My stance isn't based on "occasional diving." It's based on doing field work on the reefs everyday for months at a time and the published literature on the subject.
First, I think it's important to define what a thermocline is so we're sure everyone's on the same page since from the sounds of it, we aren't. A thermocline is when a body of water is
stratified by temperature and there is a sharp demarcation (of ~10F on the reefs) within a few feet. It happens at >90ft around reefs. Above that the water is said to be "well mixed", but that
does not mean that it's a uniform temp or that large, fast changes don't occur there. Leichter wrote about how above the thermocline more than half of the yearly variation can occur in 1-20 minutes (at a fixed point) and then stay that way for up to 4 hours before returning to the original temp in just as little time. This spring when I was working in The Bahamas it wasn't uncommon for the temp to change 4-6 degrees within a few seconds while we were measuring a single coral head. Another guy that was working there logged >10F changes in under 15 minutes on his data loggers. Those changes were large and dramatic, but were
not due to thermoclines.
I have no doubt that corals can be killed at 82. It's been replicated not only in the hobby, but in scientific experiments. The key to doing it though was to keep the corals under cool, stable conditions beforehand. Corals kept under "normal" conditions didn't even show metabolic stress up to short term temps around 88.
Maybe these fluctuations are used by corals to improve health, who knows.
It has been shown multiple times that fluctuations are important for the resistance of corals to thermal stress, but still they're regarded as stressful themselves within the hobby. Most papers on the subject make mention of how fluctuations are an important part of maintaining coral and overall reef health.
And wild corals die at the same temperature we see most coral mortality at in our tanks. Havent you heard, the reefs are dying? Just because nature is all messed up, doesnt mean I want to copy that.
There is a problem that with shifting baselines, but when people are writing about the temps on reefs they take precautions not to get "messed up" data. They take measurements for long periods and don't measure during bleaching or otherwise abnormal years. They also try to pick healthy reefs. The data of what's average also predates widespread bleaching events.
Not me. I haven't lost a coral to higher temperatures and have seen many swings to 85f.
As for the reefs dying, I would imagine they have been accustomed to a 'base' temperature within which the regular swings occur, for yeons. Now that that base temperature is fairly quickly increasing, something has got to give.
Right. It isn't increasing temperatures that induce bleaching, but temperatures increasing beyond the norm for the corals. They can be acclimatized to lower temps and then they bleach at lower temps or they can be acclimatized to higher temps and they bleach at higher temps (within limits). Many people run their tanks cool and get into trouble in the low 80s. I run my tank warmer and it's gotten to above 90 once when the a/c went out without any obvious impact (including a few of Walt's corals). The absolute physiological limit for most hard corals is >88 degrees and then it takes a few days at those temps to show signs of stress. Stress below that is usually a result of the coral's history.
No, it doesn't hurt to keep your tank in the 70s if that's where it will naturally go. However, lots of people spend a lot of time, money, and electricity trying to keep their tanks from reaching entirely natural and harmless temperatures in the low to mid 80s. What happens when something goes wrong for them though? Their temp climbs and their corals suffer. On the other hand they could have just let their tank sit at 82 where it naturally wants to be and in the event of a failure, nothing happens.
Most of the livestock in the hobby is still coming from areas that are "average" and that still includes the majority of corals unless the market has really changed in the last few years. There is no reason to try to cater the entire tank to corals from one region when there is no reason they can't do just as well at the temps the rest of the animals are from.
For anyone who wants to investigate reef temps for themselves, here are just a few of the many articles on the subject (in no particular order).
Coles, S.L. and B.E. Brown. 2003. Coral bleaching-capacity for acclimatization and adaptation. Adv. Mar. Biol. 46:183-223.
Kleypas, J. A., J. W. McManus, and L. A. B. Menez. 1999. Environmental Limits to Coral Reef Development: Where Do We Draw The Line? American Zoologist. 39:146- 159.
Wood, R. 1999. Reef Evolution. Oxford University Press. Oxford. 414 pp.
Wolanski, E and G.L. Pickard. 1983. Upwelling by internal tides and Kelvin waves at the continental shelf break on the GBR. Australian J. Mar. Res. 34: 65-80.
Leichter et al. 1996. Pulsed delivery of subthermocline water to Conch Reef (Florida Keys) by internal tidal bores. Limnol. Oceanogr. 41: 1490-1501
Leichter, J.J., B. Helmuth, and A. Fischer. 2006. Variation beneath the surface: quantifying complex thermal environments on coral reefs in the Caribbean, Bahamas, and Florida. J. Mar. Res. 64(4): 563-588.
Castillo, K.D. and B.S.T. Helmuth. 2005. Influence of thermal history on response of Montastraea annularis to short-term temperature exposure. Mar. Biol. 148(2): 261-270.
Quinn, N.J. and B.L. Kojis. 1999. Subsurface seawater temperature variation and the recovery of corals from the 1993 coral bleaching event in waters off St. Thomas, USVI. Bull Mar. Sci 65:201-214.
Hoegh-Guldberg, O. and G.J. Smith. 1989. The effect of sudden changes in temperature, irradiance and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata (Esper 1797)and Seriatopora hystrix (Dana 1846). Exp. Mar. Biol. Ecol. 129:279-303.
West, J.M. and R.V. Salm. 2003. Resistance and resilience to coral bleaching: Implications for coral reef conservation and management. Conserv. Biol. 17:956-967.
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