[Coral-List] diver distance

Eugene Shinn eugeneshinn at mail.usf.edu
Wed Aug 14 16:51:12 EDT 2013

Dear Listers, I just spent more than a month diving in the Keys and 
photographing the corals I have photographed yearly since 1960. Un 
fortunately there has been no noticeable improvement at those sites 
(Grecian Rocks and Carysfort Reef). Have spent last few days catching up 
on e-mail and scanning the various coral list threads. As a geologist it 
was easy to identify with the comments by Dennis Hubbard. His Caribbean 
work indicates a coral growth hiatus at 3,000 and 6,000 years bp (before 
present). In the Keys we have documented a hiatus in /Acropora 
cervicornis/ growth centered around 3000 years bp as well as one 
centered at 4,500 ka. The 6 ka hiatus found by Hubbard is not present in 
Florida Keys reefs because the shelf was being flooded and corals were 
just beginning to grow at that time. This knowledge is based on Carbon 
14 dating of corals at the base of coral reef cores. Although dating 
verifies two major breaks in coral growth there were likely many more 
during the past 6 ka (thousand years) when the so-called Florida reef 
tract was being flooded during the world-wide Holocene rise. Frequent 
waxing and waning occurred during that time because approximately 98 
percent of that 150+ mile long reef tract has built up no more than 2 m 
of coral accumulation. That knowledge is based on more than 20 years of 
high resolution seismic profiling and ground truth coring 
<http://pubs.usgs.gov/pp/2007/1751>. In fact, a core adjacent to the 
Aquarius habitat revealed only 15 cm of accumulation during the past 6 
ka. In other words biologists studying that area have in fact have been 
studying a hardground community that looks like a coral reef. That's ok 
because they would get the same biological results studying corals etc. 
on a true coral reef accumulation.

True reef accumulations do occur in the keys but they are mainly in the 
form of coral spurs and occur mainly on preexisting Pleistocene bedrock 
highs. These are the named reefs that have kept pace with sea level rise 
(about 2 percent of the reef tract). It is the cores of these 
accumulations upon which our knowledge of Keys coral reef history is 
based. Clearly there were climate related events (increased hurricanes? 
fluxes of cold water? warming seas? diseases?) that kept the other 98 
percent of the reef tract from developing along with those on the 
bedrock highs.

Yes, People have likely affected growth in the past 50-years but that 
does not prove natural events were not equally harmful. In the long 
term, as Hubbard says, nature has likely been more harmful than people.

During this latest expedition I observed formidable examples of shifting 
baselines or should we now call it the "new normal"? During our short 
visit to Grecian Rocks we observed the arrival and departure of at least 
5 large catamarans each of which disgorged 20 to 30 snorkelers, not 
including the numerous private boatloads of snorkelers. I estimate that 
around 200 swimmers snorkeled the leeward side of Grecian Rocks during 
that day, and this was not even a weekend! I can only surmise that the 
snorkelers were happy with what used to be a thriving reef and clearly 
oodles of money was flowing into the Keys economy. After observing all 
this activity the question of how close a diver should approach corals 
seems moot. If the water is clear the divers appear to be happy and the 
keys economy is flourishing like I have never seen during my 60 some odd 
years of diving there. I wonder what it was like at the really popular 
reefs such as Molasses reef?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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