water motion on a reef

[salty joe]

I've heard a lot of times that the water motion on a reef calms at night. But always from someone who heard it from someone else. Does anyone know for a fact if this is true or another myth?

 

[billsreef]

Often it does calm down as the temperature differential between land and sea is reduced from nighttime cooling of the land.

 

[jglackin]

I was just in Indonesia diving on a live aboard for a week. I did several night dives. I really didn't see any difference. We dove one spot during the day and then again at night and the flow was incredibly strong day and night. So much so, when I got home, I removed the night cycle from my ACIII.

Maybe the water calms down at the surface, due to lunar activity, but underneath the surface, the water still flows.

 

[greenbean36191]

It depends where you are. If waves and wind-driven currents are the dominant form of water motion, then like Bill said, you'll often get less at night. In other places tidal currents dominate and there will be no difference.

 

[MCsaxmaster]

Agreed with above: where waves are important, calmer nights than days are common, but not a rule. During the right season there might be pretty constant offshore wind, creating constant, large waves. Wave action is only the dominant form of water motion in shallow water and on exposed reefs though. Tidal flushing is very imporant most places, and that doesn't decrease at night (periodicity governed by the tide). Local currents can be very important as well.

 

[jglackin]

Oddly enough, I saw no SPS corals in shallow water. Most of the SPS corals I saw were in water 20 feet or deeper. Most of our dives were at about 60 feet. Where I saw most SPS, I believe that crashing waves or calm seas would have little impact on the flow they had running past them. I think that most of their flow is regulated by the tides.

 

[greenbean36191]

Again, that's going to depend where you are. In the Bahamas I've seen Acropora palmata sitting in calm water (though it wasn't the norm) and Porites is quite common in protected water there. In Hawai'i I've seen Pocillopora, Montipora, and Porites all thriving in calm water. Parts of the Red Sea in Jordan have thriving Acropora sitting in calm areas. All of these are in under 10 ft of water in areas where diel changes could have a very real difference. Again, there are some general trends, but it's hard to make any definitive statements.

That said, I see no benefit to the corals by reducing flow at night and in fact that's probably one of the worst possible times for them to experience lowered flow.

 

[jglackin]

Is there any such thing as calm water in the seas? All sea water is in motion or it will get stagnant. What may feel like calm water to you may actually be a strong flow. Or, if you are there at the period between the tides when the water is shifting from flowing in (high tide) to flowing out (low tide), it may appear to be calm, but the flow in the ocean is rarely (if ever) less than what we have in our tanks.

 

[greenbean36191]

Keep in mind that we're talking about protected areas where wind-driven motion dominates. When the wind stops, the water stops or at least slows. It's well documented that water on protected areas of the reef can stagnate when this is the case. The project I'm working on right now is actually looking at the impact of flow on thermal stress, so in my reading today I just happened across a paper that listed a measured range of water velocities on a reef of 0-12 cm/s.

See:
Kraines, S., Yanagi, T., Isobe, M., Komiyama, H., 1998. Wind-wave
driven circulation on the coral reef at Bora Bay, Miyako Island.
Coral Reefs 17, 133â€"œ143.

 

[Reef-LeGeNd]

yes it is. I have lived on one.

 

[jglackin]

<a href=showthread.php?s=&postid=12437363#post12437363 target=_blank>Originally posted</a> by greenbean36191
Keep in mind that we're talking about protected areas where wind-driven motion dominates. When the wind stops, the water stops or at least slows. It's well documented that water on protected areas of the reef can stagnate when this is the case. The project I'm working on right now is actually looking at the impact of flow on thermal stress, so in my reading today I just happened across a paper that listed a measured range of water velocities on a reef of 0-12 cm/s.

See:
Kraines, S., Yanagi, T., Isobe, M., Komiyama, H., 1998. Wind-wave
driven circulation on the coral reef at Bora Bay, Miyako Island.
Coral Reefs 17, 133â€"œ143.

I am not looking to argue with you, but I can't access this document to read it in full detail, but the abstract seems to contradict your claim:

Sea level, current and wave measurements were carried out within and outside an open-ended bar reef lagoon on the East African coast. The oceanic tides in this area are semidiurnal with a tidal range varying from 3 m at spring to 1 m at neap. Reef submergence relative to the mean sea level is 0.7 m. <b>Waves with significant wave height of typically 0.5 m and periods of 8â€"œ10 s continuously break on the reef-top and on the reef-edge during most of the tidal cycle</b>. This results in a wave set-up on the reef-top and a lagoon-ward flow across the reef. The set-up drives a time-dependent circulation, which is larger during neap tide when the reef is moderately submerged, than during spring tide. The wave driven circulation, <b>though small compared to the tidally driven circulation</b>, is still important because it hinders inflow of polluted water from tidal creeks that enter the ocean north and south of the lagoon itself.

This tells me that the tidal circulation is the primary source of circulation, not the wind.

Can you post the entire article? I am interested in reading it.

 

[greenbean36191]

I don't have the entire article in electronic form, but I was not trying to make the same argument as the article anyway. It's actually looking at how wind piles water up in the lagoon and how that flushs the lagoon. I was simply using their empirical measurements of velocity to back up my more subjective experiences that water on parts of reefs can indeed be truly calm. I didn't cite that paper in particular because it was the best on the subject, but rather it was the one I happened to be reading and it had empirical numbers. ATM I'm in the middle of writing a research proposal, so I don't have a huge amount of time to be digging around for better, unrelated papers.

