[Coral-List] Coral bleaching and light levels

Thomas Goreau goreau at bestweb.net
Fri Feb 13 14:28:47 EST 2009

On Feb 13, 2009, at 12:19 PM, Pam Hallock-Muller wrote:

> Tom:
> I happen to disagree that solar radiation is not involved in the  
> increase in bleaching over the past 3-4 decades.  The relationshop  
> is complicated, but several studies have found that corals are  
> showing photo-oxidative stress around the summer solstice, which is  
> when the forams show their peak bleaching (the forams are much more  
> tolerant of thermal stress).
> Pamela Hallock Muller, Ph.D., Professor
> College of Marine Science
> University of South Florida
> 140 Seventh Ave. S.
> St.Petersburg, FL 33701-5016
> Phone: 727-553-1567
> FAX: 727-553-1189
> e-mail: pmuller at marine.usf.edu
> Website: http://www.marine.usf.edu/reefslab
> < 
> Hallocketal_Anuario_2006_1_108_128 
> .pdf 
> > 
> < 
> HALLOCKetal 
> ._ICRS_2006.pdf><Hallock2000_micropal.pdf><TalgeHallock2003.pdf>

Dear Pam,

Thanks for sending these fascinating papers. I'm swamped now, but I'll  
try to read them carefully and slowly later. I suspect that forams may  
be much more susceptible to light than corals because they have lower  
levels of screening pigments in their tissues?

I'm not at all saying that light is not a contributing factor to  
bleaching, but that I think that increased temperatures, not increased  
light is the key factor in global mass bleaching. We have been able to  
predict the location, timing, and intensity of bleaching accurately  
enough for nearly 20 years from SST data ALONE. Although it is  
certainly true that light and temperature tend to covary. So when it  
is sunny as well as hot, bleaching is faster. But there aren't ANY  
examples of mass coral bleaching that I know of that took place when  
temperatures were BELOW the thresholds, much less those that can be  
explained by high light alone! We have plenty examples of bleaching  
when SST was hot but while clouds were high and light was low. In the  
1994 South Pacific bleaching we did carefully look at cloudiness data  
as well as SST and found that while it was hot it was also cloudy and  
rainy and low light conditions, so we rejected light as a factor. And  
anyway it can't be argued that light levels are increasing in a way  
that could explain this, unless some other unknown factor is  
simultaneously reducing the tolerance of corals to light.

See my response to Dunne, below.

Best wishes,

Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net


It has been well known for about 50 years from direct experimentation  
for at least that sudden changes in light level, both high and low,  
can induce bleaching. I do not at all deny that high light exacerbates  
the rate of bleaching at high temperatures, which I measured directly  
20 years ago. But I do not think anyone can seriously claim that  
increased light has caused the mass bleaching that we have seen in the  
last three decades, although it was obvious that it was a contributing  
factor from field evidence in the very first mass bleaching events,  
but it plays a secondary role, not a primary one. You should not  
confuse "blanching" (what happens to corals that are exposed to full  
sunlight at extreme low tides) with "bleaching" sensu strictu as Brown  

The question is whether light levels have changed at the surface in a  
way that could explain mass bleaching apart from changes in  
temperature. Variations in cloudiness are not new, although they are  
becoming more extreme. But there is a strong and clear tendency to  
increased cloudiness over the oceans, so this is resulting in reduced  
light stress, not more!

