Nearshore vs. offshore bleaching

Bruce Carlson carlson at
Wed May 16 15:20:34 EDT 2001

RE: Offshore vs nearshore bleaching

Bernard Thomassin disagreed with the general statement that bleaching is 
usually more severe nearshore.   I concur.

The other day, Gregor posted a message that "The pattern of bleaching 
follows a consistent trend globally that suggests that following bleaching 
events, reefs located in areas with less water column mixing are usually 
the worst affected. Typically these are inshore reefs where both nutrient 
flux and absolute levels of nutrients are likely higher than outer reefs."

I wrote to Gregor saying that my data indicate that both nearshore and 
offshore reefs in Fiji (south of Viti Levu) suffered significant bleaching 
last year.  When I revisited my transects this year I found that both areas 
suffered high mortality (>95% Acropora dead), but I also found significant 
regrowth ("phoenix" corals) and significant recruitment in the lagoons 
(Acropora spp.), whereas the remote offshore barrier reef showed virturally 
no survival and no new recruitment (the "virtually" means that whereas last 
year I counted on average over 100 acroporid colonies per 30 x 1 meter belt 
transect, this year I found only one tiny survivor on one transect and one 
tiny recruit on the other transect).  The nearshore patchreefs and barrier 
reef where recovery is good, are located near the mouth of a large river 
and the water in this lagoon area is typically turbid most of the 
time.  The remote barrier reef is typically in a pristine ocean 
environment, often crystal clear, and subjected to large open ocean swells 
much of the year.

Gregor wrote back to me and qualified his statement:

"What I was referring to was that in "moderate" bleaching events, when 
there was less than say 50% mortality, the inner reefs typically showed 
higher mortality than the outer reefs due to mixing at the outer reefs..... 
When it is a" bad" event there is little difference as you have seen in 
mortality.  But your observations of faster recovery on the inner reefs are 
interesting and one wonders if this has more to do with a higher number of 
nearby parent colonies, water retention in those areas which facilitated 
recruitment, rather than with the nutrients being a beneficial stimulus. 
But all these are open questions deserving of more research."

So what is the "global trend"?  Do nearshore reefs or remote barrier reefs 
suffer higher mortality and lower recovery rates?  Or should we even try to 
make such a generalization with so many unique factors at each locality?


At 12:33 PM 5/16/2001 -0300, Bernard THOMASSIN wrote:
>To: Debbie MacKenzie <<debimack at>
>From: Bernard THOMASSIN <<thomassi at>
>Subject: Re: nutrient deficiency and bleaching -and- Perhaps you need =20
>to do a bit more reading ...
>Dear Debbie and all colleagues interested by coral bleaching,
>To the comment:
> > How come that bleaching is usually more severe nearshore, where
> > nutrients are enhanced to levels, which in turn can become detrimental
> > to many coral reef organisms, which are highly adapted to exist in
> > oligotrophic conditions? Could that maybe relate to some patchiness,
> > too: too much 'food' and maybe toxic substances?
>I don't agree with this opinion taking as example that occured around
>Mayotte Island in the North of the Mozambique Channel, SW Indian Ocean,
>where I studied since 1983 several bleachings of various intensity.
>Here the huge bleaching event of the April-June 1998 (when an warmer
>mass of oceanic seawater coming from the North reached this SW Indian
>Oc. area) -the bleaching was undubfully caused by the seawater
>temperature increase : T=B0 C reached up to 32=B0 C in ocean open sea and
>stayed as during near 3 months, it was the corals from the outer slopes
>of the barrier reefs (187 km long) that bleached  and then died, mainly
>in the shallow depths (3m down to 15-20m  - but encrusting corals at
>down 30m also bleached -) : all the tabular and branched Acroporids,
>all the Pocillopora, some Diploastrea, some massive Porites (but on
>some of them parts were kept alive, if most of the colonies died). Even
>Sarcophyton and Sinularia bleached, as well as the large sea-anemones
>as Heterotactis magnifica, and some Tridacnids. So, consequently, the
>barrier reef slope coral communities were destroyed at more than 85
>On the slopes of lagoonal reefs, as well as on the slope of the
>fringing reefs, also the bleaching occured, but on the fringing reefs
>in muddy environments of deep coastal bays, most of the corals
>My opinion (exposed in one of our Bali's Conf. posters) is that corals
>living in clear oceanic waters on the barrier reef slopes or lagoonal
>reef slopes near large passages, live in oceanic seawaters showing more
>constant parameters (according to the seawater temperature they are
>more "stenothermes"). In contrary, corals living in nearshore
>environments where seawater parameters are more variable (increase of
>temperature due to closed environments, or decrease of temperature due
>to cool groundwater seepages ; salinity variations due to rainfalls and
>river flows ; variation of the turbidity due to alluvial inputs
>associated with rainfalls or to phytoplankton blooms ; bacterial
>attacks from terrigeneous materials ; etc...).=20
>In fact coastal populations of corals (for the same species) are more
>resistant to all the possible stresses that coral populations living in
>more stable and constant seawater conditions.
>In this conditions I disagree with your opinion.
>But be very carefull with the biology/physiology of corals. I begin to
>believe that the same species of corals have not the same biology (and
>physiology) in region located fare away. So extrapolations of results
>from one area to another one are not possible. This is true for the
>biologists and ecologists, but also for our paleo-geologist
>This can explain the opposite views between different researchers!
>Have a good day.
>Sincerely yours.
>CNRS-UMR 6540 "Dimar",
>Centre d'Oc=E9anologie de Marseille,
>Station marine d'Endoume,
>Chemin de la batterie des Lions,
>13007 Marseille,
>(33) 04 91 04 16 17 (ligne directe)
>(33) 04 91 04 16 00 (standart)
>mobile (33) 06 63 14 91 78
>fax  (33) 04 91 04 16 35 (=E0 l'attention de....)
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