[Coral-List] Parrotfish, nutrients, and control of algae

Thomas Goreau goreau at bestweb.net
Fri Nov 9 22:20:55 EST 2007

Dear lovers of coral reefs, not weed reefs,

There have been a number of interesting responses (7 of them below)  
to my post on control of algae, and since this is probably my last  
opportunity to post anything on the list server this year, I'll have  
to answer them all together. There are also interesting responses to  
the  Mumby paper in the Caribbean press, which uniformly report that  
"international scientists say" that if we stop eating parrotfish the  
reefs will recover, indicating that they all took the paper's policy  
prescription to mean that stopping grazing by stopping fishermen is  
the solution to reef ills. These folks should look at the Maldives,  
where the locals don't eat reef fish at all, and where the reefs far  
from people are free of weedy algae, and all those near sewage  
sources are overgrown with the usual weedy slime, and as you would  
expect, the areas of high nutrients and algae have the highest  
Diadema and algae grazing sea urchins too, for example next to the  
airport (T. Goreau, 1997, Damage to Maldivian reefs from mining, sea  
level rise, sewage, and global warming: recommendations for coral and  
shore protection, Report to the Ministry of Fisheries and  
Agriculture, Male, Maldives, 18p.).

Algae now dominate most reefs in many parts of the world where they  
did not in the past, and we can learn far more from their spatial  
distributions when we understand the detailed ecological preferences  
that make algae species so sharply zoned, more than we can learn from  
any other major reef component,  but only a handful of people know  
how to read the nutrient zonation preferences of reef algae species.   
Because reef ecology has diverged so far from reef biogeochemistry in  
recent decades, there has been a strong tendency to "explain" reef  
ecology purely from ecological data, in which algae have been lumped  
together, at best into crude morphological classes, and where the  
unique information on reef gradients the algae provide are entirely  
obscured. There are thousands of papers out there that "explain"  
algae by counting fish and sea urchins, and either never measure  
nutrients at all, or do so using methods that are so insensitive that  
they are only unwittingly reporting the detection limits. This is  
akin to the old saying that to someone whose only tool is a hammer,  
all problems look like nails! But is is absurd to think that only  
grazing controls algae abundances. There is a vast literature on the  
effects of nutrients, light, and temperature, and the very different  
responses to these factors of each separate species, and any algae  
specialist would regard these as the first parameters to include in  
any model that purports to describe algae growth.

In the 1980s, when I helped Brian Lapointe do work on the nutrient  
responses of many Jamaican algae species (only a portion of which was  
published), we found huge responses to nutrients at the concentration  
ranges that we found in coastal waters, in fact the problem was to  
find any water clean enough in nutrients for control experiments in  
which algae were not already growing nearly flat out to contrast with  
the nutrient additions and combinations. None of the nutrient papers  
were cited in Mumby or indeed in almost any other "top-down" control  
paper (or if they do, the chemical analytical methods or  
interpretation are seriously flawed). There is a vast number of  
papers in the phycological literature relating algae growth rates to  
nutrients, but most are for a single species under well defined  
experimental conditions, and the various algae species cover a huge  
range in nutrient response depending on whether they are adapted to  
oligotrophic, mesotrophic, or eutrophic conditions (so there is no  
single functional relationship to use in a model, instead there are  
very many, requiring that each group of algae with different growth- 
nutrient curves be modeled separately).

Since coral reefs turn eutrophic at nutrient levels too low to affect  
any other marine ecosystem, we are increasingly seeing the  
oligotrophic algae vanish and dominance by the most aggressive weeds,  
whose nutrient uptake capacity allows them to take over any place  
with nutrient inputs. It is worth noting two of the papers that most  
widely claim to reject the role of nutrients in fueling algae growth.  
One of them is the highly funded ENCORE study, in which dozens of  
Australian researchers dribbled P and N into an Australian reef  
lagoon and published papers saying that adding nutrients made no  
difference at all, but ignored the fact that the system was algae  
dominated and had nutrients above the coral reef eutrophication  
thresholds in the first place! (Bell, Lapointe, & Elmetri, 2007,  
Reevaluation of ENCORE: Support for the eutrophication threshold  
model for coral reefs, Ambio, 36: 416-424). Another is a (peer- 
reviewed) study by a group that nailed lawn fertilizer spikes into  
the Florida reef, and claimed that the nutrients actually killed the  
algae. They did not know that the fertilizer spikes contain time  
released chlorine emitters, to kill all the algae that would  
otherwise grow all over the spikes and prevent the lawn grass from  
getting the goodies.

