Coral bleaching

[siops]

Hi Charles,

In a reef tank, what causes corals to bleach? Is it lack of lighting, water temperature, bad water parameters?...others?

Thanks.....

[charlesr1958]

That term is usualy associated with a long term rise in average water temperatures. Say on a day to day basis, a coral out on the reef sees a night time temp of about 80F, then during the day it rises to 84 and for a few hours, it may get flushed with 86 when the tide drops and brings hot water out from the shallows, this short term increase causes corals no harm as they can deal with it, but should the long term average rise a few degrees for a number of days to weeks, depending on the species of coral and what type of zooxanthellae they have (heat resistant or not), the zoox can start producing toxic compounds and the coral ejects their zoox as an attempt to save their own lives.
This same effect on the zoox can also happen when the corals are placed in stronger light intensity, causing their zoox to go into over-drive, ramping up photosynthesis too much which the zoox (being an algae) then produces compounds to limit their own damage from too much light and again, effects the corals which respond by expelling the zooxanthellae.
The opposite of not having enough light can bring about the same "bleached" appearance but not because the zoox have produced toxins, but because the zoox are unable to photosynthesize and die off, but usualy the coral will allow the zoox to multiply their numbers in order to have more zoox gathering what little light there is first, which gives the coral a brown appearance, this strategy works most of the time, but again, it depends on just how low the light intensity has gotten.
Hope that helped.

Charles

[siops]

Thanks a lot....now i understand more how such things happen...

[gonzaimaki]

[charlesr1958]

Many times, but not all of the time, corals can regain their zooxanthellae by capturing them again out of the water or by having just enough remaining in their tissue to start another culture. During the time that the corals are without their zoox, extra feeding may help them endure. We of course should also ensure that what ever conditions caused them to bleach is not allowed to repeat.

Charles