[Coral-List] Coral bleaching and ocean acidification

Dwight Gledhill Dwight.Gledhill at noaa.gov
Fri Feb 13 15:35:55 EST 2009

Both climate-scale threats of thermal stress and ocean acidification 
pose a daunting challenge to tropical coral reefs. Attempting to decide 
on the gravest threat between the two may represent a futile effort. 
Both processes are intrinsically linked perhaps not only by means of 
their dependence on atmospheric CO_2 concentration, but more recent 
evidence suggest that thermal stress and high-CO_2 may act 
synergistically to lower bleaching thresholds [Anthony et al., 2008]. 
Consequently, we enthusiastically endorse Dr Goreau’s sentiment that 
CO_2 stabilization should be proactively sought after should we desire 
not to experience the first coral reef hiatus in modern human history 
(see Ridgwell & Zeebe, 2005 for some context). Considering the 
sustenance, coastal protection, and geochemical services currently 
afforded to human civilization by these ecosystems, this would be a most 
unwelcome occurrence.

That said, there are some common misconceptions about the concerns of 
ocean acidification to coral reefs and inaccuracies regarding the sea 
surface carbonate chemistry as presented by Dr Goreau that we seek to 
clarify in this response. A common misconception is that the primary 
threat ocean acidification poses to coral reef ecosystems is 
dissolution. As Dr Goreau correctly points out, it will be several 
centuries before our current CO_2 trajectories would induce dissolution 
in most oceanic tropical surface waters (not accounting for upwelling 
regions that are already impacted by high-CO_2 levels e.g., Manzello et 
al., 2008). It is for this reason that while effects of rising CO_2 on 
ocean chemistry have been discussed for at least three decades [e.g., 
Broecker /et al./,1971; Bacastow and Keeling, 1972] it has only recently 
been identified as a critical concern. The recent concern is rooted in 
experimental observations beginning in the 1990’s that reveal that the 
degree of carbonate mineral supersaturation (saturation state) imparts a 
direct control on calcification rate [e.g., Gattuso et al., 1998; 
Marubini et al., 2001, 2002; Reynaud et al., 2003; Marshall and Clode, 
2002; Ohde and Hossain, 2004; Borowitzka, 1981; Gao et al., 1993; 
Langdon et al., 2000, 2003; Langdon and Atkinson, 2005; Leclercq et 
al.,2000, 2002; Anthony et al., 2008]. Additional experiments have since 
identified responses beyond calcification effects that could 
significantly impact community structure in other ways [e.g. Kuffner et 
al., 2008]. Therefore, while most oceanic tropical surface waters will 
remain supersaturated with respect to aragonite for many years so as to 
preclude actual dissolution, most experimental evidence suggest that the 
rate of reef accretion will decline proportionally to declining 
aragonite saturation state perhaps compromising reef resiliency in the 
face of other acute threats (e.g bleaching, diseases, potentially 
increasing storm intensity, rising sea level). Any decline in 
calcification and, thus reef-building, is of grave concern for the 
persistence of reef systems because rates of accretion on healthy, 
undisturbed reefs are known to only slightly outpace rates of reef loss 
(i.e., physical and biological erosion: see Glynn, 1997 for review)

There are further complications to even this nuanced consideration 
however. Most reefs are not solely composed of aragonite but are instead 
much comprised of more soluble high-Mg calcites [Morse et al., 2006]. 
Furthermore, the diurnal amplitude in pCO_2 levels within the reef zone 
can be 10 times that of the oceanic waters and often exhibit a step-down 
in overall saturation state due to calcification and respiration 
processes. As a result, there may be periods at night when many reefs 
currently exhibit net dissolution [Yates and Halley, 2006]. Yates and 
Halley (2006) found that significant amounts of sediment (not coral) 
dissolution are already occurring (up to 71% of day time calcification 
rates), and suggest that by the year 2100, atmospheric pCO2 will exceed 
the average pCO2 dissolution threshold of 585 microatm. This suggests 
that net sediment dissolution could exceed net calcification by 2100. 
So, while coral skeletons will likely not dissolve within the next 
couple of centuries, the sediments are already dissolving and this 
dissolution rate will likely exceed the calcification rate most of the 
time in the next century exacerbating the decline in rate of reef 
accretion. So while the primary threat from ocean acidification is 
commonly accepted to be the decrease in the rate of coral calcification, 
the significance of the dissolution problem may be greatly understated 
despite the fact that oceanic tropical sea surface waters will remain 