With regard to papers that claim to have removed algae using  
herbivores, my view is that these require very close examination.  
I've looked at many places in the Caribbean where the Diadema have  
recovered from the 1983 mortality to very high densities, as well as  
places with no recovery. The places with no Diadema recovery are free  
of weedy algae if there are no nutrient sources, whether there are  
large parrotfish around or not, and all the places with known  
nutrient sources are algae overgrown, whether or not Diadema and  
parrotfish are present. The claim that Diadema recovery "controls"  
algae requires examination on only the smallest spatial scale,  
because it does not apply on the larger scales. The Diadema sit fat  
and happy inside a narrow grazing halo, usually only a few inches  
wide, in which they have gobbled all the algae, and don't need to go  
any further to find food, so that the algae canopy rises sharply into  
weedy forests just outside these little halos, which the urchins are  
clearly unable to control. So if you measure on a small enough scale,  
algae and urchins are inversely related, but on a larger scale they  
are positively correlated. Basically grazing is able to control algae  
only when nutrients are very low, but when they are high grazing is  
unable to the trick.

It is certainly true that one can find high algae abundances in  
places where there is no obvious sign of anthropogenic sources of  
nutrients. However in my experience, with sufficient searching one  
can always find a nutrient source, although it may be natural. In  
Indonesia one can see reef eutrophication caused by upwelling of cold  
nutrient rich waters, and last year we found that remote areas of the  
Turks and Caicos Islands without human nutrient inputs that were  
algae-dominated had green water from phytoplankton blooms caused by  
localized, but spatially consistent, upwelling patterns. I suspect  
that this is likely also the cause in Great Astrolabe Reef in Fiji.

The St. Croix observations by Longin remind me of Barbados, where the  
huge schools of big parrotfish were able to graze all the macro-algae  
but not the dense microalgal turf that covered all dead corals, which  
grew back as fast as it was chomped on, but which in recent years has  
shifted to cyanobacteria that nobody seems to want to eat, not even  
Diadema or parrotfish. I'd expect these reefs to deteriorate until  
the nutrients are removed, whether or not grazers are introduced.

With regard to the detailed clarification on the model that Peter  
Mumby has kindly and thoughtfully presented (unfortunately, the  
publication didn't allow adequate discussion of how the model is  
constructed), it is clear that this model could be improved along the  
lines he identifies. The discussion on hysteresis is very useful. The  
model does provide insight into the role of grazing, because that is  
what it was designed to do.  But the model contains too little of the  
nutrient driving forces that control algae growth rates to allow it  
to test the role of nutrient inputs, because in effect it makes  
grazing the sole controlling factor on algae growth. And it is odd to  
see reefs dominated by algae described as "resilient" because the  
algae won't go away by themselves, as "phase shifters" predict could  
happen. A model that conflicts with empirical observation should only  
be trusted over the data if it is so complete a description of ALL  
the relevant factors affecting the variables of interest that you  
would rather trust the model than your own lying eyes. I personally  
don't believe in "phase shifts" (a nonsensical phrase in the context  
it is widely used in reef ecology) or multiple steady states (another  
model artifact in most cases), but in time varying responses to  
multiple time varying driving forces, of which nutrients are pretty  
certainly the missing factor explaining coral or algae dominance.

