[Coral-List] Coral bleaching and light levels

Richard Dunne RichardPDunne at aol.com
Sat Feb 14 09:13:03 EST 2009


I have seen a great deal of poor work submitted to scientific journals 
over the years, some of which has unfortunately survived peer review and 
appeared in print. In some cases I have questioned in print the quality 
of those publications, as several authors can bear testament to. So you 
are incorrect to assume that I am not careful and critical of published 

Unpublished work such as your own is quite impossible to assess - so I 
am right to be highly sceptical of its value. I hope that I am not being 
personal in suggesting that a little less time writing to Coral List and 
a little more time spent researching and publishing would I am sure 
greatly benefit all of us. Anecdotes are best left to the dinner table 
and bars.


Richard P Dunne

Thomas Goreau wrote:
> I think you are missing my point. 
> We have for at least half a century from experimental work that 
> extreme changes of light in either direction can stimulate bleaching 
> (these are well known to aquariasts) and no one disputes that doldrum 
> calm, cloudless, sunny conditions make bleaching happen faster when 
> the temperature is above the threshold. But you are not willing to 
> accept that light is key secondary stressor and insist it be regarded 
> as a PRIMARY stressor, that is to say, sufficient BY ITSELF IN THE 
> But every known mass bleaching event (discounting local events which 
> could have been caused by freshwater, sediments, or other local 
> stresses, which are only a few percent of all reported events) has 
> been predicted FROM TEMPERATURE ALONE. There is no evidence that 
> average light exposure is increasing globally in reefs (although of 
> course it fluctuates strongly and this is highly relevant to local 
> bleaching patterns, as has been well known since the 1980s), and there 
> is excellent and unambiguous data that ocean cloudiness is INCREASING. 
> I am simply saying that there are no known mass bleaching events at 
> temperatures below the threshold temperatures which could be 
> attributed to high light. 
> I used neutral grey filters to reduce all light using full sunlight, 
> half sunlight, and quarter sunlight in tanks at various temperatures. 
> Peter Glynn used both neutral grey filters with and without UV 
> specific filters. You seem to believe (almost) everything you see on 
> paper and nothing that experienced observers have personally seen! I 
> did not get around to publishing my data, so you will naturally 
> discard that evidence no matter how strong it is.
> On Feb 14, 2009, at 5:23 AM, Richard Dunne wrote:
>> Tom
>> 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 
>> unscientific.
>> 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 bleaching.
>> 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 express. 
>> 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.
>>>>> _______________________________________________
>>>>> Coral-List mailing list
>>>>> Coral-List at coral.aoml.noaa.gov
>>>>> http://coral.aoml.noaa.gov/mailman/listinfo/coral-list
>>> = 
> 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
> = 

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