[Coral-List] Coral bleaching and light levels

Richard Dunne RichardPDunne at aol.com
Sat Feb 14 05:23:23 EST 2009


1. I am sure that Brown or her co-workers were not confused in their 
descriptions of bleaching if that is what you mean, nor that aerial 
exposure is always required, nor that these were "extreme" low tides 
(they occur every month). A re-read of those papers would help to 
clarify your interpretation. Your term "blanching" is semantic and 

2. "*Sudden* changes in light level" are not a prerequisite to coral 
bleaching. It is a complex combination of dose and irradiance, as well 
as spectral composition.

3. The light and temperature interaction does not necessarily require 
the presence of "high temperatures". At normal or slightly elevated sea 
temperature high light can cause bleaching. It is the interaction that 
is important. The fact that no-one can directly attribute incidences of 
"mass bleaching" to light is more a factor of the lack of adequate light 
data (cf the ease with which temperature data can be collected). Absence 
of evidence is not evidence of absence. The words of Glynn (Global 
Change Biology 1996 495-509) are still pertinent: "a lack of long term 
in situ physical data and systematic monitoring of coral health have 
frequently frustrated efforts to link unequivocally mass bleaching 
events with extreme environmental anomalies". For light data, that 
problem still persists.There are also papers where elevated temperature 
bleaching did not occur because of reduced light (e.g. Mumby et al MEPS).

4. Surface light is not just the important factor as far a corals are 
concerned - it is the underwater light field modified by a host of 
physical factors which changes not only the power, angular 
distiribution, but also the spectral power distribution in a very 
complex way.

5. The shorter wavelengths of UV are and have always been a largely 
irrelevant factor in the bleaching of adult corals in the natural 
environment. Gleason & Wellington's 993 Nature paper started an 
unfortunate hare running but was in error (comment by Dunne Nature 
1994).  This is a very complicated area but there are many subsequent 
publications some of which, regrettably, continue to muddy the water. UV 
experimentation for corals is fraught with problems, hence the lack of 
quality data.

6. Light does not just contribute to the "small scale patterns". 
Regional changes in meterology (as I mentioned before) can result in 
effects at scales of thousands of square kilometers (e.g., the change in 
the timing of the onset of the monsoon over the entire SE Asia). 
Similarly, sea level changes induced by effects such as the Indian Ocean 
Dipole affect whole ocean basins including large areas of shallow 
fringing reefs that predominate in SE Asia.

7. I am aware of Glynn's work with UV filters (Glynn et al 1992 Proc 7th 
Int Coral reef Symp) but not the PAR filter work to which you refer. In 
his UV paper he purported to find effects due to UV but there are 
problems with the experimentation which I wrote to him about in 1994.  
It would be most helpful to have the reference for the PAR filter work 
in case I have overlooked it.

Thankfully, we are now seeing useful wider geographic datasets and work 
on light emerging (e.g. the work of Hendee and others) which may help us 
to better understand wide scale bleaching patterns. And as the 
photophysiologists unravel the mechanisms of bleaching at the cellular 
level it appears that the normal modus operandii of temperature is 
likely to be damage to the mechanisms of photorepair in PSII, in 
otherwords light is required to be present at sufficient irradiances in 
the first place to cause the photodamage. Certainly, extreme 
temperatures alone can cause other forms of cellular disruption, but it 
is debatable that this is happening in most examples of widespread 

I do not seek to downplay the important role of sea temperature in coral 
bleaching, but light is an equally important consideration, albeit less 
easy to manipulate experimentally and more complex to understand.

Whilst I appreciate the historical anecdotal information that you  post 
to Coral List , it is the published science on which I prefer to rely, 
and I have doubts that this is in agreement with the views you often 

all the best

Richard P Dunne

Thomas Goreau wrote:
> 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 
> does. 
> 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
> 617-864-4226
> goreau at bestweb.net <mailto:goreau at bestweb.net>
> http://www.globalcoral.org
> 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
>> Thomas Goreau wrote:
>>>> From: Thomas Goreau <goreau at bestweb.net>
>>>> Date: February 13, 2009 10:11:18 AM EST
>>>> To: billraymond10 at yahoo.com
>>>> Subject: Re: [Coral-List] Coral bleaching and ocean acidification
>>>> Dear Bill,
>>>> Absolutely! But it is a secondary factor.
>>>> 20 years ago I did a bunch of experiments in Jamaica with corals in  
>>>> tanks at different temperatures and light level combinations.  
>>>> Bleaching took place only above a certain temperature, but the more  
>>>> light the faster it happened. Peter Glynn did similar experiments in  
>>>> Okinawa and reached the same conclusions. The reason I did this was  
>>>> our field observations in the first Caribbean bleaching event in  
>>>> 1987, where shaded corals were much less bleached, in fact where  
>>>> they were shaded by overhangs you could see the shadow, and we've  
>>>> seen that everywhere in the Indian Ocean and Pacific since too.
>>>> But the discussion was about bleaching versus acidification, so I  
>>>> did not get into secondary factors affecting bleaching. And  
>>>> temperature has been rising worldwide whereas light levels are  
>>>> basically unchanged because solar radiation variations are so small.  
>>>> In fact surface light has been going down for decades because of  
>>>> atmospheric pollution, but now starting to rise slowly as  
>>>> atmospheric sulfur and nitrogen pollution are controlled.
>>>> Hans Creek is an interesting area, and you may know that I proposed  
>>>> it as a coral reef restoration site. I still hope this is possible,  
>>>> but it really depends on blocking the marina and golf course. Which  
>>>> I guess is still a pending issue. There is a small chance that I may  
>>>> get back to BVI later this year.
>>>> Best wishes,
>>>> Tom
>>>> 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
>>>> 617-864-4226
>>>> goreau at bestweb.net
>>>> http://www.globalcoral.org
>>>> On Feb 13, 2009, at 9:57 AM, Bill Raymond wrote:
>>>>> Tom Goreau:
>>>>> Don't you consider solar intensity a factor in coral bleaching? In  
>>>>> Tortola I found bleached corals in late October, offshore, but  
>>>>> mangrove-shaded Siderastreas in warm Hans Creek
>>>>> were not bleached.
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