[Coral-List] Coral bleaching and light levels

Thomas Goreau goreau at bestweb.net
Sat Feb 14 08:32:44 EST 2009

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
>> 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
goreau at bestweb.net

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