[Coral-List] Parrotfish, nutrients, and control of algae
Thomas Goreau
goreau at bestweb.net
Sat Nov 3 22:15:28 EDT 2007
A recent paper published in Nature uses a mathematical model of coral
cover, macroalgae cover, turf algae cover, and grazing by parrotfish
and concludes that only parrotfish grazing can prevent algae from
overgrowing and killing corals. It blames fishermen for catching
parrotfish and causing algae growth, and makes the policy
recommendation that fishermen should be stopped in order to let the
corals recover. These conclusions have been widely covered in the press.
However close examination of the model reveals that these conclusions
are no more than a restatement of the original assumptions built into
the model. As a someone with experience doing mathematical modeling
in astronomy, spatial population distributions, biogeochemistry,
atmospheric chemistry, and paleoclimatology, I am acutely aware that
no model is better than its assumptions, and if these don't
adequately describe reality, the results are simply intellectual
artifacts rather than providing insight into how nature works. If we
misunderstand the key controlling factors, the management
prescriptions we make cannot possibly work.
The model published in Nature allows corals to die only by being
overgrown by algae, and by "natural" coral mortality, which is
equated with hurricane destruction, that is to say, it does not
include mortality from heat shock or new diseases, the major causes
of coral mortality in most places in the last few decades. The model
specifies that algae grow at a constant rate, and can only die by
being grazed. There is no allowance for algae fragmentation by waves
(anyone diving after a storm knows the bottom can be covered with
algae ripped loose), nor is there any allowance for intrinsic factors
that may vary the rate of algae growth. Now it is long known that
benthic algae growth can vary by orders of magnitude depending on
nutrient concentrations, but nutrients nowhere figure in the model as
a factor affecting algae growth. Hence the model's conclusion that
only grazing can limit algae growth, as was assumed in the first
place. This tautology somehow escaped the peer reviewers.
The model predicts that the more parrotfish the less algae, but
anyone who has actually watched the long term changes in algae and
parrotfish knows that as algae populations increase, so do the
numbers of parrotfish. The model uses the misnamed "phase shift"
interpretation of the long term changes in algae, corals, and fish in
Jamaica that attributes algae abundance to Diadema die off and
overfishing, and which blames the fishermen for eating all the
herbivorous fish. But in fact, long term observations of changes in
reefs all around Jamaica show that algae overgrowth took place at
different times in different places over a 40 year period, and every
place they followed local population growth and sewage inputs to
coastal waters, but did not follow overfishing or Diadema mortality
except coincidentally at a few places, such as Discovery Bay that
went eutrophic at the same time (T. J. Goreau, 1992, Bleaching and
reef community change in Jamaica: 1951-1991, SYMPOSIUM ON LONG TERM
DYNAMICS OF CORAL REEFS, AMERICAN ZOOLOGIST, 32: 683-695), and that
algae growth was strongly linked to excessive nutrients from land
based sources (T. J. Goreau & K. Thacker, 1994, Coral Reefs, sewage,
and water quality standards, PROC. 3D. CARIBBEAN WATER AND WASTEWATER
ASSOCIATION CONFERENCE, WATER AND WASTEWATER NEEDS FOR THE
CARIBBEAN: 21ST CENTURY, Kingston, Jamaica, 3:98-116 and many papers
by Brian Lapointe). In fact in this period Jamaican fish populations
changed from being dominated by fish and invertebrate eating species
to near complete dominance by herbivores, the exact opposite of what
the hypothesis of top-down control of algae by herbivores, like this
recent model, predicts, but fully consistent with the bottom-up
hypothesis that algae productivity, and herbivore populations, are
controlled by nutrient inputs.
The practical management question is: how can weedy algae be
controlled before they smother coral reefs? To my knowledge there are
only two published cases of weedy algae being removed from coral
reefs on a large scale, one of them a short term success but a long
term failure, while the other has been sustained.
In Kaneohe Bay, Oahu, Hawaii, sewage was pumped onto the reef, algae
spread out from the sewage outfall and overwhelmed the reef, and
since there was no doubt that the nutrients had caused the algae, a
long sewage outfall was built to place the problem much further away.
As the nutrients fell the algae died back, and the coral gradually
recovered. After the point source of nutrients was removed, the
suburbanization of the watershed caused uncontrollable increases in
non-point sources of nutrients from lawn fertilizers, golf courses,
road runoff, and other nutrients that were not flushed down the
sewer. These have caused the system to again go eutrophic, and the
algae have again smothered the reef. There is a large Hawaiian
literature on this that is readily available. It shows that
controlling nutrients gets rid of algae, but only if it is sustained.
A more successful long term case is a bay in Jamaica that I got
cleaned up 10 years ago by diverting all the land based sources of
nutrients and recycling them on land. Within weeks the red and green
weedy algae smothering the reef began to die back, and two months
later they were gone (T. Goreau, 2003, Waste Nutrients: Impacts on
coastal coral reefs and fisheries, and abatement via land recycling,
28p., UNITED NATIONS EXPERT MEETING ON WASTE MANAGEMENT IN SMALL
ISLAND DEVELOPING STATES, Havana, Cuba). I have just revisited this
site 10 years after, and the weedy algae are still gone, with the
algae now dominated by the oligotrophic calcareous algae.
In my experience the only way to get rid of weedy algae is to starve
them of nutrients, and then they very quickly die. But all excessive
nutrients must be controlled, and they must remain controlled. This
requires adherence to the coral reef specific nutrient standards
proposed by Brian Lapointe, Mark and Diane Littler, and Peter Bell.
We can blame the victims by stopping fishermen from eating, but this
will not work because it is based on a seriously flawed understanding
of what controls algae growth.
The Turks and Caicos Islands are the first place in the world to
propose coral reef specific water quality standards, and the only
place in the world to require that all developers build sewage
treatment plants and recycle all of their waste water on their own
property. We will not see the algae die back in eutrophic reefs until
other countries follow their example and all the sources of
anthropogenic nutrients are identified and controlled.
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
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