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Coral: life's a bleach... and then you die

Posted on 13 January 2011 by Rob Painting

Despite what you may read or see in the mainstream media, out in the real world, massive and rapid changes are taking place in many ecological systems as a result of global warming. The Earth seems to be already convinced of global warming and is responding quickly.

Perhaps the most significant, and likely most enduring, are the shifts taking place in the Earth's oceans. Whilst many readers may have read or heard about Ocean Acidification, there are numerous other changes taking place in the oceans which should be equally as concerning. One such phenomena to appear in the last few decades is mass coral bleaching, a consequence of the continued warming of the oceans. Once vast stretches of colourful reefs teeming with marine life are being reduced to lifeless rubble covered in seaweed or slime. Many areas are not recovering, and the scale and frequency of bleaching worldwide is getting worse. In fact, early reports suggest 2010 may have witnessed the largest single bleaching event ever recorded.

The lowdown on coral bleaching

Reef-coral are actually a symbiosis (a mutually beneficial relationship) between the coral polyp, an anemone-like creature, and tiny algae called zooxanthellae. The coral provide shelter and nutrients for the algae , and in exchange the algae provide carbohydrates (food) to the polyp, using energy from the sun (photosynthesis) and the nutrients provided by the coral. These algae live in the skin tissue of the polyp and produce the coloured pigments which make coral reefs so visually spectacular. When this partnership breaks down the polyps expel the algae, which leads to the "bleached" effect. Although the polyp does feed using its tentacles to snare food, the bulk of its nutrition (90%+) comes from the algae, and they are a critical component of coral skeleton formation and therefore reef maintenance and growth. Without symbiotic algae, the coral can die from starvation, or become so weakened by a lack of food, that it succumbs to harmful bacteria (Mao-Jones 2010), and/or seaweeds which can poison and kill coral on contact.

Because reef-coral have adapted tolerance to a narrow band of environmental conditions, bleaching can occur for a number of reasons, such as ocean acidification, pollution, excess nutrients from run-off, high UV radiation levels, exposure at extremely low tides and cooling or warming of the waters in which the coral reside. Typically these events are very localized in scale and if bleaching is mild, the coral can survive long enough to re-acquire new algal partners. So bleaching in itself is not something new, but mass coral bleaching on the huge scale being observed certainly appears to be, and represents a whole new level of coral reef decline.

Ocean warming is driving mass coral bleaching

As coral reefs operate very near to their upper limit of heat tolerance (Glynn & D'Croz 1990), bleaching en masse happens when the surface waters get too warm above their normal summer temperature, and are sustained at this warmer level for too long. The intensity of bleaching corresponds with how high, and how long temperatures are elevated and, as one might expect, the intensity of bleaching affects the rate of survival. Small rises of 1 -2 degree C, for weeks at a time, usually induce bleaching.

This episodic ocean warming has been most pronounced worldwide during El-Nino events, when the Pacific Ocean exchanges heat to the atmosphere and surface waters. In recent years though, severe mass bleaching is happening outside of El-Nino because of the "background" ocean warming. The huge mass bleaching in the Caribbean in 2005, a non El-Nino year, and again this year is a prime example of this (Eakin 2010) . Evidence connecting warm surface waters and mass coral bleaching has strengthened to the extent that the National Oceanic and Atmospheric Administration (NOAA) has a coral bleaching alert system in place. This alert system accurately forecasts mass coral bleaching based on satellite data of sea surface temperatures.

Hot water + Coral = Dead coral

So how does hot water kill coral?. It requires both high water temperatures and sunlight. Oxygen is released as waste during photosynthesis and like all chemical processes this is affected by temperature, speeding up as more energy (warmth) is applied. When water temperatures rise too high the protective mechanisms to prevent heat damage, employed by the coral and the algae, are overwhelmed. The zooxanthellae algae produce high levels of oxygen waste which begin to poison the coral polyp. In acts of self-preservation the coral kick out the algae, and in doing so become susceptible to starvation, opportunistic diseases, competitive seaweeds and macroalgae (slime to you and me) . Coral can succumb to the effects of bleaching years later, and for those coral that survive, growth effectively ceases and full recovery can take anything up to a decade.

