[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 
      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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