[Coral-List] Macroalgae in the Keys, top-down vs bottom up

John Bruno jbruno at unc.edu
Sun Jun 22 09:47:08 EDT 2008

MACROALGAL COVER IN THE FL KEYS:  Dear Chip,  Macroalgal cover in the  
Florida Keys is only about 10%.  A recent paper based on CREMP  
monitoring indicates that it was 9.6% in 2000 (Maliao et al. 2008,  
Marine Biology).  My own unpublished meta-analysis based on CREMP data  
plus a variety of other sources indicates it was "recently" 12.2 ±  
0.4% (n=1048 quantative reef surveys performed between 1996 and 2005)  
and macroalgal cover exceeded 50% in only 2.5% of these surveys.  This  
is substantially lower than the average in the greater Caribbean of  
roughly 20% and identical to the Indo-Pacific mean.

Nobody knows what the historical baseline (i.e., the subregional  
average, not the value on a single undisturbed reef) of macroalgal  
cover was in the Keys (or anywhere else), but I doubt that it was any  
lower than 5%.  So we may have seen a rough doubling of macroalgal  
cover, but we are very far from a state that could rationally be  
described as "macroalgal dominated" or "little more than rubble,  
seaweed and slime".

Billy Causey and his team are in my view some of the world's most  
successful reef managers; the quantitative monitoring data indicates  
that they have been very successful in managing the major threats to  
reefs that they are capable of mitigating.  They obviously cannot  
prevent climate change and coral disease outbreaks, but they have done  
a good job at managing for low-ish macroalgal cover (which will  
ideally, at some point facilitate coral recovery).  Macroalgal cover  
the GBR is about 7% (based on AIMS monitoring), but those reefs are  
10s-100s of km offshore, i.e., very isolated and naturally  
oliogotrophic compared to the Keys, and didn't suffer the virtual  
extinction of their key grazer or their dominant coral species as the  
Keys did due to regional epizootics.  All things considered, at least  
in terms of macroalgal cover, the Keys are in relatively good shape.   
Lets give the Keys management team - and all the other successful  
local reef managers - some credit, base our arguments and discussion  
on the facts, and focus on the real threats to reefs.

TOP-DOWN VS. BOTTOM-UP:  Dear Imam, assuming that like most of us you  
are not swayed by the ideology and anecdotes you have been reading on  
the list in response to your query, I'd start with Alina Szmant's  
authoritative review (2002 Estuaries) of the published science on  
nutrient and grazer control of macroalgal biomass on reefs.  You might  
also look at Idjadi et al (2006, Coral Reefs), which documented the  
immediate loss of macroalgal cover (to 6%) as soon as Diadema returned  
to the scene on a reef purported to be one of the most eutrified in  
the world.  There are a slew of other peer-reviewed papers that  
document similar removal of macroalgae on reefs widely described as  
highly eutrified once grazer populations recover (e.g., Edmunds and  
Carpenter 2001 PNAS, Carpenter and Edmunds 2006 Ecology Letters, Mumby  
et al. 2006 Science, etc.).  Also see Williams and Polunin (2001 Coral  
Reefs); a very powerful and important study that documented the  
striking negative relationship (r2=0.89) between fish biomass  
(particularly Scarid biomass) and macroalgal cover.   Finally, if you  
expand your search beyond the coral reef world, you'd find that in  
estuaries and in temperate and cold water benthic systems, urchins and  
other grazers are easily able to control macroalgal production that is  
far greater, under nutrient concentrations many orders of magnitude  
higher than those ever seen on reefs.  Just think about Jim Estes'  
work on Pacific otters-you remove them, urchin populations explode,  
macroalgae disappears and what remains is an "urchin barren"; and this  
occurs in upwelling systems with lots-o-nutrients.  This was also seen  
in the Gulf of Maine after Cod were removed by fishing (at least  
before people started harvesting urchins, then the macroalgae all came  
back); see Steneck and Carlton's and Duffy and Hay's very nice reviews  
of all this the Marine Community Ecology book (eds. Bertness et al  
2001, Sinauer-http://www.amazon.com/Marine-Community-Ecology-Mark-Bertness/dp/0878930574) 
.  And look at the recent review by Heck and Valentine (2007 Estuaries  
and Coasts) which outlines all the evidence for top down control in  
estuaries around the world.

If you stick to the hard science, the answer to your question is  
fairly clear - at least as clear as anything gets in ecology.  Let me  
know if you have any trouble getting any of these papers.



John Bruno
Associate Professor
Depts. of Biology and Marine Sciences
UNC Chapel Hill

> Dear Imam,
>  The relationship between nutrients and algal growth is well  
> established, and the influences of herbivory on algal growth and  
> cover have also been demonstrated.  However, as Dr. Goreau stated,  
> the lack of successful integration of nutrients and herbivory in any  
> of those studies has contributed to a disconnect.  In Florida, where  
> a large coastal population has resulted in mesotrophic and more  
> typically eutrophic coastal waters, we have a high % of algal cover  
> and biomass.  Add the facts that we do not fish for herbivorous  
> fishes, and that we have removed a fair % of their predators; it  
> follows that Florida would have an increase in herbivorous fishes,  
> and therefore our relative herbivory.  But we are still plagued by  
> macroalgal dominance, losses of coral and Harmful Algal Blooms. This  
> suggests that nutrients are indeed an important factor shaping the  
> algal community.  Hatcher and Larkum (1983, JEMBE 69, pp61-84)  
> compared grazing and nitrogen concentrations on One Tree Reef in
>  Australia and found both grazing and nitrogen were important in  
> limiting algal growth.
> Sincerely,
> Chip
> Rex "Chip" Baumberger
> Biological Scientist, FAU
> Marine Nutrient Dynamics Dept.
> Marine Science Division
> Harbor Branch Oceanographic Inst.
> 5600 US1 North
> Fort Pierce, FL 34946
> 772-465-2400 x398

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