nutrient deficiency and bleaching -and- Perhaps you need to do a bit more reading ...

Debbie MacKenzie debimack at
Sat May 12 12:06:51 EDT 2001

Hi Christine, Eric, coral-list,

Christine, you wrote:

>I have some thoughts
>re your question whether bleaching could be caused by nutrient depletion,
>however, I am afraid they take Ove's side.

I would just like to clarify exactly what it is that Ove and I seem to
disagree on. It certainly is not the obvious fact that the majority of
coral bleaching events are "thermally induced," or at least are strongly
associated with times of warmer water. Regarding whether thermally induced
bleaching "could be caused by nutrient depletion" - my impression was that
Ove agreed with me that it could, he just thinks it's unlikely. He
certainly did not claim to have disproved it. So if we disagree, I'm
thinking it's basically on the importance of this particular idea, and
whether or not it warrants investigation. Considering the shocking extent
of the coral bleaching problem and the dire predictions that are being made
for their future by scientists like Ove, I think that any possibility,
however remote, ought to be fully investigated.


>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?

It's my impression that "thermal stress" is apt to be higher nearshore. But
you are right that those kinds of pictures are complicated by the effects
of multiple stressors, heat, pollution and fishing, so it's very difficult,
maybe impossible, to pinpoint the exact effects of each. That's why I think
that their relative impacts will be best sorted out in areas not receiving
terrestrial runoff. Especially if one wants to isolate the effects of
fishing/biomass removal alone on the health of corals.

But before you investigate the effects of too-low "nutrient" levels on
corals, I think you need to re-examine the meaning of the word "nutrient."
I discussed this in a fair amount of detail in my (admittedly too long)
essay ( ). In the oligotrophic
waters that are normally found on coral reefs, the absolute level of
dissolved nutrients found there only represents the limit of the efficiency
of the organisms in removing them from the water. The nutrient recycling
patterns on the reefs (and elsewhere in aquatic systems, although to
varying degrees) conserve the nutrients in solid form and many circular
routes can be completed without the individual nutrients passing through
the "dissolved" stage. Fishing removals can therefore result in the effects
of "nutrient" depletion being felt despite apparently unchanging absolute
levels of nutrients in dissolved form.

>From "Life and Death of Coral Reefs" Birkeland (ed), 1997, a snapshot of
the (underrated in my opinion) "downside" of the food web:

"Fish feces have been observed to be fed upon by corals (McCloskey and
Chesher, 1971) and Tovertson (1982) deduced that some fecal material from
fishes may be eaten and recycled through five fishes before it reaches the
seafloor to be consumed by corals or other invertebrates." (p 416)

....and "corals or other invertebrates" are consumed by reef fish, some
portion of "nutrients" therefore coming full circle without passing through
the "liquid" phase. So, your measurements and thinking on "nutrient levels"
needs to be expanded somehow to reflect the presence or absence of FISH, IMO.

>From the same source, p 415, 

"On coral reefs...the movements of fishes may cause enough movement of
nutrients in coral-reef ecosystems to influence the growth of corals (Meyer
et al 1983), and overfishing can have large-scale ecosystem-level effects."

Unfortunately, however, chap 10, "Effects of Reef Fishes on Corals and
Algae" notes that "the role of fish feces fertilizing the reef" represents
a "potentially important interaction between fishes and reefs" but the
author omits it from the discussion. Beyond fish feces, ammonia excreted
from the gills of fish is available for uptake and use by corals.

One person commented to me off-list: "Ove can tell you that following
bleaching a good blast of N & P will help stimulate recovery."   A "blast
of N & P" helps corals recover? No surprise...but might that "blast" have
been given naturally when standing stocks of reef fish were higher? And the
starved state of the bleached corals is not unexpected since we know that
they lost their main food-providers when the zooks left -- but it would be
very interesting to see whether or not a "blast of N & P" given
prophylactically might help. Could the susceptibility to bleaching be
lessened in this way? Maybe when the Hotspot program indicates that
bleaching risk is rising, experimental "blasts" of N & P could be tried
here and there to see if the availability of these nutrients might prevent
the expulsion of the zooks in the first place.

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
>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?

>Finally, the web page sort of reads in a sensationalist manner, in my 
>opinion, that I don't think adds to its credibility.  

I realize that, it's because "you can't please all of the people all of the
time." I have been trying to discuss this issue with scientists, but at the
same time I try to write so that my fisherman-neighbours and the general
public just might get interested and be able to plough through an article.
Sorry, but most of your scientific literature is essentially unavailable to
them, they just can't read it. 

And Ove chose to rename this discussion "Perhaps you need to do a bit more

That's OK, of course I will, but I'll never know half of what this group
knows about corals. But I would like to point out that I MAY have done "a
bit more reading" than many in this group on the finer points of other
marine-ecosystems-in-trouble. The declining abundance and stunted growth of
fish everywhere is very worrisome. In some places the declining
productivity is blamed on decreased top-to-bottom mixing patterns - yet we
have a large area in the Northwest Atlantic (Bay of Fundy, Georges Bank)
that is constantly mixed by TIDAL action - which has not changed - but the
"productivity" and growth of fish is way down. And the certainty that fish
were in trouble solely because of changes in water temperature - that's
appealing, but it's falling apart in a lot of instances. For example, a
decade ago in Atlantic Canada we had unusually cold water which caused our
cod to feed poorly and grow very slowly (all cod papers predicted that when
the water warmed up the fish would feed better and grow more quickly).
However, in recent years the water has warmed to a point above the
long-term mean...and growth of cod is still inexplicably dropping. And
declining growth has been noted in our deep water fish stocks, living down
on the "slope" where temperature variations are in hundredths of degrees
rather than tenths...and no-one tries to stretch it far enough to blame the
slow down of those fish (exploited and unexploited) on water temperature.
It's recognized as "biomass depletion" in that case - fishing resulting in
food shortage for marine life.

I'm concerned that the coral scientists as well will belatedly discover
that the problems are not solely driven by the effects of changing water
temperature - I'm convinced that there is an "environmental impact" of
fishing that's not been recognized, and it's a generalized food shortage.

What concerns me is not just coral bleaching or coral diseases, but the
larger diagnostic problem of a whole ocean in trouble. I see a theme, a
generalized slowing of feeding and growth, and suspect it's because of
fishing-induced biomass depletion. The possibility of fishing removals
adversely impacting the "base" of the food web is adamantly denied by
(most) scientists I've corresponded with regarding the northern
fish....they try to reassure me with measurements of chlorophyll
levels....but if the corals turn out to be "hungry" because of
fishing-induced biomass depletion, it really strengthens my argument. It's
rather a huge topic though, very hard to pull it all together in one piece
of work (and I've found that most scientists are very specialized,
struggling with only one piece of the overall puzzle - that adds to the
frustration). For a bit more detail on some of the things that I have read,
and emerging themes that I think I see, check out "The Marine Nutrient

Thanks for your interest,
Debbie MacKenzie

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