Stewing Corals

[RiddleLabs]

Aloha,

I presenting evidence at the DC MACNA about corals preferentially absorbing heat from metal halide lamps in close proximity - that is, under certain conditions, corals could be as much as 10 degrees F warmer than the surrounding water temperature. There were some questions raised about the methodology, so the experiments were repeated using thermistors and.... same result. I appears that corals in less than 6" of water and directly under high wattage metal halide lamps will warm up faster than the water column. Imagine the implications. The scariest is that monitoring water temp alone may not be enough, but this is easily overcome with a little common sense and proper placement of corals.

So, there were have it. The worst-case scenario is a coral subjected to a non-shielded high-wattage (say, 400w) MH lamp.
High PAR, potential high UV and high IR energy (heat) ganging up on the poor coral. And we wonder why some corals bleach...

Dana

 

[rvitko]

Interesting but shouldn't the same effect exist in nature- like standing in the sun is hotter than in the shade? Wouldn't flow carry the heat away.

 

[Shoestring Reefer]

How does the heat up of corals vs the surrounding water compare to the heatup of rockwork vs the surrounding water?

 

[RiddleLabs]

Aloha Roger,

It is my understanding that preferential heat absorption does occur in natural reef settings, though perhaps not to the extent that I saw in captive corals. However, I did not see it when two thermistors were placed in convenient holes in Porites corals in a tide pool here on the Kona coast.

In aquaria, this absorption occurs only under certain circumstances - corals near high wattage metal halides in an efficient pendent luminaire. Probably happens on a fairly regular basis... What I don't know in this case is how skeletal density might affect heat absorption. Porites corallums, obviously, are porous. What of the dense corals, such as Pocillopora? No answer at this point, at least that I'm aware of.

Dana

 

[RiddleLabs]

Aloha Mike,

At this point, all I can say is that it is potentially possible for some corals to gain heat and become much warmer than the ambient water.

This was noticed while conducting experiments and attempting to keep all parameters as close as possible between the experimental and control corals. These small fragments (1-2" square) were not mounted to live rock (for ease of handling during the trials). So, I don't know if live rock preferentially absorbs heat, but it would seem a good bet that rock could potentially have 'hot spots' as well.

Dana

 

[Pyrrhus]

wouldnt this be a moot point with said coral being covered throughout with the best thermal conductor on earth (water) at a lower temperature? i would think any heat absorbed by the coral would be quickly dissipated into the water column unless the the metabolic processes of the coral were responsible for the increase in temperature.

or am i just missing something?

 

[RiddleLabs]

Aloha pyrrhus,

For want of a better analogy, some Porites coral (for sure, and probably others) are like bricks baking in the sun (substitute corallum and hot metal halide lamp in the appropriate places). It is reasonable to think that, even with excellent water motion, the coral would not gain temperature any more quickly than the water column. But these Porites did become much warmer than the surrounding water. I spent $1,000 to get the thermistors and data logger to verify it - It surprised me too.

I can't say much about the metabolic processes of corals and thermal gain from those processes, but last I heard corals aren't warm-blooded (or warm-fluided ;-).

Dana

 

[rvitko]

I would think you could study this in more situations easily with thermal vision. It would be interesting to survey natural reefs and aquariums with a thermal image. Do you know if in all the satellite pictures of the earth their are any thermal composites- like google earth but with thermal imaging?

 

[Pyrrhus]

would an IR thermometer like those used by restaurants be able to measure this temperature variation?

 

[RiddleLabs]

Aloha, again-

I've attached a thermograph taken with a nitrogen-cooled 'heat camera' of a Pocillopora verrucosa. The photo is fuzzy and poor (taken with a digital camera from a VCR tape played on a TV in a lighted room). But it gives an idea of the thermal variations across a coral head maintained under a metal halide lamp. If anyone is interested, let me know and I'll get a better shot.

This photo was submitted to Aquarium Frontiers about 8 or nine years ago - Craig Bingman deleted any reference to it because he felt the thermistors were a better way to go. Obviously not a high priority as it took me almost 10 years to get back to it.

Re: The IR thermometer - one was used to obtain the data presented at MACNA. There were some challenges to the information but no one could explain away the measurements I was getting. Just to be sure, I ordered a couple of thermistors with a datalogger capable of recording light intensity. As mentioned, as three techniques show corals with 'hot spots' (90 degree coral in 80 degree water, if conditions are within a rather tight parameter).

Dana

 

[RiddleLabs]

 

[mwcf]

Hello Dana.
I've a couple of questions for you...
How were the thermisters mounted to/in the corals?
Does your data logger use a constant current source for the thermistors? If so, how much?
What type of construction are the thermistors?

Here's what I'm thinking...
Are the thermistors generating heat?
Are the thermistors absorbing more light energy and therefore heating and simply not dissapating this heat as well when they are on/in the corals?
What kind of temperature deltas are you talking about?

