[Coral-List] Algae and nutrients and herbivory in oligotrophicwaters

[Douglas Fenner]

      It seems to me that the simple fact that caging experiments which
exclude herbivores produce heavy macroalgae growths within cages, while
there are much less algae outside the cage, in the same water and nutrient
conditions, demonstrates that herbivores can have strong effects on
macroalgae.  (Check out Bellwood, Hughes, & Hoey, 2006 for pictures and a 
surprising herbivore)  I don't think you have to measure nutrient levels 
accurately
for these experiments to show that herbivores have strong effects on
macroalgae.
     That said, the real world is not simple, and the fact that herbivores
have strong effects on algae doesn't mean that nutrients don't also.
I find the observation that there is tons of algae around Male in the
Maldives where the people don't eat reef fish and there are plenty of
herbivorous fish, but the city releases tons of untreated sewage, is a
convincing demonstation of the effects of nutrients.  That's at a huge
nutrient input level, and things are likely to be very different at lower
nutrient levels, like the ENCORE experiment on the GBR.  Surely there are
plenty of studies showing nutrients affect algae.
    If you have a corn or rice or wheat crop, surely the size of your crop 
depends
not only on nutrient levels in the soil, but also rainfall, temperatures,
insects, sunshine, and so on.  Saying one has an effect on the crop doesn't
mean the others don't have effects too.
    -Douglas Fenner

Bellwood, D. R., T. P. Hughes, and A. S. Hoey.  2006.  Sleeping functional 
group drives coral-reef recovery.  Current Biology 16: 2434-2439.

Speaking of the Great Barrier Reef, Robbins et al wrote,
"Our data suggest that for coral-reef sharks, immediate and substantial
reductions in shark fishing will be required for their ongoing collapse to
be reversed."
"Together, these findings indicate that extirpation of these species from
fished coral-reef ecosystems is an immanent likelihood in the absence of
substantial changes to coral-reef management."
"Inferred and projected declines such as ours appear sufficient to warrant
"Critically Endangered" status under the IUCN Red List (A3d) criteria for
this study area for both species."
Robbins, W. D., Hisano, M., Connolly, S. R., and Choat, J. H.  2006.
Ongoing collapse of coral-reef shark populations.  Current Biology 16:
2314-2319.


----- Original Message ----- 
From: "Thomas Goreau" <goreau at bestweb.net>
To: "Imam Bachtiar" <ibachtiar at telkom.net>
Cc: "coral-list coral-list" <coral-list at coral.aoml.noaa.gov>
Sent: Wednesday, June 18, 2008 6:06 AM
Subject: [Coral-List] Algae and nutrients and herbivory in
oligotrophicwaters


> Dear Imam,
>
> Very few of the studies on herbivory and algae are any good, because
> only a tiny handful have made accurate nutrient measurements. When
> this is done properly it is clear that nutrients drive algae
> productivity and herbivory is only a secondary factor, because
> grazers prefer to eat some species and avoid others. I've looked at
> the zonation of algae species in reefs all around the world and these
> are sharply zoned by nutrients in ways that clearly reveal the
> sources. Every place I've looked with nutrient sources are dominated
> by algae whether or not herbivores are present (and herbivore always
> dominate the fish where there are nutrient sources, whether these are
> from human land-based sources or from upwelling), and algae are
> absent or rare where there are no nutrient sources, again whether or
> not there are herbivores (which there rarely are).  This is the exact
> opposite of what top-downers predict.
>
> As far as coralivores go, I also think the popular conceptions are
> often erroneous. Most alleged coral eating is much less than is
> claimed. Butterflyfish mostly just suck off surface mucus and there
> is little or no physical damage to polyps. Parrotfish that are
> allegedly biting coral are almost inevitably biting algae growing on
> dead coral, and avoiding the coral tissue, as you can see if you look
> closely where they bite. The popular claims that parrotfish eat coral
> is probably largely untrue, and much of their biting of corals is
> really territorial marking the boundaries of breeding territories
> rather than for food. Almost every time you look where a bumphead
> parrotfish or Napoleon wrasse has bitten into large bubble  coral
> colonies, you will see the tubes of the burrowing clams and worms
> inside the coral that they were really eating rather than coral tissue.
>
> I'll talk a bit about this at the next Gili Trawangan workshop in
> early December. Look forward to continuing to work with you there and
> other sites around Lombok. My best to all my friends in Bogor.
>
> Best wishes,
> Tom
>
> Thomas J. Goreau, PhD
> President
> Global Coral Reef Alliance
> 37 Pleasant Street, Cambridge MA 02139
> 617-864-4226
> goreau at bestweb.net
> http://www.globalcoral.org
>