Nutrient deficiency and bleaching

Debbie MacKenzie debimack at
Mon May 14 22:43:10 EDT 2001

Hi Coral-list,

Thanks very much for your feedback. You are convincing me that, in all
likelihood, no simple correlation exists between standing stock of reef
fish and susceptibility of corals to bleaching. (Although it's possible
that the presence of fish might aid recovery, according to Alina's example
- but, maybe, maybe not?)

Tim wrote:
>One interesting observation of the 98 bleaching in western Indian Ocean was
>that the marine parks, which have a high biomass of fish, experienced the
>highest mortality through bleaching (McClanahan et al. 2001).  This is
>attributable to the fact that Acropora is more abundant in the parks where as
>it seems to be damaged by nets, coral collection and other things outside of
>the park.  Also the the very pristine and unfished Maldives and Chagos
>experienced terrible bleaching in 98 (Goreau et al. 2000, McClanahan 2000,
>Sheppard 1999).   So, these findings would support Alina's contention that
>fishing and bleaching effects are not influenced by fish abundance through
>nutrient interactions,

Ove wrote:
>Both fished and non-fished areas bleached extensive in 1997-98.  One Tree
>Island (a total preserve - not ever fished) bleached a greater levels than
>non-fished areas.

Mark wrote:
>I studied the reef fish populations in the southern Seychelles during the
bleaching in 1998, and have just returned from the Chagos Archipelago. In
both places (representing over 2% of the world's coral reefs) 80-90% of the
coral died from the bleaching. These also represent reefs with remarkably
low levels of fishing (or other environmental impacts), they had, and
actually still have, abundant and diverse reef fish populations. This would
appear to counter any corellation between nutrient mining and the impacts
of the bleaching. While you might choose to link it to a "wider ocean in
trouble" scenario, Chagos in particular is thought to have existed in
considerable isolation and with highly oligotrophic waters all around over
considerable timescales.

80-90% mortality!? - That must be very depressing to witness.

And of course severe bleaching also occurred in areas where standing stocks
of reef fish were very low. So, I guess that's not it. 

"While you might choose to link it to a 'wider ocean in trouble' scenario.."

Well, that actually is my concern, since the "wider ocean" does appear to
be in trouble by all indications. Steep drops in zooplankton levels have
been documented in some open ocean areas. One Pacific study recorded a 70%
drop since 1950. In the Bering Sea, a study on the baleen of
(plankton-feeding) bowhead whales indicates that they have been
experiencing a steadily decreasing rate of feeding success over the last 40
years. In the Northwest Atlantic, records also indicate a significant
decline in zooplankton over the last 40 years. For some reason there seems
to be less krill in the waters of Antarctica, and minke whales are getting
thinner. This leaves me wondering about the zooplankton levels in the
tropical waters - do you have any time-series data on the abundance of those?

What is the cause of these declines in other areas of the world ocean? If
it's "nutrient mining"/fishing, or some other environmental factor...might
it not be affecting the tropical ocean as well? After all, the currents in
the "wider ocean" range pretty far...

"If" the tropical open ocean zooplankton levels are also significantly less
than they once were, what effect would you expect that to have on the coral
reefs? I've read repeatedly that corals consume zooplankton washed over the
reef by ocean water (in addition to consuming the resident type), and that
a certain amount of dissolved nutrient is unavoidably lost by the reef to
the ocean water as well. What is the actual magnitude of these sorts of

The impression I've gotten from what I've read is that the zooplankton
input from the open ocean represents a net nutrient gain for the coral
reef. But that would depend on the abundance of the zooplankton - obviously
it could potentially drop below a point where the contribution of nutrients
was outweighed by the dissolved nutrients that are washed off the reef.
(The latter would not change since the same volume of water could be
expected to pass over - so if zooplankton drop below a certain threshold
the balance will be tipped, and being bathed in open ocean water, normally
a benefit to the corals, could turn into a liability.) 

