[Coral-List] Poor terminology in coral reef research 3: Top down VERSUS bottom up control of algae

Thomas Goreau goreau at bestweb.net
Sun Nov 5 10:37:45 EST 2006

This artificial dichotomy is intuitively known to be false by almost  
everyone who realizes that algae abundance is a balance between  
growth (largely fueled by the major limiting nutrients in reefs,  
nitrogen and phosphorus), and the rate at which it dies or is eaten,  
so BOTH MUST be relevant. Yet in the last 35 years every coral reef  
field course is told "OK kids, go out there and count all the sea  
urchins and herbivorous fish" because it is the easiest thing to do  
in coral ecology. Thanks to the academic publish or perish system,  
and the urgent need of faculty members and students to amass  
publications for their professional advancement, much of this winds  
up in the literature. The problem is that they then provide a "Just  
So Story", as my former advisor Stephen J, Gould put it (with credit  
to Rudyard Kipling), which purports to "explain" the abundance of  
algae, without ever measuring either the nutrient levels or algae  
uptake of them, which is serious work that is certainly not easy, so  
few do. Brian Lapointe has done the best work on this.

As a result the literature has hundreds of papers in which algae  
abundance is "explained" by lack of things to eat it for every one  
paper that shows how fast algae grow in response to the actual  
nutrient levels. In South Florida, where every sewage outfall is  
surrounded by an expanding ring of cyanobacteria smothering reefs,  
EPA points to all the papers that say lack of herbivores are THE  
cause of algae growth, to deny any linkage between nutrients and  
algae, and therefore to permit sewage plant discharges into reef  
waters (See the recent posts by Ed Tichenor).

I used to collect algae for researchers as a small boy, and I watched  
the algae species change, expand, and smother and kill our reefs in  
Jamaica. This happened over a 40 year period, took place at different  
times at different places, following local coastal development and  
sewage discharges, and had nothing to do with either sea urchin die  
off (except coincidentally at a handful of places), or with  
overfishing (when I was a boy our fish were largely carnivorous, and  
now they are almost entirely herbivorous, since there is no other  
food, the exact opposite of what the top down theory (and hundreds of  
widely cited papers) predicts, see T. J. Goreau, 1992, Bleaching and  
reef community change in Jamaica: 1951-1991, in SYMPOSIUM ON LONG  
many later papers).

Now there should really be no argument that nutrient availability is  
THE major control on algae growth rates, but that herbivores, by  
preferring the taste of some species and hating others, determine  
WHICH algae are most abundant. In my experience all around the  
Caribbean, Indian Ocean, Pacific, and South East Asia every time I  
see high levels of the algae species that are indicative of high  
nutrient levels, one can invariably find them zoned by species and  
abundance in a way that points straight to a nutrient source, whether  
or not herbivores are present. And when there is no source of  
nutrients there are very few or no algae, again whether or not  
herbivores are present. A recent paper by Vroom et al, 2006, Algae  
dominated reefs, American Scientist, 94:430-437, points to algae  
covered reefs that are very remote from human sources and concludes  
that these must be pristine reefs because there are no humans  
present, however they made no nutrient measurements. Yet all their  
photos show water that is dense green with phytoplankon, indicating  
very high nutrient upwellijng. In the Turks and Caicos national coral  
reef assessment  (Goreau et al, 2006) we found many places with high  
algae and green water that had no human sources, but in all these  
cases there is clear ecological evidence from the algae zonation for  
strong offshore nutrient sources from deep water upwelling.

I know of only a handful of places where algae overgrowth has been  
reversed or eliminated, and in every case this was done by  
eliminating the nutrient sources. Not only does this work, the algae  
starve and die back very quickly. In one bay I got cleaned up in  
Jamaica the die off was apparent within weeks and complete within two  
months, even though there were many herbivorous fish present all  
along when the sewage and detergent flowed into the sea, and they  
could not control the algae killing the corals. In contrast, when the  
herbivorous Diadema sea urchins regained their local abundances,, as  
they have in many places in the Caribbean, they are only able to  
graze small rings around themselves and had no impact on algae beyond  
that, because the nutrient fueled growth greatly exceeds what they  
can eat.

Conclusion: to prevent eutrophication one must control nutrient  
inputs, but which herbivores are present will determine which algae  
are most abundant. In any event, there is JUST NO EXCUSE for not  
stopping land based sources of nutrients from sewage and fertilizers  
in order to protect reefs from eutrophication.

Thomas J. Goreau, PhD
Global Coral Reef Alliance
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net

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