Coral resilience is futile

On a world scale coral reefs are in decline, and it makes for rather depressing reading for an avid diver like myself. Over the last 30-40 years 80% of coral in the Caribbean have been destroyed (Gardner 2003) and 50% in Indonesia and the Pacific (Bruno & Selig 2007). Bleaching associated with the 1982 -1983 El-Nino killed over 95% of coral in the Galapagos Islands (Glynn 1990), and the 1997-1998 El-Nino alone wiped out 16% of all coral on the planet. Globally about 1% of coral is dying out each year. Not all of this continual decline is solely down to bleaching of course, pollution and other human activities are also contributing, but bleaching is speeding up the loss of coral.

 

Figure 1 - from Marshall & Schuttenberg 2006 

Looking only at bleaching though, we find that the incidence of mass coral bleaching increases dramatically in the last few decades. Despite modern records being biased by better monitoring and reporting in recent times, there seem to be little evidence of mass coral bleaching further back in time when examining long-lived coral communities. Studies from around the world show no signs of bleaching dating back many thousands of years, until recent decades (Abram 2003), (Aronson 2003). In the Caribbean there are no signs of previous mass bleaching dating back 220,000 years (Pandolfini & Jackson 2006)

So where does this resilience claim originate you may ask?. Perhaps from studies that have shown some coral, in secondary bleaching events, have lower rates of death. A few coral are in more fact tolerant to bleaching, some algae for instance manufacture their own "organic sunscreen". However this a only small proportion, major reef-building coral species seem incapable of forming long-lasting partnerships with these heat tolerant algae (Coffroth 2010), and the coral polyp themselves have a very poor genetic ability to adapt to warming (Csaszar 2010). However the "resilience" fallacy arose, there's no evidence a few hardy individuals will somehow prevent the loss of most coral worldwide.

The importance of coral reefs - the oasis in a marine desert

So what does this all have to do with the average man or woman in the street?, well, as far as humans are concerned, there is a rather large dollar value attached to coral reefs. Goods and services derived from coral reefs are very roughly estimated to be between $172 to $375 billion dollars per year (Martinez 2007). Not only that, but reefs directly provide food and income to over half a billion people worldwide. The decline of coral reefs is going to not only impact those that directly depend on them for a living and sustenance, but eventually have dramatic effects on economies worldwide, and will likely drastically drive up world food prices as fish populations nosedive.

Ecologically speaking the value of coral reefs is even greater because they are integral to the well being of the oceans as we know them. It might serve to picture them as the undersea equivalent of rainforest trees. Tropical waters are naturally low in nutrients because the warm water limits nutrients essential for life from welling up from the deep, which is why they are sometimes called a "marine desert". Through the photosynthesis carried out by their algae, coral serve as a vital input of food into the tropical/sub-tropical marine food-chain, and assist in recycling the nutrients too. The reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures .

I could go on, but the similarity with the rainforest should now be clear. Eliminate the undersea "trees", which mass coral bleaching is in the process of doing, and you'll eliminate everything that depends on it for survival.

A grim outlook for coral

The critical issue with global warming induced coral bleaching, as it is for many eco-systems, is the speed of warming. They are simply not being given sufficient time to evolve tolerance. The coral's algal partners have short lifetimes and possess genetic traits which may enable successful adaptation to warming. Coral themselves aren't so lucky, somewhat in contrast to their algae, they possess a poor genetic ability to combat warming stress and have decadal lifetimes. It's likely therefore that many coral will die because the speed of warming is too great within an individual communities lifetime.

Perhaps a useful way of looking at it, is that the "bar" is continually being set higher and higher, and the recovery time between bleaching events becoming smaller and smaller. Gradually this continual ocean warming will start to impact areas which have so far escaped unscathed, and these coral will succumb too. Of course coral reefs aren't just under fire from bleaching, as mentioned earlier, humans are hurting them in many other ways. Ocean Acidification in particular is a large looming threat (Veron 2009). The increasing frequency and severity of bleaching, coupled with the persistent decline in coral around the world, should however immediately dispel any myths about coral resilience.

This blog post has been added as our 141st rebuttal, "Corals are resilient to bleaching".

