global reef area

[Joanie Kleypas]

TO REINFORCE THE MANY OPEN RESPONSES SENT TO CORAL-LIST, BELOW IS
A COPY OF MY DIRECT RESPONSE TO THE ORIGINAL TWO ENQUIRERS.  MUCH OF
THIS IS REDUNDANT, BUT AT LEAST WE'RE ALL AGREEING ON SOME THINGS.

Your question is a good one.  Estimates of reef coverage on the earth 
are only now being updated.  Below is a quick table of various
estimates of reef cover from the literature.  

============================================================================
SOURCE                   ESTIMATE     NOTE
                         10^3 km^2
-----------------------------------------------------------------------------
Smith, 1978                  617      reef coverage to 30 m depth 
De Vooys, 1979               100
Achituv & Dubinsky, 1990    2000
Crossland et al., 1991       617      used Smith's estimate to include 
                                      photic zone reefs, where sediments 
                                      80% reefal 
Copper, 1994                1500      added relict reefs, carbonate banks 
                                      and inter-reef tract to Crossland et al. 
                                      estimate 
Kleypas                   584-3930    range of modeled results based on 
                                      light-dependent reef depth limit 
Spalding and Grenfell        255      "emergent reef crest and very shallow 
                                      reef systems"
============================================================================

There is a wide range of values, due to two major factors which I outline 
below.

1.  The fuzzy definition of what constitutes a coral reef

The definition of "coral reef" is often different for biologists/ecologists
versus geologists.  The geological definition requires that the coral 
community accumulate enough calcium carbonate to form an identifiable 
buildup with structural relief relative to the surrounding sea bed.
But corals can certainly exist without forming a reef.  Geologists call
these "coral communities" rather than reefs; that is,
a coral reef has the capacity to accumulate carbonate and 
keep up with sea level rise, while a coral community does not 
have that capacity.

Hence, where the individual researcher "drew the line" 
between coral reefs and coral communities has something to do with 
his/her estimate of coral cover.

(It should be noted that from an ecological standpoint, however, 
this distinction makes little difference -- coral communities seem 
to function ecologically very much the same as coral reefs.  Thus 
conservation issues generally apply to both coral reefs and communities.)


2.  Our poor knowledge of reef distribution

Reefs are poorly charted on navigational charts.  Navigators tend
to call any structure that is a navigational hazard (reefs, submerged
rocks, shoals, etc) a "reef", while structures which exist below the
depth where they pose navigational risk tend not to be charted as "reefs".
John McManus (ICLARM, Philippines) and Mark Spalding (WCMS - World Conservation 
Monitoring Centre) are compiling area coverage estimates for the ReefBase 
project (see web site: http://www.wcmc.org.uk/data/database/reefbase.html).
Their estimates are based on these charted reefs only; but will eventually 
be the most accurate.

My own method has been to estimate reef area coverage by modeling
suitable reef habitat on the globe, based on environmental tolerances
for reef-building corals (e.g., temperature, light, salinity).
These reef area estimates vary greatly (584,000 - 3,242,000 km2) 
depending on the light level chosen in the model as "limiting" to 
reef growth.  Based on comparisons of the model predictions with
known reef locations, and the overall observation that most active 
coral reef growth is limited to around 30 m depth, the LOWER FIGURE
is likely the best guess.  This lower figure is also surprisingly
close to Steve Smith's 1978 figure, which he came up with before
any of these global marine data sets were around!  I have a paper
in review to Paleoceanography on these estimates and can send you
a draft or preprint if necessary.

added comments:
COMPARED TO MARK SPALDING'S ESTIMATE OF 255,000 KM2, THE MODELED
VALUES ARE HIGH BECAUSE THEY DO INCLUDE DEEPER SUBMERGED REEFS AND
"REEFAL SHELF"; AND ALSO BECAUSE THE RESOLUTION IS LOWER (MARKS:
1 KM; MINE: 9KM).
------------------------------------

You might want to get Mark Spalding's latest estimate!  -- YOU
ALREADY HAVE THIS BY NOW.

cheerio, 
Joanie


****************************************************************************
J. Kleypas
Climate Change Research Section
National Center for Atmospheric Research
PO Box 3000
Boulder, CO  80307-3000

PH:   (303) 497-1615
FAX:  (303) 497-1348

kleypas at ncar.ucar.edu
****************************************************************************