[Coral-List] top-down, bottom-up arguments

jmcmanus jmcmanus at rsmas.miami.edu
Wed Oct 11 15:50:21 EDT 2006

Hi all,

I agree in principle that the argument over 'which is worse" is
counterproductive. Unfortunately, we cannot yet drop the research side of
the issue. Life would be much easier if we could simply prevent both fishing
and nutrient loading on reefs. However, that usually not being the case, we
need to come up with scientific advice that managers can use effectively. 

The existing efforts to come up with maximal allowable N and P
concentrations are moving in the right direction. Unfortunately, the early
concept from Littler and Littler was that levels of herbivory interact with
levels of nutrients to result in different effects on the ecosystem. Adding
nutrients with lots of herbivory should lead to the growth of unpalatable
algae such as a proliferation of encrusting algae, which may not be as bad
in terms of reducing coral settlement. Adding the same amounts with less
herbivory would be expected to lead to proliferation of more palatable
algae, including many species of fleshy macroalgae which is problematic
where it was not previously abundant. From basic ecological theory, one
would expect that adding nutrients to a healthy food web would lead to it
being spread through the food web at many trophic levels, resulting in more
biomass overall. Adding them to a reduced food web, particularly one with
reduced herbivory, should result in a concentration of biomass at the
primary productivity level -- some kind of algal bloom. Thus, a maximal
allowance for nutrients in a healthy reef ecosystem (food web-wise) would be
expected to be higher than one for a heavily fished one. 

We clearly need to understand how much of what combinations of nutrients can
be allowed with what levels of what kinds of fishing. That is the sad
conclusion that has led some of us to start delving into multivariate
statistics and simulation modeling. However, Gene is absolutely right, that
we need larger scale experiments where they can be done without damaging
reefs. A less controllable but often more feasible approach is to avail of
natural and management induced experiments via long-term, large scale
comparative studies across multiple reef systems. Basically, we need to turn
a large number of both highly and less-disturbed reefs into a global
experiment. That is the idea behind the Comparative Analysis of Reef
Resilience Under Stress (CARRUS) Alliance several of us are trying to
gradually pull together. The focus there would be mostly on making more use
of existing well-monitored reefs. Throwing in some large-scale controlled
experiments would make the overall scheme all the more effective.

I'm not too worried about the existing nutrient minima from places like
Jamaica, because they were obtained in overfished and otherwise low-herbivoy
systems and are therefore conservative. However, we still have a long way to

Of course, too much nutrient addition can result in macroalgal growth that
will blow away herbivory capacity and damage other life forms directly.
Similarly, a reef with limited macroalgae but few fish and lobsters will
likely have other problems, such as outbreaks of boring sea urchins,
crown-of-thorns seastars, coralivorous snails, and innumerable other issues
(this is, after all, a highly co-evolved ecosystem!). One last thing to note
is that the traditional concept of 'overfishing' roughly equates to removing
more than half of the biomass of the target species. Many overfished reefs
have removed more than twice that, and the range of target species is in the
many hundreds around the world (not to mention by-caught species). Take any
highly co-evolved ecosystem and take out half of its higher trophic level
biomass and tell me that you do not expect problems! 

So DeeVon is right about there not really being an argument of one or the
other cause being worse. However, we still have to figure out a lot more
about what we need to know to set nutrient and fishing limits.


John W. McManus, PhD
Professor, Marine Biology and Fisheries
Coral Reef Ecology and Management Laboratory (CREM Lab)
Director, National Center for Coral Reef Research (NCORE)
Rosenstiel School of Marine and Atmospheric Science
University of Miami, 33149
Office: 305-421-4814/4820, Fax: 305-421-4910, Website: www.ncoremiami.org
If I cannot build it, I do not understand it. -- Richard Feynman, Nobel

-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov
[mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of DeeVon Quirolo
Sent: Tuesday, October 10, 2006 12:41 PM
To: Paul Hoetjes
Cc: coral-list at coral.aoml.noaa.gov
Subject: Re: [Coral-List] top-down, bottom-up arguments

Thank you Paul for your clarity and understanding of the compelling 
need to deal with both top down and bottom up impacts to coral 
reefs.    The time for debate about one over the other should be 
officially declared closed.  Best, DeeVon Quirolo, Reef Relief

At 09:17 PM 10/9/2006, Paul Hoetjes wrote:

>    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
>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
>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
>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
>this discussion as a threshold for a phase shift to algal dominance.
>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
>happen as well. The problem, as I see it, is that likely several factors
>determining whether nutrient enrichment is deleterious to a reef and reef
>corals and whether that enrichment leads to algal dominance over corals,
>those factors have yet to be determined for most if any reefs. There must
>some mechanism that caused the reefs in Keneohe Bay to become eutrophic and
>covered in macroalgae while similar enrichment off One Tree Island caused
>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
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