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Comments 51 to 66 out of 66:

  1. According to data from paper Declining Coral Calcification on the Great Barrier Reef, De’ath, Lough and Fabricius, 2009., GBR calcification decline looks like this.
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  2. MikeG and Rob, Thanks for your feedback regarding the urchins. Wish that I had taken the time to look at the urchins closer, but at the time I was more concerned about avoiding their spines ;)
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  3. HR @48, "Apologies Albatross for mixing up denier and skeptic but it makes little difference." Actually, it does make a difference, but whatever. With regard to you're personnal observation about Mauritius in 2001. There's a report here covering the period you were there. They seem to rule out ocean changes associated with climate change. Again you misrepresent what I said. I said "The reefs were not in good shape-- I did not blame that on AGW. Thinking about the poor health of the reef and the abundance of urchins made we wonder about the potential role of urchins taking advantage of the unhealthy corals. That is why I asked Rob and MikeG, who clearly are very well versed on the subject. To be candid I am having trouble accepting the sincerity for your apology, especially when it is followed by another misrepresentation of what I said and even an incredibly cheap shot about my honeymoon. In fact, I would argue that the latter breaks the comments policy.
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  4. Arkadiusz @ 51 GBR calcification decline looks like this. Maybe it would if De'ath et al made all the errors that Professor Ridd and his colleagues did, that simply wasn't the case. But again you continue to be off-topic, this post is about bleaching the calcification "argument" is in the works.
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  5. AS @ 51 No, that graph is NOT what De'ath et al's data show. It took a bit of hunting to figure out where that graph even came from, but eventually I found it here: More Coral Reef Shenanigans It's a perfect example of why you should RTFP and understand what the authors actually did before you declare them wrong. Apparently, what the creator of that image did was use the raw data from only the NEW sample sites, which were only a small subset of the total number of samples used by De'ath et al. The majority of the of the data were from Lough's previous samples which included hundreds of corals. The assertion that there were only 9 samples in the early 2000s is false. At the end of the study period in 2005, there were still 21 corals in the sample and 77 for the previous year. At the peak, there were ~300 corals in the sample, not the ~60 shown in the graph. Not only is the creator of the graph not using most of the data De'ath did, they also didn't correct for distance from shore and sea surface temperature. As Lough and Barnes (2000) found in the paper you cited, skeletal density increases with distance across the shelf and calcification increases with SST (which co-varies with latitude). Failure to correct for these variables introduces a spatial bias as corals from different parts of the reef enter the record- e.g. years with more corals from the North would increase the average calcification rate and make it look like calcification was increasing over time, even in the absence of any real temporal trend.
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  6. Albatross - Re your sea urchin question. Ran across this by chance: Sea Urchins Destroy Reef Building Algae in Overfished Sites on Kenya's Coast The relevant portions: The authors found that reefs with large numbers of grazing sea urchins reduced the abundance of crustose coralline algae, a species of algae that produce calcium carbonate. Coralline algae contribute to reef growth, specifically the kind of massive flat reefs that fringe most of the tropical reef systems of the world. Overall, reefs with more sea urchins grew significantly slower than ones with more complete fish communities.
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  7. Eric (skeptic) issues a challenge to find "... external costs [for] which a libertarian solution is impossible". It is probably a hard challenge to meet in that libertarians are noted for simply not counting externalities as costs. It is likely, therefore, that Eric will simply not count the costs and certainly not the full costs of anything we present. Never-the-less, I would pose any example of major ecosystem breakdown as meeting his challenge. I shall take an example that is close to me, the Great Barrier Reef, although the potential collapse of the Amazon makes an even better example. The Great Barrier Reef is threatened by two climate change issues, rising sea water temperatures and reducing ph levels (acidification). It has been shown that temperature increases greater than 2 degrees above the industrial average, combined with atmospheric CO2 concentrations greater than 500 ppm are likely to lead to the destruction of most corals world wide, and all large reef structures. Disturbingly these effects are expected right on the "safe" threshold of CO2 increase that the international community is targeting, and failing to achieve. Unfortunately, this means the best chance for survival of the Great Barrier Reef is that the scientists studying this issue are wrong. That is, of course, possible. Scientists are wrong from time to time - but you don't get rich by betting against the consensus of scientific opinion. Consequently I think Queensland's largest conventional economic asset (coal) is going to destroy Queensland's actual greatest asset (outside of its population). So, the challenge to Eric is to cost out such a loss in a way that is not transparently undervaluing the reef.
