Ocean Warming and Coral Health

James C. Hendee hendee at AOML.ERL.GOV
Thu Jun 22 15:00:12 EDT 1995

The following was originally posted under the news group  
sci.geo.oceanography by Dr. Hugh Easton in response to Dr. Ray  
McAllister, and is re-posted here for information and comment. 

In article <Pine.A32.3.91.950604144824.7461D-100000 at seminole.gate.net> 
           mcallist at gate.net "Ray McAllister" writes: 

> Josh, your analysis is much more believable to me than the global warming  
> hypothesis. So far we have no definitive evidence for ocean warming and  
> may have to wait till ATOC to get anytrhing dependable.  

That is no longer true, at least according to an article, "Drying out the 
tropics", in the 6 May issue of New Scientist. Apparently there has been a  
recent shift in thinking among climatologists about the stability of 
tropical climates. More importantly for the purposes of our discussion, 
substantial warming has been measured in tropical oceans. 

"The 1980's were the warmest decade on record, and this was primarily 
because temperatures rose in the tropics. ... The tropical ocean  
temperatures were between 0.25 and 0.75 C warmer [from 1981 to 1990] 
compared with 1951 to 1980. Since 1976, the eastern tropical Pacific has 
been more than 0.5 C warmer than in the previous decades" 

I have also got some material which explains why high water temperatures 
adversely affect coral reefs: 

"Because reef-building corals are dependant on their plant partners, they 
need shallow, sunlit waters for photosynthesis to occur most rapidly. These 
conditions are also essential for the successful deposition of the 
corallite by the coral polyp. The optimum temperature for this to take 
place is between 26 C and 27 C (79 F and 81 F). Once the temperature falls 
below 23 C (73 F) or rises above 29 C (84 F), the rate of calcification 
rapidly decreases and the forces of erosion overwhelm those of growth and 
repair. Prolonged temperature changes therefore spell doom for a reef. ... 
   There is another ominous threat to reefs connected with rising sea 
temperatures that has recently come to light. Coral colonies have been 
known to occasionally lose their zooxanthellae. The ability of the 
zooxanthellae to produce oxygen by photosynthesis increases with 
temperature. It appears that if the zooxanthellae produce too much oxygen 
during photosynthesis, then toxic by-products result that are damaging to 
the tissues of the coral polyp. Thus, the zooxanthellae may be lost through 
the damaged wall of the polyp back into the ocean. The coral colony turns 
brilliant white as it now lacks any pigmentation. The process is known as 
coral bleaching. Close inspection of a bleached coral colony at night will 
reveal that coral polyps are still present, but transparent. Such a coral 
may recover by obtaining new zooxanthellae from the surrounding water but 
it is more likely, however, that it will die. 
   Outbreaks of coral bleaching have been recorded world-wide and cover 
enormous tracts of reef. At least three major occurrances since 1979 have 
been reported by researchers. Particularly alarming is the fact that 
episodes of bleaching coincide with either the hottest season for that 
area, or unusually hot conditions due to other factors." - Reef, pp 31 & 
58-59, Jeremy Stafford-Deitsch, Headline 1991. 

So unusually high temperatures have two effects on corals. At temperatures 
above 29 C corals growth is no longer fast enough to keep up with erosion, 
and at temperatures substantially higher than the coral is used to, oxygen 
damage and coral bleaching occur. Recent research has confirmed that 
high water temperatures are the primary cause of coral bleaching: 

