[Coral-List] Coral diseases and algae

John Bruno jbruno at unc.edu
Wed Feb 25 18:48:58 EST 2009

Dear Jack,

A few clarifications to your comments about our paper:

> Date: Mon, 23 Feb 2009 17:39:31 -0500
> From: "Jack Sobel" <jack_sobel at verizon.net>
> Subject: Re: [Coral-List] Coral diseases and algae
> Gene Shinn's initial posting on this topic and the responses from  
> Alina
> Szmant and Tom Goreau to it inspired me to acquire and read the recent
> article titled, "Macroalgae Has No Effect on the Severity and  
> Dynamics of
> Caribbean Yellow Band Disease." (Ivana Vu et. al, 2009, published in  
> PloS
> Feb 09 Vol 4 Issue).  While I agree that it is a worthwhile  
> contribution to
> this subject and to the literature, it is hardly a refutation of the
> well-documented concept that removing fish and invertebrates that eat
> macroalgae, can lead to proliferation of macroalgae, which in turn  
> can lead
> and/or contribute to a decline in coral health.

I totally agree. But since you read the paper, you know we didn't  
claim to refute such a broad and practically untestable hypothesis.

> Despite the unfortunate title of this article, it provides no proof  
> that
> fish and invertebrate removal, resulting in macroalgal  
> proliferation, is not
> a contributor to the decline in coral reef health.

Sir, you are correct.  But again, we never claimed otherwise.

> Nor does it even prove
> that such removal plays no role in Caribbean yellow band disease (YBD)
> outbreaks or proliferation, let alone any other disease impacts.

Not being a scientist, you understandably might not know this, but no  
experiment can "prove" anything.  Scientists try to disprove  
hypotheses.  In this case we attempted to disprove the hypothesis that  
macroalgae measurably affect several metrics of yellow band disease  
severity in Montastraea.  All of our experiments and results suggested  
this was not the case, i.e., we could not reject the null hypothesis  
of no effect of macroalgae.

> Tom
> Goreau's prior response touched on some of the evidence for links to  
> other
> diseases.

With due respect to Tom, there is exactly one published paper (Nugues  
et al 2004) that supports a mechanistic link between macroalgae and  
coral disease and it has nothing to do with fishing and DOC (the  
species of algae that the experiment suggested acts as a vector is not  
controlled by herbivorous fish).  The papers Tom cited (Kuntz et al  
2005, Kline et al 2006, Smith et al 2006) did not measure anything  
about infectious disease as a response variable.  They simply measured  
mortality.  No disease was detected and the studies were not about  
infectious coral disease.

>  However, even with respect to YBD, there are limits to what you
> can conclude about the role of fishing based on this study.

Certainly.  And as you stated in your post, I think we made that  
abundantly clear.  So why the beef? (slow day on CSPAN?)

> In their study, they showed that although the algal treatments  
> increased DOC
> in the immediate vicinity of the algal packets, this increase was  
> localized
> and could not be detected nearby at the corals being studied.  Can  
> we rule
> out a DOC mechanism, if the treatment did not raise DOC around the  
> corals?

Well in this series of experiments, yes we can.  The big problem is  
that nobody has any idea how much macroalgae, under what flow and  
grazing conditions, etc, is required to boost DOC and what the spatial  
and temporal extent of the effect is.  Ours was, remarkably, the first  
study to demonstrate that macroalgae even effect DOC concentration on  
a reef.  This is very obviously a gaping hole in the FAD hypothesis.   
It is really hard to design a relevant field experiment to test the  
FAD when nothing is known about the real world links between fish,  
algae and DOC.  For example, I think it is equally likely that fish  
and grazing would increase DOC by damaging algae and promoting known  
DOC leaking groups like turf algae.  At this point we just don't know.

> Similarly, with respect to "the role of benthic macroalgae acting as
> pathogen reservoirs or vectors", the experimental design may have also
> limited the ability to pick up such a role by (1) preventing contact  
> between
> the macroalgae and corals and

Well, again, we pointed this out.  The purpose of the study was not to  
test the role of macroalgae as vectors.

> So, I think I would agree with Gene that additional research is  
> needed in
> this area,

Yes, clearly.  This is a list of outstanding questions I made in my  
post on ClimateShifts (http://www.climateshifts.org/?p=1120) about the  

There are several unresolved issues though.  First, could other algal  
species or other combinations of algae cause or exacerbate yellow band  
disease? What about other host species and other coral disease  
syndromes?  Could there be other unexplored factors that are also  
necessary for infection to occur (e.g., high temperature)?  And how is  
the concentration of relevant forms of DOC related to fish, fishing  
and algae?   All of these questions remain unanswered.  Leaving lots  
of room for future field experiments by ambitious students.

> but I think it would be premature to conclude that removing fish
> and invertebrates from coral reefs is not having an impact on coral  
> reef
> health,

Brilliant deduction.  I assume you'd acknowledge that we didn't  
suggest otherwise.  Our conclusion was that: "within the context and  
duration of our three experiments, our results suggest that macroalgae  
has no effect on the severity and dynamics of CYBD. Therefore,  
limiting macroalgae is unlikely to reduce the prevalence of CYBD and  
possibly of other important coral diseases."

John F. 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

Kline, D. I., N. M. Kuntz, M. Breitbart, N. Knowlton, and F. Rohwer.  
2006. Role of elevated organic carbon levels and microbial activity in  
coral mortalty. Marine Ecology Progress Series 314:119-125.
Kuntz, N. M., D. I. Kline, S. A. Sandin, and F. Rohwer. 2005.  
Pathologies and mortality rates caused by organic carbon and nutrient  
stressors in three Caribbean coral species. Marine Ecology Progress  
Series 294:173-180.
Nugues, M. M., G. W. Smith, R. J. Hooidonk, M. I. Seabra, and R. P. M.  
Bak. 2004. Algal contact as a trigger for coral disease. Ecology  
Letters 7:919-923.
Smith, J. E., M. Shaw, R. A. Edwards, D. Obura, O. Pantos, E. Sala, S.  
A. Sandin, S. Smriga, M. Hatay, and F. L. Rohwer. 2006. Indirect  
effects of algae on coral: algae-mediated, microbe-induced coral  
mortality. Ecology Letters 9:835-845
Vu, I., G. Smelick, S. Harris, S. C. Lee, E. Weil, R. F. Whitehead,  
and J. F. Bruno. 2009. Macroalgae has no effect on the severity and  
dynamics of Caribbean yellow band disease. . PLoS One 4:e4514.

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