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  8. Tom, thanks for the opportunity. As I said to scaddenp, for an economic solution, the reef and damages to the reef need to be valued. To say most corals and all reefs will be destroyed is not realistic or useful. The drop from 8.2 to 8.1 preindustrial to present is within natural variability (7.5 to 8.5 diurnal, seasonal and other variations), and is within coral tolerance (1 point of pH). There are many sources online about keeping coral in tanks and in most cases, CO2 is added to offset calcium added for faster growth. Predicted drop by 2100 is to 7.8 or 7.9. Temperature is a different story as we will gradually shift into a regime where temperature fluctuates above about 30C which starts to kill the coral. The reef has economic value as you suggested and some of that value should be used for its protection. The simplest solution is to use some of the proceeds from the reef for upstream shading to cool the water. This can be done with the cloud making ships suggested for other purposes or other means. If pH needs to be changed that is also possible although more difficult. What we can do with the reef being a limited and valuable resource does not apply to the atmosphere. Studies (e.g. http://www.arts.usask.ca/economics/faculty/papers/Bruneau_Echevarria_dp_2003-5.pdf) show a positive relationship between environmental quality and per capita income although with a small elasticity of income in the case of global environmental quality and CO2. Enhancing that elasticity requires a global perspective since nothing is accomplished when a consumer substitutes some other coal for Queensland coal. My suggestion, like in the other thread, is to deregulate and privatize coal substitutes and apply taxation and other policy where it can be done reasonably (i.e. keeping in mind the positive relationship between income and environmental quality).
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  9. Eric (skeptic) @58, I have to admit that I am disappointed with your response. To start with your first step was to argue that the scenario was "not realistic". I do not know your qualifications as a marine biologist, but as I am going to have to take somebodies word on this, I'ld rather take that of genuine experts rather than some random guy on the internet. What is more, your two arguments are respectively irrelevant and wrong. The diurnal and seasonal fluctuation in pH is irrelevant for much the same reason that diurnal and seasonal fluctuation in temperatures are irrelevant to the threat posed by rising temperatures. Certainly they vary over a large range, but the increase in pH between the industrial and preindustrial means the lower range in pH values enters adverse extremes more frequently. In contrast, your argument from aquarium practise appears to be simply wrong. Googling on the issue shows that CO2 is used to dissolve aragonite into water to enhance coral growth, but that it is important that the CO2 is not allowed to enter the aquarium itself. This advice is:
    "Be careful not to inject excess CO2 directly into you tank as it can act as a fertilizer and help create an algae bloom. The effluent (liquid coming OUT of the calcium reactor) may have some residual CO2 in it. Its best to "blow it off" by letting it drip through something above the water where the CO2 can be exchanged for O2. We actually run our calcium reactor effluent into our mud filters where our macro algae is growing. That way any CO2 that makes it out of the reactor is rapidly used up by the macro algae, typically Chaeto sp., before it makes it into our main systems. You don't want CO2 to enter you main tanks, as it is acidic and will rapidly lower your pH."
    (Source) So while CO2 is helpful for dissolving aragonite (exactly the problem for coral reefs), enriched CO2 in the main tank causes decreased pH and algal blooms. More concerning is your indication of the libertarian solution to the problem. You first invent the existence (without any papers demonstrating feasibility or lack of adverse secondary effects) of a relatively cheap technical solution. And then you require the funding of that solution from the economic activity associated with the reef. In real life, if such a relatively cheap technical solution exists (a very big if), it is probably true that the fisherman and tourism operators on the reef will bear the financial burden of implementation. It is hard, however, to see that as a libertarian solution. This cost is imposed on the tourism operators and fisherman as a result of externalities from the fossil fuel industry. Your solution, in other words, allows agents other than those directly effected to impose costs on those directly effected without their permission. This is, in fact, a feature of most libertarian "solutions" I have seen proposed, but it contradicts the purported libertarian principles. Further, given the size of the reef and the number of independent operators involved, the implementation of your hypothetical cheap technical solution will require government regulation in any event, so this is not in anyway a market based solution. Finally, your solution to save the reef requires businesses which are already facing a cut back in incomes because of declining reef quality to take a further cut back in income in an experimental attempt to save the reef which may or may not destroy the reef through secondary effects so that the causes of the problem, the fossil fuel industry, do not need to take a cut in income. Please, I beg you, write to the various fishermen and tourism operators on the great barrier reef outlining your plan to save your income. The political fight to combat global warming in Australia could do with the kind of bolstering your solution would provide it.
