Coral Bleaching in the TEP

[Fernando A. Zapata Rivera]

CORAL BLEACHING IN THE TROPICAL EASTERN PACIFIC

	Regarding the recent report on coral bleaching in the Pacific
coast of Panama made recently in this list by Baker, Mat, and Glynn, I
would like to share my own observations from the Pacific coast of
Colombia.

	During the course of ongoing studies on coral reefs of Gorgona
Island (2 58 N, 78 10 W) being carried out by two teams from Universidad
del Valle, I have made three visits to the island between May 24 and
September 16, 1997. During the first visit (May 24-29), I made extensive,
albeit non-quantitative, observations on coral cover and "health" at six
different reef sites around the island, and did not observe any signs of
significant or widespread coral bleaching. However, my colleague Jaime
Cantera did call my attention to the whitened, most distal portion of
branches of  some patches of Pocillopora sp., which he interpreted as
coral bleaching (the lower portion of colonies had normal coloration). To
me this was not different to the usual whitened branch tips normally seen
in the Pocillopora capitata species "complex". On the other hand, I did
observed a very small number of partially to completely bleached colonies
of Pocillopora sp., but these were always unattached, large fragments and
seemed to have suffered from some unknown physical disturbance. Thus, I
did not attribute this bleaching to sea water warming. Sea surface
temperature remained constant throughout our visit at 29 degrees celsius.

	During the second visit (July 27-August 1) I made observations at
four sites and only in the two largest reefs I saw some signs of
bleaching. These were similar to the bleaching observed during the first
visit, except that the number of large, unattached fragments of
Pocillopora sp. that were bleached was more noticeable (i.e., it appears
that the number of bleached fragments had increased). I also found one
small (15 cm diameter), unattached colony of Pavona varians completely
bleached, without live tissue, and with beginning algal overgrowth. At
that time it again seemed to me that these cases of bleaching could not be
attributable to sea warming. Occasional SST measurements made revealed
that water temperature was 29 degrees.

	During the last visit (September 10-16) it became apparent that a
bleaching episode was on its way based on observations at the two major
reefs of Gorgona. The bleaching at the branch tips of Pocillopora sp. was
now more obvious, extending downward about 2 cm from the tip, and patches
with this type of bleaching at the tips were encountered frequently. These
patches appear to be monospecific judging from colony morphology and
coloration, and were surrounded by high cover of other, normal looking
Pocilloporids, suggesting that some species of Pocillopora are more
susceptible than others to bleaching. I believe that bleaching at the tips
of branches was not caused by aerial exposure during extreme low tides
(which causes similar bleaching) since it was observed in both shallow and
deeper sites on the reef, and some normal-looking patches were located
next to and at the same depth as bleached patches. No algal growth was yet
apparent at the bleached tips of branches on any of these coral patches.
Additionally, partial bleaching was now readily seen on species of massive
corals (large Pavona gigantea, and P. clavus). Some colonies had up to 30%
of their surface bleached. Nonetheless, the extent of coral bleaching at
the scale of an entire reef was clearly low, although it was not
quantified. Water temperature measurements made varied between 28 and 29.5
degrees.

	The main point I would like to make is that there appears to be a
trend towards increased bleaching with time as SST is maintained
abnormally high. According to Dr. Alan Strong's "Hotspots maps" (see
http://wesley.wwb.noaa.gov/eileen/orad/const1.html), SST at Gorgona was at
least 2.5 degrees celsius above the norm on the week of August 11, 1997.
Whether this trend continues at Gorgona remains to be seen, but it is
interesting that significant bleaching is already being reported elsewhere
in the tropical eastern Pacific.


Fernando Zapata
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Fernando A. Zapata		     Ph.:  (92) 339-3243
Departamento de Biologia	     Fax.: (92) 339-2440
Universidad del Valle                International (+57-2)seven-digit number 
Apartado Aereo 25360                 E-Mail:fazr at biologia.univalle.edu.co
Cali, Colombia                       
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