Bleaching & dissolved oxygen

Peter Craig Peter_Craig at
Sat May 19 09:43:30 EDT 2001

Coral list,

In discussions of bleaching, low levels of dissolved oxygen (DO) are 
occasionally mentioned, but I have been surprised by the tolerance of some 
nearshore corals in American Samoa to extreme ranges of DO.  

At one site, a diverse and healthy-looking assemblage of 52 coral species 
tolerated DO's ranging from 15 to 233% saturation, with negligible bleaching 
(about 1%).  (The DO instrument was still in calibration after these 
measurements were taken and the data were similar to those of a second 

The study site is a large and 'pristine' backreef moat on a fringing reef.  At 
low tide, the 1-meter deep moat is isolated from ocean flushing, thus it is 
subject to wide daily fluctuations in temperature, brief exposures (hours) to 
very high temperatures, extreme fluctuations in DO saturation (15% at night, 
233% daytime), and changes in pH (7.9-8.5).  During the daytime, steady streams 
of oxygen bubbles float to the surface (so some supersaturation seems 
reasonable) although there is little macroalgae present.

I am aware of only a little literature on naturally occurring DO levels in 
tropical reefs (eg, Kinsey & Kinsey 1977, Tytler & Davis 1984), or its 
relationship to bleaching or effects on coral reef organisms, so comments would 
be welcome.

Peter Craig
National Park of American Samoa 

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