coral bleaching and temperature

[whalloper]

Hello,

As the temperature has risen over the past few weeks, some of my corals, especially my montiporas, have started to bleach. Im not positive the temperature increase is the reason. In the winter my tanks usually stay around 79 +/- 1 degree. Right now, one is 83.4 and the other is 82.7.

All other parameters are ok except mg which is a little low. I was raising it in one tank and that was about the time I noticed the bleaching. It could have been a coinsidence, but I stopped adding the Kent supplement just in case. I didnt raise mg in the other tank, and I have bleaching, so probably not the cause.

So, are those temps high enough to cause bleaching? Or, could low oxygen (windows shut to run ac) be the cause?

One final weird note, I have corals that I have fragged in the same tank and some are bleaching, while others look perfectly healthy right next to each other!! This hobby is so confusing

[greenbean36191]

Yes, given the normal temp, your current temps are high enough to cause bleaching if they have been that high for a few weeks. Poor aeration could be a contributing factor as well.

Bleaching and thermal stress is a really complicated subject because it's a interplay between lighting, temperature, water flow/aeration, and acclimatization.

My suggestion is that you lower the lighting intensity and increase water flow for the next few weeks while the corals acclimatize to the new temps. Those temps are almost the optima for hard coral growth, so there's nothing wrong with keeping the it like that long-term. You just have to ease your corals into it, otherwise you can get bleaching as you've seen.

[whalloper]

how do you think I should lower light intensity...decreasing hours? If the halides run for 8 hours currently, how many hours should I run them?

Any ideas why some frags (of the same species) bleached and some didnt?

[greenbean36191]

I would either raise the lights a few inches or put a few layers of window screen between the tank and the lights. Decreasing the photoperiod only decreases the duration of the lighting, not the intensity. Intensity is what you need to regulate.

Tolerance to bleaching conditions is a product of prior exposure to temperature and lighting regimes, not genetics. Corals of the same species have very different tolerances for bleaching conditions depending on what conditions they experienced previously. Even different parts of a single colony can have differential tolerance due to different microhabitats. This is pretty well documented with Porites. For instance, if you had a branching Acropora, the tips normally get the highest lighting and the lower portions are relatively shaded. If you snip the ends off and move them, you could be increasing the lighting to more shaded interior parts of the colony, exacerbating the heat issue. Meanwhile the tips, which are used to higher lighting, have no problem with the heat.