[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  
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  
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,  
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
Global Coral Reef Alliance
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net

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