My own experience, in seeing a Jamaican bay that is still clean of  
weedy algae 10 years after we got rid of them, where the Elkhorn and  
other corals are coming back,  and where the algae are now the good  
old low nutrient calcareous beach sand producers and no longer the  
eutrophic weeds that smothered the bay when we began, convinces me  
that nutrient control is the fastest and most effective way to get  
rid of algae. Please note that I don't dispute that herbivores should  
be encouraged, only that the most effective way to get rid of algae  
is to starve them of nutrients, and unless this is the focus of  
control strategies, they are unlikely to work well.   What won't work  
is to try to pluck them off, for they grow right back (I used to weed  
algae from reef patches in Jamaica for years, trust me, it's a fool's  
errand). For a third proposed way to get rid of algae, see my recent  
comment below, which SCIENCE refused to publish (There were also two  
requests from sadists who suggested I go publish a paper in Nature or  
Science, but I'm not masochistic enough to waste my time when there  
is ZERO chance of it getting past the kinds of peer reviewers who  
have rubber stamped all the trendy fad papers they have published in  
the last few decades on "phase shifts" and "resilience" in reefs. )

Coral reef eutrophication, nutrients, and vacuum cleaners

"Call the Hose Brigade!" (Random samples, 10 August, 2007, p. 729)  
describes an effort to remove a massive nuisance algae bloom killing  
corals in Kaneohe Bay, Hawaii, by sucking it up with huge barge- 
mounted vacuum cleaners. Unfortunately this will give only temporary  
results and will fail in the long run unless the nutrient excess that  
fuels the rapid growth is removed. Kaneohe Bay is a classic example  
of coral reef eutrophication: benthic algal blooms caused by point  
discharges of sewage killed the reef in the 1970s, and died back when  
the outfall was removed, allowing the reef to gradually recover (1).  
With continued suburbanization of the watershed, uncontrolled non- 
point nutrient discharges to the bay from golf courses, lawn  
fertilizers, and road runoff have again raised the nutrient  
concentrations (2,3) above the thresholds for nuisance algae (4-6).  
Besides the temporary success in Kaneohe Bay, there are very few  
examples of algae being successfully removed. In one bay in Jamaica  
where all the land-based nutrients were diverted, nuisance algae that  
were choking the reef began to die back in weeks, and only a few  
dying clumps off weedy algae remained two months later (7). If algae  
are starved of nutrients, they die very quickly, and will not return  
unless nutrient thresholds are again exceeded. But no amount of  
sucking them off will work when they grow right back because they are  
overfertilized. It is the suckers paying for this well intentioned,  
but ultimately futile effort, who will be hosed unless the underlying  
causes of eutrophication are removed.

1.	A Banner, Proc. 2nd Int. Coral Reef Symp., 2, 685 (1974)
2.	E Laws, C. Allen, Pac. Sci., 50, 194-210 (1996)
3.	S. Larned, J. Stimson, Mar. Ecol. Prog. Ser., 145, 95-108 (1996)
4.	P. Bell, Wat. Res., 26, 553-568 (1992)
5.	B. Lapointe, N. Littler, D. Littler, Proc. 7th Int. Coral Reef  
Symp., 1, 323-334 (1992)
6.	T. Goreau, K. Thacker, Proc. Carib. Water & Wastewater Assoc., 3,  
98-116 (1994)
7.	T. Goreau, UN Expert Meeting on Waste Management in Small Island  
Developing States, 1-28 (2003)
John Bruno's notion that algae overgrowth of corals is good after  
bleaching, because it shades them and alleviates light stress on top  
of thermal stress, ignores the fact that most of the overgrown coral  
is killed from lack of light.

As far as reprints go, some of our more recent papers are posted on  
www.globalcoral.org, but we unfortunately lack the resources to scan  
and post the older ones.

Best wishes,

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

Dear Dr Goreau

I have read with a great interest your recent message
posted on the Coral-list (related to Mumby's paper in
Nature), and I am very interested in reading the
papers you cited. If pdf reprints are available, could
you please send them to me?
Thank you very much in advance for this courtesy.