There is in fact a large literature on changes in global surface  
irradiation, and the results have shown that solar output is  
essentially unchanged but that changes in atmospheric transparency  
have had a global impact. This was stimulated by the widespread  
prediction that decreases in the ozone layer caused by increases in  
atmospheric nitrous oxide and halocarbon gases should result in  
increases in surface UV. But the long term data conclusively showed  
the opposite, that UV had been decreasing for decades. The same turned  
out also to be true for surface visible light, and also for total  
irradiation as measured by global meteorological pan evaporation  
rates. The explanation was global dimming caused by sulfur and  
nitrogen aerosols generated from combustion. As industrial pollution  
has begun to be controlled, global dimming has decreased, allowing a  
slight increase in surface irradiation in recent years. This is a  
double whammy because global dimming had masked an appreciable portion  
of the global warming caused by CO2 in the short term, so that now  
that these pollutants are reduced, global warming rates will greatly  
accelerate. Increased pollution has been proposed by some as a devil's  
bargain to geo-engineer against global warming, but this is a fool's  
errand because the CO2 remains in the atmosphere absorbing heat for  
centuries, but the aerosols are rained out in days to weeks (or a year  
or so in the stratosphere) so you would need to pollute with aerosols  
at exponentially increasing rates just to stay even. There are many  
excellent papers on this, but Meinrat Andreae provides the clearest  
recent explanation.

At any rate, light cannot explain the large scale spatio-temporal  
bleaching patterns we see, although it clearly contributes to the  
small-scale patterns. Juvenile corals, which prefer to settle in dark  
undersides, are protected for that reason. Peter Glynn did excellent  
work with various filters in which he showed that it was  
photosynthetically active radiation was causing the increased rate of  
bleaching in high light exposed corals above the bleaching threshold  
temperature, and that UV increases made no difference. This is as  
would be expected by the very high levels of UV screening pigments.  
Charlie Mazel and I dived on severely bleached reefs at night in 1990  
measuring UV absorbance and fluorescence of bleached corals, and we  
found that even totally bleached corals were completely opaque to UV  
light. So increased UV cannot be the cause of the large scale field  
bleaching, even though it can be an experimental cause of bleaching.

I'm fully aware of the microphysical patterns you mention discussing  
light attenuation and scattering. But the reason I do not think they  
are relevant is because what I have seen is that waters in most reefs  
are far more turbid than they used to be and light penetration is  
greatly reduced. In many places I see large areas of deep reef 100-200  
feet down that used to be healthy and are now dead because they are no  
longer getting the light they used to, for example in Jamaica and  
Panama. In places where as a boy I would watch my father diving 300  
feet below from the surface, the waters that used to be clear and blue  
are now dark and green. The whole reef light looks completely  
different. So I am sure that increased light exposure is not the cause  
of mass bleaching.

You may think that the changes global atmosphere patterns are  
"simplistic" and not "scientific" with regard to microphysical water  
measurements, but I can assure you that they are well documented in  
the atmospheric literature (having degrees in Planetary Physics (MIT),  
Planetary Astronomy (Caltech), and Atmospheric Biogeochemistry  
(Harvard) and having worked on these issues from the top down as well  
as the coral's bottom up view).

Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net

On Feb 13, 2009, at 12:25 PM, Richard Dunne wrote:
> Tom
> It is wrong to state so unequivocally that light is "a secondary  
> factor". It can be a primary factor as publications have shown (e.g  
> Brown et al - plenty of pubs). It is much more scientific and  
> accurate to say that it is an interplay between light and  
> temperature - see the diagram and discussion in the Fitt et al.  
> Review in Coral Reefs. Sometimes light is more important and  
> sometimes temperature.
> It is also incorrect to say that "light levels are basically  
> unchanged". Changes in regional meterology occur as a result of  
> climate change which can lead to coral reefs being exposed to much  
> higher seasonal irradiance which can tip the balance between corals  
> bleaching or not. In addition, changes in sea level can be a very  
> important factor, particularly for shallow reefs. The underwater  
> light field is a lot more complicated, and affected by many physical  
> parameters (e.g., cloud cover, water surface roughness, sediment  
> levels, water depth, bottom type, zenith angle of the light field,  
> differential spectral attentuation, etc). Your statement about  
> surface light going down (evidence?) is far too simplistic to be  
> accurate.
> I know this is just Coral List but please can we be a bit more  
> scientific.
> Richard P Dunne

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