Reefs and Bioherms

Mark and Arnaz Erdmann flotsam at
Fri Jun 2 11:41:38 EDT 2000

Dear Colleagues,
 I refer to the recent thread regarding definitions of reefs and bioherms, and
in particular whether azooxanthellate deep water corals form "reefs" or
or "groves". As a PhD candidate in modern coral reef ecology, my dissertation
committee mandated that I read J.A. Fagerstrom's excellent 1987 book "The
evolution of reef communities" (John Wiley and Sons) in order to gain a
geological perspective on reefs. I believe that text to be highly relevant to
this discussion. In Fagerstrom's glossary, the definition for reef reads
"countless authors, almost exclusively geologists, have defined organic reefs;
these definitions are so inconsistent, contradictory and inconclusive that it
would be pointless and futile to include them here". Rather, Fagerstrom
an entire chapter to the subject "What is a reef?", including a discussion of
"what is NOT a reef".  A brief passage from that chapter is perhaps
to the coral list discussion:

"A typical marine organic reef is the product of a vigorous biological
to a relatively restricted set of interdependent environmental factors. During
its growth, a reef progressively alters its environment...The chief
manifestations of this biological response are the relatively dense packing
rapid growth rates of predominantly sessile, colonial, and highly gregarious
organisms. Typically these organisms secrete calcareous (siliceous in a few
taxa) skeletons, which, by virtue of their large size, dense packing, and in
situ interlocking growth, form a rigid structure having a ridge or moundlike
shape and positive topographic relief with respect to the unconsolidated,
usually carbonate, sediments of the adjacent seafloor..."

Throughout the book, the recurrent themes for reefs include:
-rigid framework
-dense packing of sessile (often colonial), gregarious organisms whose
skeletons are intergrown in living position
-positive topographic relief

Fagerstrom (p 63-64) considers the Atlantic deep water coral "aggregations" to
be aphotic zone reefs, in that they are often large,  rigid structures formed
from skeletons of gregarious organisms intergrown in living position with
significant topographic relief.

Regarding the term "bioherm", Fagerstrom traces this term to the geologists
Cumings and Shrock (1928) as a "dome-, moundlike or otherwise circumscribed
mass built by sedentary organisms and enclosed in normal rocks of different
lithology". Seemingly implicit in this definition is that the mound is usually
covered in lime mud. He also refers to a related paper that may be of interest
to this discussion: 

Cumings, ER. 1932. Reefs or Bioherms? Geol. Soc. Am. Bull. 43: 331-352

Perhaps this information sheds some additional light on this debate. While the
Webster's New Collegiate Dictionary definitions of reef quoted by Alina Szmant
("a chain of rocks or ridge of sand at or near the surface of water" or "a
hazardous obstruction") are appealing in their simplicity, I believe it is
quite an injustice to ignore the significant body of scientific work
(admittedly mostly geological) that has been devoted to the question of "what
is a reef?"


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