Nearshore vs. offshore bleaching

Debbie MacKenzie debimack at
Thu May 17 12:03:45 EDT 2001

Dear Coral-list,

Here's what I'm thinking now: To be healthy corals need access to their
normal supply of zooplankton from open ocean water. Without it, they'll
live for a while, but their resistance to an array of
malnutrition-aggravated conditions drops. Corals can derive limited benefit
from land-source nutrient input and fish poop, but all will feel the ill
effects of a dropping supply of oceanic zooplankton...."IF" this is now a
fact of their existence.

This may help explain why corals have done so badly in some of the pristine
open ocean locations. In addition to missing the open ocean zooplankton,
they are deprived of any benefit they might have derived from land source
nutrients, so their position becomes doubly precarious (and possibly worse
yet again if the fish poop feeding option has also been removed...although
Alina made an interesting observation that corals prefer to consume the
feces of planktivores vs. herbivores...plankton clearly still being on the
the "keys.")

If you will permit me to compare codfish and corals briefly, I'd like to
point out that the "better inshore/worse offshore" trend in nutritional
success is predicted by the "overall marine biomass depletion" theory, and
is very well demonstrated in the condition of Canadian groundfish stocks
and other marine animals. It is a common theme that I've noted repeatedly
in the "starving ocean," and it pops up quite often on my website.
Basically, natural processes in the sea do not work quickly enough to
counteract the nutrient loss incurred by fishing, so input sources from
outside the system become increasingly important. Nutrients in terrestrial
runoff provide only low-grade nourishment for the coastal system, but this
does confer a survival advantage (when it does not kill). The other coastal
nutritional advantage is in the productivity that occurs in the intertidal
zone - might that help explain the slight advantage enjoyed by the corals
inside the lagoons? - and, doesn't the water actually get hotter in the
lagoons? I guess, I don't know.)

Your detailed observations of the coral bleaching pattern are very
interesting. It seems to me that they might be reasonably consistent with
my interpretation - what do you think?

Bernard concluded that:

>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.
(The same is clearly true for our groundfish stocks, their resiliency to
various stressors, i.e. fishing pressure and climate change, is much higher
when they are located nearer to outflows of major rivers or mainland
shorelines. So is their growth rates, this is well documented...the
connection to enhanced feeding opportunities is rather hard to deny.)

Bruce wrote:

>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.

As I mentioned a few days ago, lacking the normal degree of zooplankton
contribution, "large ocean swells" would become a nutritional liability to
corals, as dissolved nutrients would continue to be washed away. Might the
water have become "too clean?"

John wrote:

>During the recent
>severe bleaching event in Palau, we noted that corals in the lagoon close to
>the main island of Babeldaob were basically not impacted, while much of the
>coral (particularly Acropora) on the barrier reef was hammered.  Could
this be
>attributed to the possible lowering of nearshore water temps from runoff?

Does runoff have the effect of lowering the water temperature? I don't
know, but it does have the unarguable effect of raising the level of
dissolved and particulate "nutrients" in the water - no?

Bob wrote:

>"There are consistent habitat differences in bleaching resistance at a given
>locale; corals in habitats that are more variable or more prone to
stresses of
>various sorts, including thermal (such as shallow water or fringing
reefs), tend
>to be less bleached than those in more normally equable environments."

...which sounds like the shallows are warmer. Thermal stress higher, yet
resistance to bleaching higher also??

Peter wrote:

>My work was conducted in the Iwayama Bay,
>> secluded lagoon waters nearshore Koror.  I noticed that the large beds of
>> Anacropora were not impacted nearly as much if they were situated along a
>> channel (where runoff enters).  

Jacques wrote:

>Like Bernard Thomassin and others I found  that bleaching had been
>more severe on the outer ocean facing subvertical slope of the reef (about
>100% from surface down to about 35 metres deep) than in the lagoon. In fact
>the less affected zone we observed was a wide patch reef  between 3 and 15
>m deep in the shallower part of the lagoon with "only" 50 to 60%
>destruction. This was the only place on that atoll where Mussids and some
>Faviids were still alive.
>In fact the place were young colonies were more abundant were small reefs
>near the city-island of Malé, and subject to pollution and man
>disturbance...In Gaidoo, however,  all branching species had beeen wiped

For corals, the best survival odds were nearer the source of pollution? And
the worse were facing the open ocean? As I have repeatedly pointed out (to
others) regarding the groundfish picture, a decreasing survival gradient as
one moves away from the shoreline seems to reflect the disadvantage
associated with decreasing availability of coastal-source nutrients much
better than a temperature gradient.

I'm also interested in the reason for the heightened susceptibility of the
branching corals (they have also been heavily impacted by infectious
diseases as well as bleaching, have they not?)

Gregor wrote:

>reason that the Acropora go first is simply that the water usually heats from
>shallow to deep, thus it is the common shallow water corals that get
nailed first
>-- such as Acropora.

Does the water usually heat from shallow to deep? The temperature
connection can get confusing. 

Does Acropora typically live in the warmest spots? Or is their
vulnerability related to the possibility that they naturally rely more
heavily on the zooplankton nutrition provided by the seawater? Looking at
pictures of Acropora, the branching design seems to be meant to maximize
the feeding benefit derived from the passing seawater, since contact with
the water seems to be maximized. If the passing water didn't contribute the
usual amount of zooplankton, yet washed away the usual amount of nutrients,
the fast-growing, branching coral might find itself in the worst trouble.
Their strategy of maximizing exposure to the passing water, one that
previously allowed their dominance - has it lately backfired? Have you
considered the problem from this angle?

So, bottom line, last question: where are the time-series data on tropical
ocean zooplankton levels?

Debbie MacKenzie

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