Bleaching & dissolved oxygen
Peter Craig
Peter_Craig at nps.gov
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
instrument.)
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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