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

[Christopher Paul Jury]

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
> <eshinn at marine.usf.edu>
> Tel 727 553-1158---------------------------------- 
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