YM Bozec

Hi Dr. Goreau,

My name is Eliza Heery and I work for Ken Sebens in Washington  
state.  I was pleased to see your comments yesterday on the coral  
listserve regarding the Mumby et al. paper that just came out in  
Nature.  Having recently completed graduate work in mathematical  
biology and population dynamics, their approach interested me very  
much.  However, I was also quite concerned about several of the  
assumptions that were implicit in their model.

Out of curiosity, I have been recreating their simulation model in R  
in order to see how it might behave given alternative assumptions  
about nutrient loading, coral growth rates, etc.  It seems  
particularly problematic that they model the colonization of  
macroalgae based solely on data they've collected in Belize (Mumby et  
al. 2004), where the population density of people is considerably  
less and where sewage treatment is taken quite seriously.  Since they  
eventually validate their model based on data from Jamaica, I was  
wondering if you might know of any data sets that relate nutrient  
loading to macroalgae colonization along the Jamaican coast.  This  
would help me tremendously in reparameterizing their current model,  
and I feel it might influence simulation results considerably.

Thanks for your time.  I look forward to hearing about any leads you  
might have.

Eliza Heery

eliza.heery at gmail.com

In favor of parrotfish and other herbivores....

I have no doubt that land-based nutrient input is often the cause of  
algal reef takeovers....but not always.

Two of the three inhabited islands within Fiji's Great Astrolabe Reef  
have a total population under 300 and those two islands control  
useage of 90% of the lagoon and reefs.  Agriculture is liimited to  
family garden plots.  There are no roads or vehicles.  Fishing for  
sale in Suva markets is the principle livelihood.  (The third island  
has a larger population and similar gardens, but does not have  
fishing rights on the largest part of the reef and lagoon.)  The  
reefs are 35 n.m. from Viti Levu with a broad and deep passage  
separating them from Suva's nutrients.

The 2000 bleaching event left the lagoon and reefs with less than 17%  
live coral cover.  Algal growth has increased dramatically since  
then.  There is coral recruitment, but recruits as well as older  
corals are often invaded by various blue-gree algae (Lyngbia spp. are  
common) and ascidians.

With little to no nutrient input from the villagers,  I can only  
suspect that over-fishing compounded the damages of the bleaching  
event.  The fish population is much smaller than it was 15 to 20  
years ago but it has been a number of years since anyone looked at  
that.  (Jennings, S. and N.V.C. Polunin: 1996. Impacts of fishing on  
tropical reef ecosystems.  Ambio      25:44-49
Jennings, S. 1998. Artisanal Fisheries of the Great Astrolabe Reef in  
Fiji: monitoring, assessment and management. Coral Reefs and PIMRIS  
(LB 2376 .F5 A8 - 1994-3)

Peter Bell found no nutrient input into the lagoon from one of the  
two islands in 1991 where the human population has decreased from 125  
to 85 since then.

Unfortunately the U. of the South Pacific has had to close the field  
station within this reef system, so further studies cannot be  

Joan Koven
Astrolabe, Inc.
601 Springloch Road
Silver Spring, MD 20904


I understand and agree with your explanation but in St. Croix, where  
my home has been since 1986, there has been a large upsurge in the  
use of gill nets on the reefs in recent years (something we are  
fighting to ban) and the decreases in parrotfish and surgeonfish  
(favorite food fish) are dramatically obvious. We also have had a  
long-term sewage problem until recently. In the past two years I've  
also seen a dramatic increase in cyanobacterial overgrowth of the  
Montastrea complex reefs. How do you think this is impacting our  
coral cover and algae populations? What would you expect me to see  
happening in the coming years as a result of this intensive fishing  
pressure?      Thanks very much, Lonnie Kaczmarsky, Florida  
International University

First, is anyone aware of an example (preferably published) where
macroalgae have alleviated some stress or otherwise benefitted corals
in terms of growth, survival, etc?  I have heard people talk about
observing that shading by macroalgae can reduce bleaching and
bleaching-related mortality, but don't recall seeing any studies on
this and have not been able to dig any up using various search engines.

