Coral starving and survival

Debbie MacKenzie debimack at
Tue Mar 26 19:18:43 EST 2002

Dear coral list,

The question of whether or not corals suffered heat-induced mortality
during the Medieval Warm Period is an interesting one, and is one line of
investigation that might shed some light on the recent coral mortality. It
might help to validate or weaken the conclusion that today's bleaching
events are purely cases of exceeding thermal thresholds characteristic of
these species. But I don't think it is necessary to pursue the MWP angle to
investigate whether or not elevated temperature is the only stressor
involved in inducing today's mass bleaching events.

Surely appropriate experiments could easily be devised to test the
hypothesis that corals have varying amounts of food available, and that the
pattern of food availability might be correlated to their ability to
withstand transient episodes of thermal stress. You have several methods of
measuring "condition," reflecting adequacy of food input, in corals - I
can't imagine that it would be exceedingly complicated or expensive, but I
don't know much about that side of your work...

I agree with Pedro that it doesn't matter much exactly "what" the corals
feed on - zooplankton, bacteria, particulate organic material - the bottom
line is the nutritional status/reserves and resiliency that they might
obtain. (If you looked at my barnacle article, which shows a declining
trend in a sessile plankton feeder in Atlantic Canada, you'll see that
barnacles here appear to be relatively very well fed by the organic
material in a heavily sewage-polluted harbour - they are benefitting from a
lot of food that is not-quite-zooplankton - but this is clearly allowing
them to withstand "whatever" it is that is causing the disappearance of the
barnacles on the open coast. (
) - I know, it's not coral, but I think there are interesting similarities.)

How have you tried to disprove the hypothesis that thermal stress is the
primary cause of mass bleaching?

It occurred to me, and I asked Ove Hoegh-Guldberg about this last year on
this list...that an interesting test might be transplanting corals between
the various regions of the Great Barrier Reef. Now, maybe I've got this all
wrong, but I've gotten the impression that there are species of coral whose
range spans the whole GBR (right or wrong?). If so, individuals of a given
species living in the different regions of the reef would seem to have
differing thermal thresholds - according to Ove's 1999 paper "Climate
Change, Coral Bleaching and the Future of the World's Coral Reefs," the
southern region temperature has averaged approx. 26C for the last century
and the bleaching threshold there is 28.3C, while the central region has a
long term average approx. 27C and a bleaching threshold of 29.2C, and the
Northern region has averaged about 28C and has a bleaching threshold of
30C. The recent rising trend in water temperatures has raised each region
approximately 1 degree C on average and in recent warm years the seasonal
highs have been hitting the bleaching thresholds in all three
regions.  Everything has recently been ratcheted up by one degree, and the
long term norms in the three regions vary by about one degree. So...the
average temperature now in the central region is about 28C, which has been
the long term average for the Northern region, and the average temperature
in the southern region is approximately the same as the long term average
that was well tolerated by corals living in the central region (27C).
Therefore moving corals from the Northern and central regions to the
regions further south should place them within the temperature ranges that
they've successfully tolerated for at least a century...and the transplants
should therefore demonstrate superior tolerance to heat stress and
bleaching the next time that the "hotspots" hit the areas - that is, if the
whole pathology is the simple result of an environment that exceeds their
natural thermal tolerances. Does this make sense? Why or why not? I've read
about other experiments that involved coral transplantation...

Another question that comes to mind is, since many global warming scenarios
predict shifts of species to higher latitudes (and some other marine
species appear to be doing just that), why are the most southerly corals on
the GBR also vulnerable? Have they not been living at the southerly limit
of where these species can survive, the range limit based most likely on
their lower thermal tolerance level? And should not at least some of them
therefore be looking better, or expanding southward, since conditions have
gotten warmer?


Debbie MacKenzie

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