[Coral-List] The myth of 100% coral cover
Eugene Shinn
eshinn at marine.usf.edu
Fri Mar 23 11:34:56 EDT 2012
Listers, I would like to amend comments I made in Vol. 43, Issue
19, regarding the myth of 100% coral cover. After looking at some
outcrop photos, I decided that, yes, there was/is such a thing as
100% coral cover. Outcrops demonstrating 100% cover occur on the
flanks of lake Lago Enriquillo in the Dominican Republic. This lake
was originally a 70-km-long blind-end embayment with only one
entrance to the ocean on the south coast of the DR. The blind-end
cul-de-sac was, and still is, surrounded by arid mountainous terrain
and is sporadically subject to torrential runoff. About 7,000 years
ago (based on C-14 dating), sedimentation blocked the mouth of the
cul-de-sac and stopped circulation with the ocean, allowing
evaporation to lower water level some 42 meters or more. Lowering
water level exposed what had once been lush Holocene fringing reefs.
Erosion from continued runoff continues to cut steep-sided arroyos or
channels oriented perpendicular to the lake's receding shore. One can
now walk these channels with vertical 30- to 40-foot-high walls on
either side and observe what was once the marine shore (fringing
reef) and on downward through what was a lush coral reef and on to
the lower limits where reef facies grade into fine-grained sediment.
I had the good fortune to accompany Dennis Hubbard and his
students on such a paradigm-changing walk following the last ICRS.
Dennis had been studying these exposed Holocene reefs, as have Lisa
Greer and her students, for the past 2 decades. If one wants to see
an area where there was likely once 100% coral cover, this is the
place.
This trip was a game changer for me. Who would have believed
such lush coral growth, some of it consisting almost entirely of
Acropora cervicornis, could have flourished in a long narrow
blind-end cul-de-sac with no apparent circulation and where reef
builders had to contend with sporadic outwash from the surrounding
hillsides and mountains? There was little circulation with the open
ocean and little wave action in this cul-de-sac, especially along the
northern leeward shore. Clearly conditions were not what most coral
researchers/activists preach is absolutely necessary for coral
growth. Of course humans were probably absent. Hudson, Lidz,
McIntyre, and I had observed an earlier game changer in the mid-1970s.
Our USGS group experienced that during our first expedition to
Belize back before the Caribbean-wide demise of Acropora began. We
found A. cervicornis growing right up to red-mangrove roots around
inshore mangrove islands, and our 20-foot-long vibracores
demonstrated there was no hard substrate. There is no evidence that
nearshore staghorn growth ever existed in the Florida Keys. The upper
few meters of our cores revealed only staghorn sticks growing on
staghorn sticks floating in a matrix of silt and sandy sediment.
Similar conditions must have existed at Lago Enriquillo just 7,000
years ago. It should be mentioned that we all know from the coring
work of Precht and Aronson that staghorn around those patch reefs and
islands in Belize died in the late 1970s (they were thriving in 1976)
and were replaced by Agaricia sp., and later they too died and were
replaced by fleshy algae.
What can these observations teach us? First, we all know that
corals in Belize, Florida, San Salvador, and pretty much the entire
Caribbean began dying in the late 1970s. Their demise was punctuated
in 1983 and '84 along with the die-off of Diadema and the advent of
seafan disease. Second, we really do not understand coral demise
other than that it is heat- and disease-related (and likely involves
sewage in some areas). Third, we are wrong if we say corals cannot
thrive in areas of periodic natural sedimentation and poor
circulation. Fourth, we are right if we admit we really do not know
what constitutes "critical habitat" for coral growth. And finally,
yes, there were and probably still are areas where there is what
appears to be 100% coral cover. How you actually measure 100% cover I
do not know. 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
<eshinn at marine.usf.edu>
Tel 727 553-1158----------------------------------
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