Nearshore bleaching photos - Fiji

[Ove Hoegh-Guldberg]

Dear James,

Interesting, useful photos.  Without knowing the precise oceanographic
conditions of the area at the time, the suggestions below will be speculations
at best.  But, however, here goes (my two cents worth):


Your question - Photo 1596 shows an Acropora sp. bleaching from the center out.
Why is this?

I would suspect that the secondary variables light and flow (perhaps trapped by
the morphology of this flat Acropora colony) have conspired to increased the
effect of the warmer than normal conditions.  We should also be mindful that
most of the growth occurs on the outer edges (that are less affected), and that
the number of zooxanthellae are lower there as well.    The latter might mean
less oxidative stress per host cell and hence less bleaching.


Photos 1594 and 1595 show a monospecific stand of Acropora.  I would expect
either the whole colony to bleach, or perhaps the tops of branches - however
the coral is only bleached in large "patches."  Why is this?

Two interpretations:  (1) One is that there are clonal (genetic) differences
either in the host or the zooxanthellae that create slightly different
temperature thresholds for bleaching stress (see paper by Pete Edmunds) OR (2)
slight variations in flow have interacted with the effects of temperature -
producing different patches.

Some of the photos show many different colonies in one patch reef.  Why is one
coral bleached, and it appears that an identical species next to it is
not (1599 and 1600).  Why did one coral bleach, and the other did not?

As above:  Either it is genetics or it is local secondary factors that vary
across the reef.  History - feeding, reproductive condition, interspecies
aggression etc) might also play a role in determining behaviour under thermal
stress.


Cheers,

Ove

Professor Ove Hoegh-Guldberg
Director, Centre for Marine Studies
University of Queensland
St Lucia, 4072, QLD

Phone:  +61 07 3365 4333
Fax:       +61 07 3365 4755
Email:    oveh at uq.edu.au




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