[Coral-List] Artificial Reefs

[Douglas Fenner]

   I think one other distinction needs to be made.  And that is the
difference between a coral reef as a geological structure, and a coral reef
as a living ecosystem.  People commonly use the term "coral reef" to mean
either or both.  They are two quite distinct things, although the living
ecosystem builds the geological structure, so the are very much connected.
    Since they are two distinct things, you can have both together, or just
one or the other present.  So you can have a geological "coral reef" which
is an accumulation of calcium carbonate that was produced by a coral reef
ecosystem in the past, but which no longer has a coral reef ecosystem
living on it today.  A Pleistocene coral reef which has almost no corals on
it today, such as Gene was pointing out in Florida, would be an example.
It might be called a "hardground" or a "dead coral reef."  On the other
hand, you can have a coral reef ecosystem, without a geological coral
reef.  So an ecosystem of living corals and other organisms, in which
corals are abundant enough to make it a coral reef ecosystem instead of
say, an algae bed, but which is growing directly on rocks that are not
carbonates.  So there is no carbonate buildup beneath the corals, yet there
are enough corals that there is a coral reef ecosystem.  As John says, that
is often referred to as a "coral community."  And then you can have both
together, which is what most of us are most familiar with.  But they are
two distinct things, and are separable.
      I would think that something humans build and put into the water
could easily become a coral reef ecosystem if enough corals settle on it,
but would require a lot of time to become a coral reef geological
structure, the corals would have to build so much carbonate that the coral
carbonate was what the structure was mostly made of.  Could certainly
happen, but would take a lot of time.
      The other aspect is the "reef" aspect.  A "reef" being a natural
structure in the water that is shallow enough you can run your boat aground
on it.  "Coral reefs" can be considered a type of reef that is made of
coral.  However, this is a relatively superficial use of terms that doesn't
acknowledge the distinction above between a geological structure and an
ecosystem.  Plus it gets into the problem of what to call a geological
structure with a coral ecosystem on top of it, which doesn't reach shallow
enough depths to run a boat aground on.  There are places like that, surely
plenty.  In my view, they should still be called "coral reefs" even if they
are too deep to run a boat aground on.
      Then there is the distinction that we have become aware of, that
there are deep water, cold water, places that have corals growing, and a
structure built by corals.  Those corals do not have zooxanthellae, they
have to catch food.  The corals are typically thin branching species, and
that ecosystem has a very low diversity of coral species, probably most are
dominated by just one species on a structure.  But the structures they
build (I think they're called "bioherms") can be fairly large.  But clearly
this is a distinct, though somewhat similar, thing to the tropical, shallow
water, coral reefs most of us work on.  Often they are called "deep water
reefs" or "cold water reefs" or something like that.
      Hope that helps a little.    Cheers,  Doug


On Tue, Oct 29, 2013 at 4:04 AM, Eugene Shinn <eugeneshinn at mail.usf.edu>wrote:

> John, That is a well reasoned response to my questions, thank you and
> congratulations. I especially appreciate your comment about Pleistocene
> coral reefs. I continually rant about how little growth there is on the
> so-called Florida reef tract that has had 6,000 years to grow and have
> often suggested it be called a hard bottom coral community rather than a
> reef. In this case the hard bottom is slowly forming  on a thick
> Pleistocene age coral reef so geologically speaking it could all be
> lumped together and called a coral reef. It is a coral reef which
> contains an  80 to 110 thousand year old hiatus between a Pleistocene
> coral reef tract that was exposed and populated by air breathing plants
> and animals when sea level dropped a few hundred feet  below present.
> The mystery is how did this thick coral reefs develop during the
> Pleistocene in a location where where corals have been doing poorly for
> the past 6,000 years? I can only guess that it was warmer during the
> Pleistocene interglacial period. Gene
>
> --
>
>
> No Rocks, No Water, No Ecosystem (EAS)
> ------------------------------------ -----------------------------------
> E. A. Shinn, Courtesy Professor
> University of South Florida
> College of Marine Science Room 221A
> 140 Seventh Avenue South
> St. Petersburg, FL 33701
> <eugeneshinn at mail.usf.edu>
> Tel 727 553-1158
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