Alina expressed the opinion that "overfishing" is having a very serious
effect on coral reef health. She recognizes the importance of fish in
nutrient recycling - yet she's not getting the impression that fishing as
such on an individual reef, is having the kind of direct detrimental
nutrient impact that I suspected. There's got to be a good reason for her
conclusion. It seems that the consensus here is that coral bleaching is
being induced by something "environmental," a feature of the seawater
itself - and temperature appears to be blatantly obvious as the #1
enviro-trigger. But is there another one?

The lack of "fishing effect on coral nutrition" on the local scale of a
given reef -- really should not surprise me, it's reminiscent of the lack
of "fishing effect" elsewhere on individual fish stocks. Downturns in
fisheries everywhere are frequently giving the impression that they are
being caused by environmental factors - factors that invariably cause
decreased growth, but also there are dramatic species shifts - and changing
temperatures alone cannot possibly explain all these. (Nor can pollution,
nutrient or chemical.) The changes give the impression that they are being
driven by something affecting the whole ecosystem - my question is "could
that 'something' be overall biomass depletion?" Could that be a systemic
effect of fishing?

I have another question regarding the bleached corals -- Why do you suppose
that the faster growing species are the most susceptible to the bleaching?
(Of course, my simplistic view is that faster growing ones need more food
to be available, therefore they feel the pinch first. But maybe there's
something else about their metabolism that makes them more susceptible to
higher temperatures. Have you investigated this question?)

Now I'm repeating myself, but a couple of days ago I wrote:

>Eric wrote:
>>The patchiness of bleaching was discussed on the list a while back, and 
>>stagnant areas due to flow dynamics even around a coral  colony can
result in 
>>local conditions that exacerbate bleaching.  
>That sounds reasonable, but which feature of "local conditions" is most
affected by stagnant flow, "nutrient" levels or water temperature? My hunch
is that still water would be more prone to becoming extremely
nutrient-depleted rather than extremely warm, but I DO NOT KNOW! Do you?

I'd like to read your discussion of the reasons for the patchiness of
bleaching, but couldn't locate it in the list archives. When was it, and
are the messages sorted by thread anywhere? It seems to me that there has
to be a very important clue in the details of the "patchy" pattern, and
also in the order in which corals fall ill during the course of a bleaching
event. What is it that allows some coral colonies to resist the effects of
warmer water longer than others?

Alina wrote:

>I have
>done experiments in which I starved corals in the lab for weeks and months
>and did not observe any change in zooxanthellae density.  I did however
>induce severe bleaching of corals in the lab of corals maintained for only
>2-3 weeks at 30 oC while controls at 28 oC were fine.  

Did you starve them till the point of death? I've read that food starvation
does cause loss of zooxanthellae in similar manner to what is observed in
mass bleaching events (as do several other stressors, of which you all know
more than I only point being that simple food starvation is one
thing that "could" cause the observed phenomenon, including reduced thermal
threshold.) And if you starve them in an aquarium, do you change the water,
to simulate the washing away of nutrients that occurs on the reefs? If not,
maybe they'll last a lot longer in your experiment than they will in the

And the bleaching that occurred at 30 C after 2-3 weeks, did you get the
impression that they had run out of fuel? Or what? - what changed? How
could they tolerate the elevated temperature for a few weeks, if it's a
temperature that they cannot stand? What allowed them to survive it for as
long as they did? Technically, I doubt if it could be called "heatstroke,"
which would normally cause organisms to sicken in hours rather than weeks.
Have you tried varying the amount of food available to the corals
undergoing these temperature trials?

I realize that if it helped at all, food could only do so much to raise the
thermal threshold, these organisms obviously have a temperature ceiling
that's getting them in trouble (...but I'm still bugged by the question of
whether or not they're feeling the added effects of a nutritional threshold
as any of you get the impression that their heat tolerance must
have been higher in the past?) And I asked Ove the other day if he'd
considered transplanting some corals on the Great Barrier Reef to slightly
higher latitudes where today's temperatures might match their normals of a
couple of decades ago - I think that could be an interesting test also, it
might help reveal what, if any, other secondary environmental factors are
at play.

Debbie MacKenzie 

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