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  10. Tom, do you believe that the reef tourism industry would be unwilling to consider ideas that could save their reef from sporadic heat events? Also the "fossil fuel industry" and tourism industry have a lot of overlap, so the tourism industry is responsible for their direct and indirect emissions which come from shore tourism industry, airlines and other CO2 emitters. The solution can be organized by a consortium of operators. There should be no secondary effects since I am simply proposing cooling the water upstream when it gets too warm.
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  11. Eric(sk) "...cooling the water upstream when it gets too warm." Upstream? Where on earth is upstream for warm water in relation to the GBR? If you know a way to cool any significant portion of the eastern Pacific ocean currents that warm the GBR waters, everyone would like to hear about it. (Tasmania would like to hear about it too, because their fisheries are also affected by warmer waters.)
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  12. Duh! That would be the western Pacific next to eastern Australia.
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  13. adelady, from "Changes in fish assemblages following 10 years of protection in Tasmanian marine protected areas": "Notable results include a statistically significant increase in abundance of Latridopsis forsteri and large fish (> 300 mm) when examined across all reserves relative to controls, and a 10-fold increase in the abundance of large fish and a doubling of per site species richness of large fish within the Tinderbox Marine Reserve relative to controls." No mention of climate in the abstract. On the reef, from what I read my solution of water temperature is too simplistic for the inshore reef, mainly because of the proximity to land, shallow water next to land and effects of runoff which will make it difficult to prevent temperature spikes. Offshore there is the East Australian Current most responsible for many temperature fluctuations.
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  14. Eric, Good to see the Tasmanian marine protected areas are doing their job. For climate impacts of the warmer current going south there's this. And that's pretty well word for word from the abstract - full paper paywalled. Good for sea urchins. Not so good for the seaweed areas they invade.
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  15. From that abstract: " ocean warming off the east coast of Tasmania at 3-4 times the global average is the result of intensification of the East Australian Current." A complex current with natural http://journals.ametsoc.org/doi/pdf/10.1175/1520-0485%282001%29031%3C2956%3AMTEACI%3E2.0.CO%3B2 and AGW http://eprints.usq.edu.au/1070/ factors. There's no simple explanation of the consequence of CC because the driving forces for the current are not simple. I still think mitigation is plausible since there is an well defined upstream source for the warm water.
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  16. Eric (skeptic) @60: 1) I am sure the those sections of the tourism industry sufficiently profitable to not be driven bankrupt by the expense would much rather pay for a geo-engineering fix than go bankrupt. I am also certain that they would rather not have to pay to save the reef by ensuring the reef was not under threat to begin with. Neither of these facts alters the fundamental injustice in libertarian terms of a cost being imposed on the tourism operators involuntarily by the actions of others. You suggest there is a lot of overlap between the fossil fuel industry and tourism. Unless there is complete overlap, and their most certainly isn't, that is irrelevant. Further, the tourism industry is not an unusually great emitter of fossil fuels, coming well behind mining, heavy manufacturing, aluminium refineries, and indeed, ordinary business travel in that regard. 2) Your claim that there would be no secondary effects is dubious at best. The paper from which your proposal comes states:
    "If our technique were to be implemented, global changes in the distributions and magnitudes of ocean currents, temperature, rainfall and wind would result. Even if it were possible to seed clouds relatively evenly over the Earth's oceans, so that the effects of this type could be minimized, they would not be eliminated. Also, the technique would still alter the land–ocean temperature contrast, since the radiative forcing produced would be only over the oceans. In addition, we would be attempting to neutralize the warming effect of vertically distributed greenhouse gases with a surface-based cooling effect, which could have consequences such as changes in static stability, which would need careful evaluation. Thus, it is vital to engage in a prior assessment of associated climatological and meteorological ramifications, which might involve currently unforeseen feedback processes. It is important to establish the level of local cooling which would have significant effects on ocean currents, local meteorology and ecosystems. This will require a fully coupled ocean/atmosphere climate system model."
    (Source) Changing the distribution of ocean current, rainfall and wind will certainly have secondary effects, and while a purely regional usage of the scheme only may have those effect, the paper indicates that a global implementation would have those effects. "Changes in static stability" are particularly concerning to me. Your scheme involved changing the condensation rate of water in a cyclone prone area during periods of ideal conditions for cyclones. But yet you assure us without support from the modelling in the paper on which you are relying that "There should be no secondary effects". 3) Because there most assuredly will be secondary effects, a consortium of private individual or companies will certainly not undertake this sort of large scale geo-engineering. They could not bear the costs, nor shield themselves from any potential legal action as a result of secondary effects. This is quite apart from the issue of free loaders.
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