"Major outbreaks of "bleaching" of coral reefs in the past decade were 
almost always caused by unusually high sea temperatures, probably linked 
to global warming, according to the first global study of the phenomenon. 
... Goreau, who is president of the Global Coral Reef Alliance, and his 
colleague, Raymond Hayes, former chairman of marine sciences at the 
University of the Virgin Islands, discerned a rising tide of coral 
bleaching between 1983 and 1991. They compared its incidence with satellite 
data on average ocean temperatures (Ambio, vol 23, p176). 
   In 1983, coral was bleached throughout the Pacific, from the Java Sea 
to Costa Rica. Bleaching was widespread in the Caribbean in 1987, and 
recurred every year until 1990 - a year of record temperatures and 
bleaching in the Gulf region of Oman. Bleaching was again widespread in the 
Pacific in 1991, from Thailand to Polynesia. 
   Bleaching occurs in all waters, from the warmest that can sustain coral 
reefs to the coldest. Goreau and Hayes found that the effect is triggered 
not by any specific temperature, but by anomalous warming locally. 
Bleaching invariably followed the warmest period ever recorded in an area. 
The threshold appeared to be a monthly mean more than 1 C higher than the 
long-term average. "Above [that temperature] bleaching always took place," 
say the researchers, "and below it did not." 
   Most bleaching in the Pacific occurred in 1983, 1987 and 1991, when 
changes in ocean circulation - known as El Nino events - caused warmer 
water than usual to spread across the tropical Pacific. However, El Nino 
is unlikely to have caused the bleaching in the Caribbean, where the two 
researchers say that a strong warming trend persisted throughout the 
1980s." - A paler shade of coral... New Scientist 11/6/94, p19. 

I have been collecting the CAC ENSO indices for the last year and a bit. 
These include the monthly average temperatures for various sectors of the  
Pacific. Here are the Nino 4 and Ship Track 6 figures which refer to 
temperatures in the equatorial Pacific. 

                             Anomaly  SST(C) 
mar 94 
 Nino 4                        0.2   (28.3) 
 Ship Track 6 (Hawaii-Fiji)          (28.4) 
apr 94 
 Nino 4                        0.3   (28.6) 
 Ship Track 6 (Hawaii-Fiji)          (28.5) 
may 94 
 Nino 4                        0.6   (29.1) 
 Ship Track 6 (Hawaii-Fiji)          (29.0) 
Jun 94 
 Nino 4                        0.6   (29.1) 
 Ship Track 6 (Hawaii-Fiji)          (29.0) 
Jul 94 
 Nino 4                        1.0   (29.4) 
 Ship Track 6 (Hawaii-Fiji)          (29.3) 
Aug 94 
 Nino 4                        1.0   (29.4) 
 Ship Track 6 (Hawaii-Fiji)    1.1   (29.4) 
sep 94 
 Nino 4                        1.0   (29.3) 
 Ship Track 6 (Hawaii-Fiji)    1.6   (29.7) 
oct 94 
 Nino 4                        1.1   (29.4) 
 Ship Track 6 (Hawaii-Fiji)    1.5   (29.6) 
nov 94 
 Nino 4                        1.2   (29.5) 
 Ship Track 6 (Hawaii-Fiji)    1.5   (29.8) 
Dec 94 
 Nino 4                        1.3   (29.5) 
 Ship Track 6 (Hawaii-Fiji)    1.9   (30.1) 
  (the procedure for calculating the indices was changed at this point) 
Jan 95  
 Nino 4                        1.1   (29.2) 
Feb 95 
 Nino 4                        1.0   (29.0) 
Mar 95 
 Nino 4                        0.2   (28.3) 
 Ship Track 6 (Hawaii-Fiji)          (28.4) 
Apr 95 
 Nino 4                        0.5   (28.8) 
 Ship Track 6                  0.5   (28.9) 
May 95 
 Nino 4                        0.5   (29.1) 
 Ship Track 6                  0.5   (29.2) 

>From these figures you can see that 29 C was exceeded for most of the period 
covered, and for several months the temperature anomaly was greater than 1 C.  
>From here it would appear that the "global warming hypothesis" (your words 
not mine) has a lot going for it. 

>         The idea that Acropora species are endangered by a combination of  
> factors that synergistically damage this genus is more convincing to me.  

For scientific or aesthetic reasons? 

> What about A reticularis and A hyacinthus in trhe Pacific? 

Judging by the figures above, I would imagine that they are in pretty bad 
shape. Anyone care to comment? 

>         Bu the way, in Florida I am not so sure that A cervicornis is  
> threatened more than palmata, but no studies, just observation. Thanks to  
> all who posted on this subject. 
> Ray McAllister, Prof (Emeritus) Ocean Eng., FAU, Boca Raton, FL 33064 
> Diving Dinosaur, Geologist/Oceanographer/Ocean Engineer, 43 years SCUBA 
> mcallist at gate.net (305) 426-0808, Author Diving Locations, Boynton/Dania 

Hugh Easton                             <hugh at daflight.demon.co.uk> 

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