[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


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 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)
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University of South Florida
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