With regards to Dr Goreau’s assessment of tropical surface water not 
being in equilibrium with atmospheric CO_2 we would encourage him to 
consult the recent literature reporting the results from the Bermuda 
Atlantic Time Series Study (BATS) [ Bates, 2007] where it is reported 
that “surface seawater dissolved inorganic carbon (DIC) and pCO­_2 
increased annually at rates similar to that expected from oceanic 
equilibration with increasing CO_2 in the atmosphere.” Furthermore, this 
is consistent with the global synthesis of Takahashi et al., (in press) 
as well as the recent findings for the Greater Caribbean Region based on 
underway pCO_2 data collected aboard the Explorer of the Sea’s since 
2002 [Gledhill et al, 2008, http://coralreefwatch.noaa.gov/satellite/oa] 
where we find aragonite saturation states declining at about 3% per 
decade. We find similar correspondences between atmospheric CO_2 and sea 
surface carbonate chemistry at the Hawaiian Ocean Time-series over the 
past 20 years [Doney et al., 2009]. One should also consult the 
pioneering work of Sabine et al (2004) which specifically quantifies the 
uptake of anthropogenic CO_2 in the global surface oceans and 
demonstrates that even in the surface waters of the equatorial Pacific 
there has been an uptake of more than 40 micromoles/kg of anthropogenic 
CO_2 . The impacts of which are detailed by Feely et al (2004) which 
describe these changes as being perhaps the most significant in surface 
carbonate chemistry in more than 20 million years.

Finally, it may be worth considering that ocean acidification represents 
a chronic, long-term threat, whereas, to-date, thermal stress has been 
short-term, albeit highly acute. Connell’s (1997) pioneering long-term 
studies of coral reef response to both acute and chronic disturbances 
have shown that reef systems are more vulnerable to chronic disturbance 
and can recover from short-term, acute disturbances. Implicit in this 
statement is that the frequency of recurrence for the short-term, acute 
disturbances is not too rapid. If so, then acute disturbances then 
behave more like a chronic disturbance and allow little if any recovery 
of the reef community. Time will tell what the actual frequency of 
bleaching events will be and how corals respond, but one thing that 
isn't questionable is that the effects of ocean acidification will be 
cumulative, chronic and long-term; affording no recovery periods or 
respite from the globally-imposed declines in calcification.

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Takahashi, T., S.C. Sutherland, R. Wanninkhof, C. Sweeney, R.A. Feely, 
D.W. Chipman, B. Hales, G. Friederich, F. Chavez, A. Watson, D.C.E. 
Bakker, U. Schuster, N. Metzl, H. Yoshikawa-Inoue, M. Ishii, T. 
Midorikawa, C. Sabine, M. Hopemma, J. Olafsson, T.S. Arnarson, B. 
Tilbrook, T. Johannessen, A. Olsen, R. Bellerby, H.J.W. de Baar, Y. 
Nojiri, C.S. Wong, and B. Delille (2007): Climatological mean and 
decadal change in surface ocean pCO_2 , and net sea-air CO_2 flux over 
the global oceans. /Deep-Sea Res. {in press] {This paper is posted on 
the Deep-Sea Research Website}

/Note: this was a joint response authored by the following:

/Dwight K. Gledhill, NOAA Coral Reef Watch, (dwight.gledhill at noaa.gov)
Richard Feely, NOAA PMEL, (Richard.A.Feely at noaa.gov)
Derek Manzello, NOAA AOML (Derek.Manzello at noaa.gov)
Kim Yates, USGS ,(kyates at usgs.gov)
Rik Wanninkhof, NOAA AOML ,(Rik.Wanninkhof at noaa.gov)


Dwight K. Gledhill, Ph.D.
I.M. Systems Group at National Oceanic and Atmospheric Administration (NOAA)
National Environmental Satellite, Data and Information Service (NESDIS)
Coral Reef Watch
Physical Scientist/Oceanographer
E/RA31 SSMC 1 rm 5306
Silver Spring, MD 20910-3226

Email: dwight.gledhill at noaa.gov
Ph: (301) 713-2857 x137; Fax: 713-3136

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