[Coral-List] top-down, bottom-up arguments
Paul Hoetjes
phoetjes at cura.net
Mon Oct 9 21:17:19 EDT 2006
Fascinating. Everyone seems to agree that both nutrient loading and
grazing are important factors in the macroalgae coral/ ratio (and
let's not forget about coralline algae) on a reef. There is a diagram
in: Lapointe, B.E. Nutrient thresholds for Bottom-up control of
macroalgal blooms on coral reefs in Jamaica and Southeast Florida.
Limnol. & Oceanogr., Vol 42, No.5 part 2: The ecology and oceanography
of Harmful algal blooms (Jul 1997), 1119-1131, which nicely sets out
these interlinked factors. What are we really discussing here? Whether
grazers are just this little bit more important than nutrients? What
does it matter? Overfishing is bad, nutrient loading is bad, who cares
whether one may have effect a little sooner than the other or even
sooner if they are combined. The point is both should be avoided as
the plague by anyone who wants to protect the coral reefs, and neither
should be played down or trivialized. Doing so will play into the
hands of those who would like to overfish or to load nutrients into
the ocean because it will make or save them lots of money.
Are Diadema important for a reef? Of course they are. How could a
species whose total biomass may well have been greater than the
combined biomass of all the present day grazers put together (speaking
from Curacao experience, I don't know if such densities were normal
elsewhere), NOT be a major factor in an ecosystem. Was their
disappearance bad for the reef? Presumably. Will they be able to
counteract anthropogenic nutrient enrichment of reefs? How can they?
They don't ascend to heaven, they remain on the reef, as do their
waste products. So the nutrients will keep building up until
eventually the reef dies anyway.
Is nutrient loading really bad for coral reefs? How can it be
otherwise for an ecosystem adapted to an extreme oligotrophic
environment. Which is not to say you might not have coral reefs in
somewhat eutrophic circumstances if everything else is right, but it
is saying that such circumstances are probably a marginal environment
for coral reefs. It means that wherever the waters used to be
oligotrophic and are now becoming more and more eutrophied, the reef
is probably being stressed or worse. Perhaps theh reef can survive for
years depending on how big the nutrient sink is, but indefinitely? So
please don't say nutrient enrichment sometimes is and sometimes isn't
harmful to the reef. It ALWAYS is in the end. Unless it has evolved
eutrophication tolerance in the last 50 years.
Best regards,
Paul Hoetjes
Christopher Paul Jury wrote:
/usr/bin/arc: /usr/bin/arc
Gene,
I sympathize. Actually, the ENCORE project administered in microatolls off
of One Tree Island, GBR is probably right along the lines of what you are
thinking.
Koop, K and many others. 2001. ENCORE: The effect of nutrient enrichment on
coral reefs. Synthesis of results and conclusions. Marine Pollution
Bulletin. 42(2): 91-120.
In the initial low-loading phase of the study ammonium was elevated to 11.5
umol/L at every low tide and phosphate to 2.3 umol/L. The following year
nutrient loading was more than doubled to 36.2 umol/L ammonium and 5.1
umol/L phosphate.
Despite expectations, there were essentially no differences in algal
biomass, productivity, coverage, etc. for any of the groups studied
(phytoplankton, macroalgae, endolithic algae, coralline) in nutrient
enrichment treatments vs. controls. There were also few if any differences
in coral survivability, calcification, linear extention, etc. attributable
to nutrient enrichment in the low-loading phase. There were some differences
in the high-loading phase, but they seem species-specific if anything and
are less than totally satisfying. Additionally, this is well above the
nutrient concentrations reported even on most (though not all) polluted
reefs or areas of strong upwelling, so it is difficult to extrapolate what
these results suggest for a reef experiencing moderate nutrient enrichment.
The nutrient concentrations attained in this study are obviously well above
the 1.0 umol/L nitrogen and 0.1 umol/L phosphorus that has been suggested in
this discussion as a threshold for a phase shift to algal dominance. Despite
this there was no indication that the system was moving towards algal
dominance at any phase, and just the opposite in fact.
It seems clear that one cannot possibly say that nutrient enrichment always
leads to harm to reef organisms or that surpasing a 1.0 umol/L DIN and 0.1
umol/L DIP threshold always leads to algal dominance on reefs. Clearly this
is not the case and there are many reports that demonstrate this. However,
taking into account such case studies as the reefs of Kaneohe Bay, Hawai'i,
many of those around Jamaica, and in various other areas it is clear that
one cannot possibly claim that nutrient enrichment never leads to harm to
reef organisms or to algal overgrowth over corals. Clearly this can and does
happen as well. The problem, as I see it, is that likely several factors are
determining whether nutrient enrichment is deleterious to a reef and reef
corals and whether that enrichment leads to algal dominance over corals, but
those factors have yet to be determined for most if any reefs. There must be
some mechanism that caused the reefs in Keneohe Bay to become eutrophic and
covered in macroalgae while similar enrichment off One Tree Island caused no
such harm. That, it seems to me, should be what folks are working
on--determing why one reef becomes eutrophic and another does not, not
whether nutrient enrichment is potentially harmful or not. We have already
answered that question: sometimes yes, sometimes no.
Best regards,
Chris Jury
Center for Marine Science
Universty of North Carolina Wilmington
5600 Marvin K. Moss Ln
Wilmington, NC 28409
Gene Shinn writes:
As a person who began diving in the Florida Keys in 1950, I have
appreciated the series of nutrient/Diadema discussions set in motion
by Martin Moe. Hopefully all this fuss will stimulate someone to do
large scale experiments, especially ones that examine the effects of
various nutrient levels on corals in controlled conditions. Will no
agency will fund such a study? Incidentally, a similar top-down
versus bottom-up battle over the health of Kelp beds is raging in
California (see letters in Science vol. 313, 22 September 2006, pages
1737-1739). It will be interesting to see what comes from all this
debate once the dust has has settled. Gene
--
No Rocks, No Water, No Ecosystem (EAS)
------------------------------------ -----------------------------------
E. A. Shinn, Courtesy Professor
University of South Florida
Marine Science Center (room 204)
140 Seventh Avenue South
St. Petersburg, FL 33701
[1]<eshinn at marine.usf.edu>
Tel 727 553-1158----------------------------------
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