[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
TERM DYNAMICS OF CORAL REEFS, AMERICAN ZOOLOGIST, 32: 683-695 and
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
President
Global Coral Reef Alliance
37 Pleasant Street, Cambridge MA 02139
617-864-4226
goreau at bestweb.net
http://www.globalcoral.org
More information about the Coral-List
mailing list