Second, is anyone aware of an example in which after herbivore
populations were replenished (either naturally or via management)
macroalgal cover/biomass remained "high"?  This question really gets
at whether "phase shifts" to macroalgal dominance are "permanent
states" or simply responses to the removal of top down control.
Every example I can think of clearly indicates that once herbivores
return (e.g., Diadema to the north coast of Jamaica ala Edmunds and
Carpenter 2001, Carpenter and Edmunds 2006) macroalge immediately
return to formerly very low cover and abundance (which also suggests
that top down rather than bottom up forces were the primary cause of
the change in macroalgal cover in the first place).

Thanks for any examples, ideas or advise you might have.


John Bruno, Ph.D.
Associate Professor
Department of Marine Science
The University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-330
jbruno at unc.edu

Dear Tom,

Thanks for the coral-list response to the Nature paper.  Very  

Do you know of anyone who has produced a quantitative dose-response  
showing the increase of algae with respect to an increasing nutrient
concentration gradient?  If so please send citation.

I am trying to collect as many dose-response curves as possible for
different coral reef stressors for my work with coral reef indexes of  

Any other citations demonstrating dose-response curves for various other
coral reef stressors would be appreciated.

Best regards,

Dr. Stephen C. Jameson, President
Coral Seas Inc. - Integrated Coastal Zone Management
4254 Hungry Run Road, The Plains, VA  20198-1715  USA
Office:  703-754-8690, Fax:  703-754-9139
Email:  sjameson at coralseas.com
Web Site:  http://www.coralseas.com


Research Collaborator
Smithsonian National Museum of Natural History
Washington, DC 20560

Dear Dr Goreau

I read with interest your message about our paper and would like to
clarify a number of misconceptions.

1) We do NOT conclude that lack of grazing is the cause of coral reef
decline nor do we conclude that parrotfish are the solution to the
problem. The model shows that reefs have a greater capacity to recover
from disturbance if total grazing and coral cover are higher.
Sensitivity analyses repeatedly showed that reefs could shift to either
a coral- or algal-dominated state when levels of grazing represented
those found today in the functional absence of Diadema. Whether the reef
will shift to either a coral or algal state depends on the initial coral
cover, the disturbance regime, the underlying dynamics of the system,
and the level of grazing. However, as grazing levels are increased by
fisheries management, the likelihood of a reef becoming entrained
towards an algal-dominated state decreases - i.e. the system becomes
more resilient. Therefore, high levels of parrotfish grazing do not
guarantee that a reef recover, but they maximise the probability that it
will. There are a number of studies from the Caribbean that show that
corals can respond when grazing levels increase, so this really isn't

2) Model outcomes are NOT an inevitable restatement of assumptions. The
model was an individual-based (or agent based) approach that assigns
empirical rates of processes to corals, categories of algae, herbivores,
and so on. In other words, we use the existing literature to build the
reef from scratch - e.g. corals recruit at the rates we see in the
field, grow at observed rates, etc, etc. This is very different from
creating an explicit mathematical relationship such that algal cover is
merely a function of coral cover. It is also important to bear in mind
that each model parameter is based on an empirical field study from the

3) Your statements about the functioning of the model are incorrect.
Corals experience both partial and whole-colony mortality at chronic
background levels observed in nature (from studies by Rolf Bak and John
Bythell). The causes of this mortality are varied and include parrotfish
predation, occasional diseases, storm damage, etc. We then estimate the
'equilibrial' dynamics of such reefs between major disturbance events -
i.e. does the reef tend to increase in coral cover, remain the same, or
decline over time after a disturbance of given size? We then add a
disturbance regime which, by way of example was hurricanes occurring
every 20 years, and assess the net outcome of disturbance and recovery.
This highlights whether the reef has the capacity to absorb these
impacts or not, which is the essence of resilience.

Other assertions on the parameterisation are also incorrect. Algae do
not grow at constant rates and they are extensively scoured (removed)
after major storms. I'll deal with nutrients separately.

