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Hi,
Is there a layperson explanation for why "small" gradual temperature rises in the oceans are responsible for bleaching reefs, when hobbyists seemingly can grow stony corals at a range of temperatures?
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| Both the ocean and our tanks are subject to rise in temperatures over the course of a day due to the sun or our lights, although the reefs actualy get hit with more sudden and higher temps due to tidal movements pulling relatively hot water out from the shallows, which does no harm as such rises in temperature are short lived, {sun goes down, tank lights go off, tide changes direction}. What does the damage, and the cause of bleaching in corals is the long term rise in daily temperature averages, as an example, lets say that the reefs and/or our tank starts the day out at 80F and over the course of the day rises to 84F, which would give us a long term daily average of 82F. But then currents change or a chiller fails and the reefs or tank temperature daily average changes to 84F, still no problem for the reefs or tank for a few days or so, but should that avearge maintain for a week or more, then the coral's zooxanthellae start producing toxic substances and the coral ejects the zoox as a means to save its own life, in doing so, the coral loses the coloration of the algae and appears white, hence the term "bleaching". Keep in mind the examples given, are just examples and are not a temperature guide for our tanks, although a daily average of 82 is good as that is the average for most of the worlds reefs, with exceptions of course depending on location, but for the typical indo-pacific reef, 82 is a good number (night low of 80 / day high of 84), if for some reason the tank's temperature rises to 86 or slightly above due to equipment failure, it is not going to harm anything for a day or two/three while the equipment is repaired or replaced. BUT... for those tanks that keep a steady, unchanging temperature, such a rise or change is much more likely to do harm simply because the corals kept have been weakened by "stability". Chuck
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Acclimitization to variability plays a role, but I think even more key is the type of zooxanthellae they harbor in terms of stress and temperature tolerance - there are also species differences in bleaching susceptibility outside the type of zooxanthellae. But, Chuck's explanation of the variations between captivity and tanks is also spot on.
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Eric Borneman
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I understand the day to day variations whether it be in nature or in aquariums.
My thought was more based on one hobbyist A running his setup at say 77 degrees. Then hobbyist B buy 25 frags from him and grow them with visual success in his setup which he likes to run at 82 degrees.
In the example which I think happens frequently, there is a setup with its own temperature swings migrating into a different environment with its own temperature swings. But in the migration there is a 5 degree immediate shift in average temperature.
How come this fast temperature shift is tolerable when in nature we aren't (as far as I am aware) seeing even a single degree changing around the equator.
Am I missing something 
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You can get bleaching from a rapid temperature increase, even the example used here, but few corals would bleach at 82F - this is pretty far below most bleaching thresholds unless you are talking about some subtropical areas.
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Eric Borneman
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| So reefs bleaching in nature due to warmer waters, were already close to an upper limit of tolerance?
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I'm thinking of running a cooler temp tank of 75F to 78F
--------------------------------------------------------------------"Corals,corals,corals-they are as fascinating as candle lights." phyto4life
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There are a lot of factors in bleaching and even bleaching itself can be minor (not even visibly apparent) to affecting some species and not others, to patchy across colonies, to only the tops getting bleached, to total reef bleaching. But, basically and in the case of mass bleaching events, it seems to be a combination of extended periods of unusually warm water, light (including UV), and stagnation (doldrums) acting synergistically and what zooxanthellae are present. There are many documented cases where the water temperature gets warm enough to cause bleaching and where wind driven waves or currents run strong, that area of the reef doesn't bleach or doesn't bleach as badly. Also, if the species have thermal tolerant zooxanthellae, their upper thermal limit may be higher than those with different types. But yes, for many reefs the combination of factors (species, zooxanthellae, location) are largely all near their upper thermal limit and prolonged warm water can do it. There have been enough large scale bleaching events now, though, that some reefs just don't or haven't recovered, and what are left are resilient or that reef gets repopulated with either lower diversity or more thermally adapted symbioses. Other reefs continue to bleach repeatedly and then get repopulated from new recruits (Fiji is a good example), but this is a dangerous long-term situation since a continuous source of new recruits is getting increasingly hard to depend on as source reefs continue to decline from so many other pressures.
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Eric Borneman
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PHYTO4LIFE (7/5/2009)
I'm thinking of running a cooler temp tank of 75F to 78F
why do you want to keep you temps so low? that is stressful for your corals.
and it may cost more money to run a chiller to maintain those temps!
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Carl-
We are all stardust
Sun powered reef
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I'm looking into maybe getting seahorses and mandarins for the seahorse I heard get less disease with low temp but feel maybe I can get away with sps at around maybe 76F-77F?
--------------------------------------------------------------------"Corals,corals,corals-they are as fascinating as candle lights." phyto4life
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