[Coral-List] Great Barrier Reef is rapidly losing coral

[Eugene Shinn]

     With all this talk about the declining Great Barrier Reef I felt 
compelled to throw in my two cents. I am confused with the way the 
term Great Barrier Reefs has been described in the recent letters. As 
I understand it the GBR is a discontinuous 2000 long strip many miles 
off shore and separated from shore by a deep lagoon with hundreds of 
patch reefs, sandy islands and the tops of hills/mountains that were 
flooded by rising sea level during the past 18,000 years. The writers 
and the recently cited biological paper (27-year decline) tend to 
lump all the varied reefs into one entity called the Great Barrier 
Reef. It is my understanding that the true barrier reef is the narrow 
discontinuous outer strip of coral and reef sand that faces the open 
ocean.
     The narrow offshore barrier I have read about is a thin veneer of 
coral capping pre existing topography. There is not a lot on this 
outer barrier for COTS to eat. Please correct me if I am wrong. The 
lush coral we all know about is around those large patch reefs many 
miles landward of the outer barrier. Are not these reefs more likely 
to be devastated by COTS?
     I certainly am not an expert on the GBR but I did do a tour there 
in 1972. Like most people I did not see the actual Barrier reefs that 
face the open ocean.  My knowledge of those outer barrier reefs is 
based purely on geological accounts. Fortunately my weeklong tour 
followed the huge starfish epidemic that occurred in the late 1960s 
and had run its natural course.  That epidemic was one of the hot 
topics at the Third International Coral Reef Symposium in Miami in 
1977. Arguments that it was human induced were similar to those of 
today except that bleaching, climate, and disease were still on the 
horizon.  Several Australian scientists went at each other with great 
vigor. Some geologists had taken cores, dug holes, and found layers 
of the distinctive skeletal parts of COTS indicating the infestation 
was nothing new. Some outspoken biologists on the other hand, 
vigorously refused to accept geological observations. They opted for 
humans as the cause and blamed the cause on collection of a snail 
that preys on starfish. The division between anthropogenic and 
natural causes seems little changed over the past 35 years.
     The first stop on my 1972 tour was Heron Island near the southern 
portion of the barrier reef complex.  Besides my amazement that 
Pacific corals could survive more than an hour of subaerial exposure 
during low tide I was greatly impressed with the lush variety of 
corals. Surprisingly people were walking and collecting (Fossicking) 
on the vast exposed reef flat. I rented a skiff and took many 
underwater photographs on the flanks of the reef area. On my return 
to the dock I told the man in charge that I had seen a grouping of 
bomies I recognized from a televised nature film but did not see the 
starfish that had been the programs centerpiece. The boat rental 
agent said, "Mate we brought that starfish from a reef 30 miles away 
and placed it there for the program."
     The next stop was Hayman Island in the Whitsunday group of 
mountaintops surrounded by fringing reefs. That stop was not very 
instructive but still there was no evidence of COTS. However, the 
next stop, Green Island, was most instructive. COTS had decimated 
hard corals around the island and the reef sand consisted almost 
exclusively of starfish fragments and spines.
     These observations suggested that COTS did not affect all of the 
hundreds of islands and patch reefs in the Barrier Reef lagoon. I do 
not know if they occurred on the outer barrier many miles seaward but 
it was clear they had not damaged the lush reefs around Heron Island.
      At the Ninth ICRS in Bali in 2000, I was surprised to see a 
session devoted to muddy water coral reefs. About 24 papers described 
muddy water coral reefs that occur closer to shore in Australia. 
These reefs are little known and tour guides are not likely to take 
tourist divers to these reefs where the water is too murky for 
underwater photography. In spite of the evidence that such reefs 
exist the myth that corals cannot live in muddy water persists. Of 
course they are not the flamboyant colorful coral species one expects 
farther off shore but as a geologist I recognize these as more like 
reefs in the geological record.
     More up to date, a recent 2012 issue of the Journal Geology 
contains a paper with the long title, "Evidence of very rapid reef 
accretion and reef growth under high turbidity and terrigenous 
sedimentation." That study describes 8 muddy water reefs near 
Magnetic Island and is based on cores and 40 radiocarbon dates. The 
study documents the surprising result that these reefs have accreted 
faster than clear water reefs father off shore! How can this be?  The 
simple answer is that the high sedimentation rate blankets dead 
corals and thus preventing bioerosion that usually reduces dead 
corals to sand and rubble. This observation is counter intuitive but 
makes good sense. It is a likely explanation for the thick, mud-rich, 
coral and Rudistid reefs preserved in the geologic record.  Of course 
it is a Goldie Locks situation. The sedimentation rate must be just 
right. Not enough to kill the coral but just enough to blanket dead 
coral surfaces and fill spaces between the corals.
     In recent years we have recognized similar examples in the 
Florida Keys. Corals offshore close to the clear Florida current have 
experienced rapid demise while corals closer to shore where 
visibility is greatly reduced have survive. It should also be noted 
that many of these near shore reefs succumbed to cold winter 
temperatures in 2010. All things considered there seems to be a 
striking similarity between the Great Barrier Reef area and the 
Florida Keys situation. In Florida the thickest reef accumulations 
(places like Grecian and Key Largo Dry Rocks) occur starting about a 
mile landward of the outer barrier we generally call the Florida reef 
tract. This outer 6,000-year-old Holocene barrier is for the most 
part is under less than 8 to-10 meters of water and less than 1-meter 
thick. The more shoreward reefs that have grown up to sea level are 
roughly 10-meters thick.  Likewise, the thickest reefs in Australia 
are also well shoreward of the outer barrier.
      With these observations in mind I encourage researchers worrying 
about the future of Australian coral reefs to be more specific about 
what they refer to as the Great Barrier Reef.  Gene

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No Rocks, No Water, No Ecosystem (EAS)
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E. A. Shinn, Courtesy Professor
University of South Florida
College of Marine Science Room 221A
140 Seventh Avenue South
St. Petersburg, FL 33701
<eshinn at marine.usf.edu>
Tel 727 553-1158---------------------------------- 
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