3) Nutrients. It's very important to understand that the model was
parameterised for a particular type of reef - seaward forereefs. These
reefs have low suspended sediment concentration and typically have very
high levels of primary production because high wave exposure maximises
the delivery of nutrients to algal fronds. Therefore, the algal
colonisation and growth rates are relatively high and considerably
greater than those from leeward reefs. Changes in algal growth rate will
affect the shape of the hysteresis curve (Fig. 2 of the paper). When we
carried out sensitivity analyses to different algal growth rates (as a
proxy for nutrient flux), we found that the conclusions held despite
perturbing the growth rates of algae. In short, the model does have the
capacity to vary algal growth rates and sensitivity analyses reveal that
20% fluctuations do not alter our main conclusions. Indeed, an earlier
study, published in Ecological Modelling, found that even a doubling of
algal colonisation and growth rates had limited impact on reef
trajectories. That does not mean that nutrients are unimportant -
rather, the model provides a framework to integrate the effects of
changes in both algal growth rate and grazing. There are some
combinations of coral cover and grazing for which changes in algal
growth rate are really quite important (as illustrated by your
examples). Like most things in nature, the relative importance of
nutrient flux and grazing varies from place to place.

We must try to incorporate the best science available to parameterise
ecological models. As new data emerge, the model (and others like it)
can be improved. For example, I have not yet seen a study that
rigorously quantifies the additive effect of rising nutrient
concentrations on the community composition of Caribbean forereefs (i.e.
factoring in other changes such as exposure, grazing, water velocity,
habitat differences, etc). If such studies are available then we can
progressively refine the model accordingly.

Lastly, I'd just like to reiterate the main take-home messages of the

A) Previous field studies have provided clear evidence of phase changes
on coral reefs, such as those that occur when the levels of urchin or
fish grazing change. What our model reveals is that phase changes can
involve at least two alternate stable states. This means that once a
threshold of say coral cover is exceeded during a sudden disturbance,
the reef can either shift towards a coral- or algal-dominated state even
if grazing levels remain unchanged. As to whether the reef moves towards
a coral or algal state depends on the initial coral cover, underlying
reef dynamics (including nutrient levels), level of grazing, and
intensity of disturbance. Alternate states occur because of feedback
processes and this means that it becomes progressively more difficult to
restore a reef as its health declines because the negative feedback
loops maintain the algal-dominated state. This process is called
hysteresis and implies that management is more likely to be successful
when applied to reefs in the early stages of degradation rather than
waiting until the reef is fully degraded. There is, therefore, a need
for urgent action.

B) Plots of hysteresis behaviour in reefs are useful if the axes
represent major issues of importance (e.g. coral cover, which is
battered by bleaching, hurricanes, etc) and processes under management
control (e.g. grazing or nutrient flux). The shape of the hysteresis
plot is influenced by the underlying reef dynamics and therefore reefs
in different environments will have different-shaped curves. Placing all
these factors on a single graph is useful because it puts everything
into context - the type of disturbance regime and the ability of
different management interventions to alter resilience. To illustrate
this concept, we applied an example of a disturbance regime to a seaward
forereef and estimated the probability that reefs would retain an
ability to exhibit net recovery. In other words, what is the probability
that a reef of coral cover X and grazing level Y in the year 2007, would
still be able to exhibit recovery in the year 2037 under the specified
disturbance regime?

I hope that clarifies some of the issues but would be happy to continue
the discussion privately.



Professor Peter J Mumby
Marine Spatial Ecology Lab

School of BioSciences
Hatherly Laboratory
Prince of Wales Road
University of Exeter
tel: + 44 (0)1392 263798
fax: + 44 (0)1392 263700
e-mail: p.j.mumby at exeter.ac.uk

Research: http://www.ex.ac.uk/msel
Free video clips of coral reefs: http://www.reefvid.org
Coral Reef Group at Exeter: http://www.ex.ac.uk/celp

-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov
[mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Thomas
Sent: 04 November 2007 02:15
To: coral-list at coral.aoml.noaa.gov
Subject: [Coral-List] Parrotfish, nutrients, and control of algae

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
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
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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