 

[rvitko]

Dana,

I just wanted to say my comments were never meant as a criticism, just thinking of ways to add data on the cheap to support the evidence you have. I figured their may be existing thermal images of reefs to look at.

 

[RiddleLabs]

Aloha Roger,

Gosh, I never took your comments as criticism... is my internet persona that evil ;-).

The interesting thing about the thermographs is that the temps are displayed on the video tape (but I cropped them out to meet the photo size limitations). In this case the temps were much warmer than the water temp (>90F coral temp v. 76 water temp).
This was the greatest delta T - some parts of the coral were much cooler.

Roger - ALWAYS challenge me if you wish! Healthy debate and constructive criticism are part of advancing our understanding of corals in captivity.

Dana

 

[RiddleLabs]

Aloha mwcf,

The stainless steel external thermistors marketed by Spectrum Technologies are advertised to accurately measure (within 0.9C)
temps across a range of below freezing to the boiling point of water. In reality, my lab checks have found the two thermistors I have to be accurate to a much closer tolerance (0.1C) as measured with a calibated 'lab' themometer and using a stirring/hot plate to warm the water. The data logger's internal thermometer unfortunatelty accurately reports heat gained within the NEMA 4 box I've mounted it in.

As for the data logger, it uses a 3V battery for power (it is a field unit).

The thermistors are mounted in a coral by the following method:
A masonry bit is used to drill a hole from the bottom up in a detached coral head. The drilled hole is just a few thousandths larger than the thermistor, ensuring a snug fit, and allows a heat measurement within just a few mm of the corals surface - the thermistor is not exposed directly ot light and measures the temp of the coral 'skeleton.'

These themistors are recommended for use in air, soil or water and have to be pretty darn accurate - their measurments are used to calculate degree-days for commercial crops, as well as forecast potential for different plant ailments using Spectrum's propietary software.

Dana

 

[grupper]

I was under the impression that water absorbed thermal IR very quickly. According to the spectra I found at: http://www.lsbu.ac.uk/water/vibrat.html water has an absorption coefficient of around 1000 1/cm at thermal wavelengths. This means that thermal IR can only go a few microns in water.

This means that the thermal image that you showed would have to be taken with the coral out of the water. It also implies that the heating has to be either from internal metabolism, or from near visiable wavelengths (200nm - 1000nm). Unless your metal halide lamp has a very strong peak around 1000 nm then this heating probably occures in nature as well as in the aquarium.


Greg

 

[RiddleLabs]

Grupper,

You are correct on two counts - the Pocillopora was out of the water for the thermography.

And, I have get this from a reliable source - there will be a peer-reviewed paper published soon on preferential heat absorption by corals in nature. The implications are staggering - bleaching has always been associated with surface or ambient water temperature - not coral temperature.

Rest assured that the Pocillopora in the thermography was not gaining heat by 'internal metabolism' (I think I know what you mean by that...).

>>that the heating has to be...from near visiable wavelengths (200nm - 1000nm).<<

You've completely lost me here... Are you saying UV and visible radiation are responsible for heating? Aren't visible wavelengths routinely defined as something on the order of 380-750 nm? Could you fill us in, please?

Best,
Dana

 

[grupper]

>>that the heating has to be...from near visiable wavelengths (200nm - 1000nm).<<

You've completely lost me here... Are you saying UV and visible radiation are responsible for heating? Aren't visible wavelengths routinely defined as something on the order of 380-750 nm? Could you fill us in, please?

You have the sample at about 6" (15cm) deep in the water, so you need the water to have an absorption coefficient that is less then around .1 1/cm. According to the water spectra that I posted, that corresponds to the 200 - 1000nm wavelengths.

You are correct that visible light is somewhere in the 400-700 nm. So I crudely called the 200-1000nm "near visable" .

Yes, I am claiming that the heating is coming from UV, visible and near IR light. The thermal IR ~10,000 nm will be completely absorbed by the water.

The metal halide lights are likely to give off a lot of energy in the thermal IR, but all of this energy will be absorbed by the first few microns of water.

 

[grupper]

Is there any value to looking at the time response of the temperature difference? (i.e. turn on the light and log how the temperature changes, then turn off the light and log quickly the temperature returns the water temperature.)

If the temperature responds with the normal exponential, then it will give you some idea about how thermally isolated the temperature probe is from the water column. If its is not a normal exponetial, then you will know the problem is more complicated then your giving it credit for.

 

[Mind_nl]

pyrrhus: water is not the best thermal conductor on earth, it's actually one of the worst thermal conductors there is!

What is the setup of the experiment? I guess with the water moving and the coral staying put the coral will recieve constant energy from the light while the individual water particles will heat up under the light and cool down while traveling thru sump/pump/etc.
The coral would be like the mentioned brick wall in the sun. While the water will let most of the energy generated by the lamp pass right thru it (water being transparant) the coral will absorb most of the wavelenghts