Coloured Pigments and Coral Bleaching?

[Alex Mustard]

To Prasanna & rest of the list,

I have observed similar pastel pink and blue coloured corals both in the
Maldives in May 1998 (at the onset of the severe bleaching event) and in
Thailand in May 2002.

Follow this link for a recent image from Thailand:
http://www.soc.soton.ac.uk/GDD/hydro/atmu/ecology/chapter7/bleach.jpg

To my mind, these pastel colours are not (only) fluorescent because it is
possible to photograph them with a standard underwater flash, which usually
overpowers coral fluorescence.

I have been told (by two independent people working on bleaching) that these
colourful pigments are actually different strains of zooxanthellae that are
more resistant to bleaching. These pastel colours seem to be a transition
stage in bleaching?

For those who are interested, I observed the recent bleaching event in
Thailand while on vacation in the Similan Islands. While corals were paler
than "normal" for at least 10 days prior (start of my holiday), I observed
very dramatic changes in colour on the 5th & 6th May. On those 2 days, I
spent about 7 hours in the water and subjectively felt I was seeing
noticeable changes in colony colour on a time scale of hours. The most
extensive bleaching was at depths between 3-6m on the reef flat, and 5-8m on
the fore reef. Acropora and Pocillopora were most bleached ~70% of colonies,
others much less ~20%. Acropora, Pocillopora, Fungia and Porites all showed
the "pastel colours". Bleaching intensified right up until my last dive of
my trip (06 May 2002).

These observations are backed up by photographs, which I am happy to share
on request, but no data (holiday!). Dive computers indicated water
temperatures between 30-32C above a thermocline at 15-25m (27-28C below).
Comparable temps to those reported by Brown et al. (2000) Nature. 404:
142-143.

Regards

Alex

> One thing that I found very interesting at the onset of the 1998 bleaching
> monitoring in Sri Lanka was that some corals took on a brilliant
> ultramarine
> blue for a couple of days before bleaching into blue. this was mostly
> observed on colonies of Acropora formosa and couple of Montipora species.
> I'm talking about whole colonies (2-3meters across) going deep blue. I
> sure
> would like to have an explanation for this if there is one.
>
> An additional note that there was a mild bleaching event that we were
> observing on several south coast reefs in Sri Lanka during April. The
> corals
> had become significantly paler overall, (60-70% of colonies) but none had
> bleached into pure white except for development of completely bleached
> spots
> ranging from 5-15 cm across on some colonies. I have not been able to
> monitor for the last month but the indications were that things would be
> easing up with the arrival of the Indian Ocean monsoon.
>
> Prasanna

--

Dr Alexander Mustard               T +44 (0)23 80 59 60 16
George Deacon Division             F +44 (0)23 80 59 62 47
Southampton Oceanography Centre    M +44 (0)78 76 52 31 10
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