[Coral-List] Reply to Gomelyuk, what is a coral reef?

[Gene Shinn]

Victor Gomelyuk, asked the question, "can anyone explain to me the 
difference between "coral reef" and "coral community" in quantitative 
terms?" That question, often stated as, "what is a reef?" has been 
argued by geologists and biologists since I can remember. Few will 
ever agree on one answer. Nevertheless, I will open this can of worms 
one more time and see what comes out. First I seriously doubt the 
differences can be explained in quantitative terms but lets attack 
the other part of the question, the difference between "coral 
community" and "coral reef" and see where it leads us.
      The biological explanation is that a "coral community" is a 
community of corals and associated reef organisms living on a hard 
rock surface. Such communities are sometimes referred to as "hard 
bottom communities." These communities do not care if they are 
growing directly on basalt, concrete, or a previously formed "coral 
reef."
      The geological definition of a "coral reef," as often argued, is 
a "coral community" growing upward on a framework built by the same 
"coral community." This geological "coral reef" could begin as a 
"coral community" and grow upward creating a fossil reef many meters 
thick. Thus, you can have a "coral community" without a "coral reef" 
but you can't have a "coral reef" without a "coral community." The 
geological record contains abundant fossil coral reefs. For example, 
the upper Florida Keys is a fossil Pleistocene age coral reef, while 
the lower Florida Keys are fossil Pleistocene carbonate sand bars.
      Here is an example of a "coral community" growing on a fossil 
"coral reef" that was not created by the present "coral community." 
During a mission in the Aquarius underwater habitat (with 3 coral 
biologists) we drilled a 50-ft deep core next to the habitat. For all 
practical purposes the area looks and functions like a coral reef and 
meter-high living coral heads are scattered here and there. The core, 
however, showed that the "coral community" was only a few centimeters 
thick. What was under it was a Pleistocene fossil reef that formed 
about 80,000 years ago. That reef had died when sea level fell. It 
had been exposed to form dry land and was capped by a soil stone 
crust. It was flooded again by rising sea level about 6,000-7000 
years ago but during the past 6,000 years the "coral community" had 
attained a thickness of only a few centimeters. Today what one sees 
is a coral reef community but geologically speaking it is not a coral 
reef, or said differently "it quacks like a duck but it is not a 
duck." Like I said in the beginning, what is a reef, is a 
can-of-worms-question.
     What we found in the core at the Aquarius site has been noted in 
many other cores taken on the outer margin of the Florida reef tract. 
The data from those cores were used to ground-truth several hundred 
kilometers of high-resolution seismic profiles throughout the Florida 
Keys Marine Sanctuary. These studies showed that the areas where 
"coral communities" have created coral reefs through continual upward 
growth during the last 6,000 years of sea level rise (places like Key 
Largo Dry Rocks and Grecian Rocks) are surprisingly small. By the 
definitions provided above these "coral reefs" occupy less than 1 
percent of the Marine Sanctuary. The rest of the Marine Sanctuary 
consists of an accumulation of lime sand, lime mud, coral rubble, and 
patch reefs, to name a few. For details on all the habitats that have 
been mapped in the Florida Keys Marine Sanctuary see, B. H. Lidz et 
al. (2005). It is a DVD presentation of USGS Professional paper 1714. 
This massive manuscript with its maps, diagrams and reviews weighed 
16 lbs so it was put on a DVD.
      So in conclusion, I doubt that the difference between "coral 
reef" and "coral community" can be distinguished in quantitative 
terms because what one would measure, whether it be species, species 
abundance, or species diversity, would likely be the same in both 
cases.
      The mystery for many of us is not what is a reef, but why so few 
of the "coral communities" (less than 1 percent) were able to keep 
pace with rising sea level during the past 6,000 years and form 
"coral reefs?" Why did the vast majority of the "coral communities" 
not form "coral reefs" when even the slowest growing corals can 
accrete faster than the well-documented rise in sea level during the 
past 6,000 years?
E. A. Shinn

Reference: Lidz, B. H., Reich, C. D., and Shinn, E. A., 2005, 
Systematic mapping of bedrock and habitats along the Florida Keys 
reef tract: Central Key Largo to Halfmoon Shoal (Gulf of Mexico). 
USGS Professional Paper 1714.
     

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No Rocks, No Water, No Ecosystem (EAS)
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E. A. Shinn, Courtesy Professor
University of South Florida
Marine Science Center (room 204)
140 Seventh Avenue South
St. Petersburg, FL 33701
<eshinn at marine.usf.edu>
Tel 727 553-1158---------------------------------- 
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