nutrient deficiency and bleaching
Alina M. Szmant
szmanta at uncwil.edu
Sat May 12 16:11:11 EDT 2001
Hello Debbie and others:
I have been following with interest the debate about whether bleaching in
reef corals could be at least partially due to coral starvation that could
result from ovrefishing. I guess I'd like to add my 2 cents to the
I've had a chance to scan thru your web papers, and I want to add my voice
to those of others concerned about the VERY serious effect overfishing is
having on coral reef health. This has become my "band-wagon" as of late
(as some of you know), and is also expressed in my Panama Symp paper
(Szmant, A.M. 1997. Nutrient effects on coral reefs: the importance of
topographic and trophic complexity on nutrient dynamics. Proc. 8th
Internat. Coral Reef Symp., Panama, June 1996. Vol. 2: 1527-1532), in
which in fact I express concern for the effects of overfshing on coral reef
trophic dynamics and nutrient cycling. There is no doubt that fish
excretion and defecation is important to nutrient cycling on coral reefs
(Szmant-Froelich, A. 1983. Functional aspects of nutrient cycling on coral
reefs. In: The Ecology of Deep and Shallow Coral Reefs. Symp. Ser.
Undersea Res. NOAA Undersea Res. Prog., Vol. 1: 133-139.) just as similar
activities by larger vertebrates are important to trophic dynamics and
nutrient cycling in terrestrial systems.
However, I do not think that it is a factor in coral bleaching. 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.
And while I do not have any data to support or refute that coral nutrition
is or is not affected by overfishing, I do think that the nutrient-related
effects of overfishing are happening more at other levels (effects on algal
dynamics, lack of herbivory, population explosions of corallivores, etc).
Further, in comparing tissue biochemical characteristics (C, N, C/N ratios
etc) of corals from reefs with more and fewer fishes (e.g. Glovers Reef and
Bahamas back in the '80s before bleaching was such a problem) we did not
find any difference that correlated with fish communities (we were looking
for differences associated with nutrient environment).
But I also agree with your statements about the important of "solid"
nutrients (see Szmant 1997), and have played around with corals eating fish
feces (they do if they are from planktivores but not herbivores). But
Florida corals have bleached and died just as much as Caribbean corals that
are much more overfished, and Florida corals have a high tissue N content
compared to some other areas where fish are more depleted. I do not think
that Florida reefs are anthropogenically nutrified (Szmant, A.M. and A.
Forrester. 1996. Water column and sediment nitrogen and phosphorus
distribution patterns in the Florida Keys, and potential relationships to
past and present coral reef development. Coral Reefs. 15: 21-41.) but but
there is plenty of particulate matter in the water there has been no
consistent difference between the bleaching of corals offshore (lower
nutrients, more fishes) and closer to shore (more nutrients and
particulates, fewer fishes).
And my best set of evidence for disbelieving the fish
presence-nutrition-bleaching hypothesis you propose is that some of the
most bleached corals in the Florida Keys during the 1998 bleaching event
were on a nearshore patch reef where the snappers and grunts literally
formed a think layer over the corals all day long (little other shelter
around) and the corals (other than bleached) appeared to be very healthy.
This patch reef had an incredible amount of algae on any non-coral
substrate wherever the fishes hung out (few herbivores on this reef), which
I attributed to the fish excretion. The corals recovered very well from
the bleaching as soon as the water temperatures started to drop in late
summer. Thus, high fish abundance and fish excretion and defecation did
not prevent these corals from suffering from bleaching when temperatures
exceeded 30 oC, but who knows, they may have helped the corals recover
Thank you for initiating an interesting debate. Over-fishing is a terrible
thing for coral reefs, and in fact, we don't know all the ways its effects
can cascade down thru the reef community. I do not doubt that coral
nutrition is affected at some level, but in my opinion not enough to be a
contributor to bleaching.
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