The sentences you've emphasized from the abstract are irrelevant to the original question though because they apply specifically to the exposed portions of the reef. No one is arguing that those areas are the ones where wind effects are dominant.

A quote from the paper though- "Circulation in reef systems is typically driven by a combination of tidal flow, wind-driven flow and flow induced by the breaking of wind-waves on the reef flat (Symonds et al. 1995; Hearn and Parker 1988)."

 

[jglackin]

<a href=showthread.php?s=&postid=12440108#post12440108 target=_blank>Originally posted</a> by greenbean36191
A quote from the paper though- "Circulation in reef systems is typically driven by a combination of tidal flow, wind-driven flow and flow induced by the breaking of wind-waves on the reef flat (Symonds et al. 1995; Hearn and Parker 1988)."

Sure, there are lots of things that have an impact on flow to a reef, but in degrees that vary greatly. As in the article, the tidal flow is by far the most predominant source of flow. Again, I am not trying to argue with you, I am just struggling with believing that there are any reefs that do not get tidal flow to the point that the water becomes calm and/or stagnant. Virtually all water in the ocean is in a constant state of motion. The tides are always moving and there is always some form of surge, wind driven or otherwise. This is my highly uneducated opinion based on my limited experience with the ocean and diving. If there is any documentation to show that there are reefs that have calm waters or, more specifically, SPS reefs that are in calm waters, I would love to read about it. I would like to broaden my views on this.

 

[salty joe]

Thanks for all those replies. The reason I wondered about water motion is I am planning a tank that will use surge 24/7. Surge will be it for water movement except for water flowing through the filtration system. At this point, I plan on a RCSD on each end of the tank and they will alternate firing on 3 hr intevals. As long as the fish have hideouts, this might work out, don't you think?

 

[jglackin]

I have a Wavebox, 4 tunze 6055s, a Koralia 4, and a Reeflo Snapper that pumps over 1000 gallons per hour. The only thing that shuts down at night is the Koralia 4.

In my mind, my tank's flow, no matter how many pumps and surges I put in it, is equivalent to a slow moving wind driven surge as described above. In other words, it isn't even close to the real flow that reefs see in the ocean. I don't believe that slowing down the flow at night is a big deal.

 

[greenbean36191]

Just FWIW I haven't forgotten about this thread. Hopefully I'll get a break tonight and be able to pull up some articles for you. If not, it may not be until next Thursday or Friday. In the meantime you might try looking at some of Brian Helmuth's work. He's done a lot of characterization of flow on reefs and I think it was him that did a profile of Carrie Bow Cay (which was dominated by Acropora until the 1980s) where he not only measured water velocity, but also whether wave oscillations or tidal currents were dominant.

 

[jglackin]

Thanks. I don't expect you to take on the responsibility of educating me, as I am sure you have other things to do, but I do appreciate anything you can provide.

 

[salty joe]

Yeah, thanks.
For some reason, nothing showed up in my email indicating the last couple responses-probably something with my computer-CPU settings remain the same.

 

[greenbean36191]

Well, I did end up forgetting about this thread for a few days after I said I wouldn't.

As in the article, the tidal flow is by far the most predominant source of flow.

On that particular reef and at particular depths. There are other areas with what are called microtidal regimes where waves and wind-driven transport dominates. Tague Reef on St. Croix is one example.

Brian Helmuth and Ken Sebens have profiled the flow regimes in Discovery Bay, Jamaica and Carrie Bow Cay, Belize- both reefs that used to be dominated by Acropora. They found that wind-driven motion dominated above 20m on the forereef and above 10m on the protected portions of the reef and the velocity was extremely variable (depending on the waves). The wind speed, which often drops at night, is one of the major modulators of wave height.

See:
Helmuth, B. and K. Sebens. 1993. The influence of colony morphology and orientation to flow on particle capture by the scleractinian coral Agaricia agaricites. J. Exp. Mar. Biol. Ecol. 165: 251-278.

Sebens, K., B. Helmuth, E. Carrington, and B. Aguis. 2003. Effects of water flow on growth and energetics of the scleractinian coral Agaricia tenuifolia in Belize. Coral Reefs. 22: 35-47.

Again, I am not trying to argue with you, I am just struggling with believing that there are any reefs that do not get tidal flow to the point that the water becomes calm and/or stagnant.

It's common enough that it has its own term- "ponding." Ponding happens in protected parts of the reef during low tide where the water essentially gets trapped in the lagoon because the reef crest almost (or in some case does) breaks the water surface. In less protected areas with microtidal regimes wind can essentially pile up water near shore and prevent it from flushing back out. In both cases, ponding causes the water motion to drop to essentially 0. There's a paper I was looking for that looks at the potential adaptability of corals to high temperatures and they found Porites in areas where the water had ponded long enough to get up to 105+(!) but still showed little or no signs of bleaching.

If I manage to remember long enough I'll get you some pics of "SPS" growing in extremely calm water in about 2 weeks. Hopefully I'll be able to make some crude flow measurements too.

 

[sjames]

alot of wave action is generated by storms that can be a thousand miles away. North shore of Hawaii is a good example.