), Early cellular changes are indicators of pre-bleaching thermal stress in the coral host. J Exp Mar Biol Ecol 364, 63-71.

De’ath G, Lough JM, Fabricius KE (2009), Declining coral calcification on the Great Barrier Reef. Science 323: 116-119

Hoegh-Guldberg O, Mumby PJ, Hooten AJ, Steneck RS, Greenfield P, Gomez E, Harvell CD, Sale PF, Edwards AJ, Caldeira K, Knowlton N, Eakin CM, Iglesias-Prieto R, Muthiga N, Bradbury RH, Dubi A, Hatziolis ME (2007), Coral reefs under rapid climate change and ocean acidification. Science 318: 1737-1742.

Gledhill, D. K., R. Wanninkhof, F. J. Millero, and M. Eakin (2008), Ocean acidification of the Greater Caribbean Region 1996–2006, J. Geophys. Res., 113, C10031, doi:10.1029/2007JC004629.

Guinotte JM, Fabry VJ (2008), Ocean acidification and its potential effects on marine ecosystems. Annals of the New York Academy of Sciences 1134:320–342

Suzuki A, Gagan M, Fabricius K, Isdale P, Yukino I, Kawahata H (2003), Skeletal isotope microprofiles of growth perturbations in Porites corals during the 1997-1998 mass bleaching event. Coral Reefs 22:357-369

Tanzil JTI, Brown BE, Tudhope AW, Dunne RP (2009), Decline in skeletal growth of the coral Porites lutea from the Andaman Sea, South Thailand between 1984 and 2005. Coral Reefs (on line first doi 10.1007/s00338-008-0457-5

Dr. Katharina Fabricius
Principal Research Scientist
Australian  Institute of Marine Science
PMB 3, Townsville MC
Queensland 4810

email: k.fabricius at aims.gov.au
Tel: (07) 47 534412
Fax: (07) 47 725852

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