[Coral-List] Algae and nutrients and herbivory in oligotrophic waters

Paul Hoetjes phoetjes at gmail.com
Mon Jun 23 11:42:30 EDT 2008

   From  some  of  the  messages  posted on this subject I am getting the
   impression  that  the  distinction  between oligo- meso- and eutrophic
   really  isn't  that important. It is all the same as long as there are
   enough  grazers because that will make the reefs all right. I'm sorry,
   but that seems like a gross missing of the point.
   Can   someone   perhaps  explain  the  tresholds  between  oligo-  and
   mesotropic  and  between  meso- and eutrophic? And can someone tell me
   whether   reefs   can  actually  flourish  under  meso-  or  eutrophic
   situations?  I  have  always  understood that for coral reefs you need
   oligotrophic  circumstances.  That  would  seem  to  indicate that any
   nutrient loading would inevitably lead to circumstances unsuitable for
   coral reefs.
   Or  is  oligotrophic such a broad term that it can absorb the nutrient
   inputs  from sewage and waste water from extensive coastal development
   and   fertilizer   run-off  from  sugarcane  fields  (e.g.  Caribbean,
   including  prime  example  Florida),  from  hog and chicken farms, and
   other unbridled and unsustainable urban/agricultural development?
   It  would  be  really  great  if we could stop all that damage just by
   prohibiting  all  fishing  and restore the grazing fish stocks, but it
   would  probably  be  better  if we just acknowledged and document that
   damage  and  work  on  stopping  the  major  causes  of  it. Perhaps a
   Caribbean  or  even  world  wide  program  to  monitor  (oligotrophic)
   nutrient   levels,   sources,  and  corresponding  coral  reef  health
   (including  fish  populations)  should  be undertaken, in the same way
   that we are monitoring coral bleaching. The problem is at least of the
   same scale!
Paul C. Hoetjes
Senior Policy Advisor
Department of Environment & Nature (MINA)
Ministry of Public Health & Social Development (VSO)
Schouwburgweg 26
Curaçao, Netherlands Antilles
tel. +(599-9)466-9307; fax: +(599-9)461-0254
e-mail: [1]paul at mina.vomil.an
-- [2]http://mina.vomil.an --

   On 6/22/2008 9:47 AM, John Bruno wrote:

MACROALGAL COVER IN THE FL KEYS:  Dear Chip,  Macroalgal cover in the
Florida Keys is only about 10%.  A recent paper based on CREMP
monitoring indicates that it was 9.6% in 2000 (Maliao et al. 2008,
Marine Biology).  My own unpublished meta-analysis based on CREMP data
plus a variety of other sources indicates it was "recently" 12.2 ±
0.4% (n=1048 quantative reef surveys performed between 1996 and 2005)
and macroalgal cover exceeded 50% in only 2.5% of these surveys.  This
is substantially lower than the average in the greater Caribbean of
roughly 20% and identical to the Indo-Pacific mean.

Nobody knows what the historical baseline (i.e., the subregional
average, not the value on a single undisturbed reef) of macroalgal
cover was in the Keys (or anywhere else), but I doubt that it was any
lower than 5%.  So we may have seen a rough doubling of macroalgal
cover, but we are very far from a state that could rationally be
described as "macroalgal dominated" or "little more than rubble,
seaweed and slime".

Billy Causey and his team are in my view some of the world's most
successful reef managers; the quantitative monitoring data indicates
that they have been very successful in managing the major threats to
reefs that they are capable of mitigating.  They obviously cannot
prevent climate change and coral disease outbreaks, but they have done
a good job at managing for low-ish macroalgal cover (which will
ideally, at some point facilitate coral recovery).  Macroalgal cover
the GBR is about 7% (based on AIMS monitoring), but those reefs are
10s-100s of km offshore, i.e., very isolated and naturally
oliogotrophic compared to the Keys, and didn't suffer the virtual
extinction of their key grazer or their dominant coral species as the
Keys did due to regional epizootics.  All things considered, at least
in terms of macroalgal cover, the Keys are in relatively good shape.
Lets give the Keys management team - and all the other successful
local reef managers - some credit, base our arguments and discussion
on the facts, and focus on the real threats to reefs.

TOP-DOWN VS. BOTTOM-UP:  Dear Imam, assuming that like most of us you
are not swayed by the ideology and anecdotes you have been reading on
the list in response to your query, I'd start with Alina Szmant's
authoritative review (2002 Estuaries) of the published science on
nutrient and grazer control of macroalgal biomass on reefs.  You might
also look at Idjadi et al (2006, Coral Reefs), which documented the
immediate loss of macroalgal cover (to 6%) as soon as Diadema returned
to the scene on a reef purported to be one of the most eutrified in
the world.  There are a slew of other peer-reviewed papers that
document similar removal of macroalgae on reefs widely described as
highly eutrified once grazer populations recover (e.g., Edmunds and
Carpenter 2001 PNAS, Carpenter and Edmunds 2006 Ecology Letters, Mumby
et al. 2006 Science, etc.).  Also see Williams and Polunin (2001 Coral
Reefs); a very powerful and important study that documented the
striking negative relationship (r2=0.89) between fish biomass
(particularly Scarid biomass) and macroalgal cover.   Finally, if you
expand your search beyond the coral reef world, you'd find that in
estuaries and in temperate and cold water benthic systems, urchins and
other grazers are easily able to control macroalgal production that is
far greater, under nutrient concentrations many orders of magnitude
higher than those ever seen on reefs.  Just think about Jim Estes'
work on Pacific otters-you remove them, urchin populations explode,
macroalgae disappears and what remains is an "urchin barren"; and this
occurs in upwelling systems with lots-o-nutrients.  This was also seen
in the Gulf of Maine after Cod were removed by fishing (at least
before people started harvesting urchins, then the macroalgae all came
back); see Steneck and Carlton's and Duffy and Hay's very nice reviews
of all this the Marine Community Ecology book (eds. Bertness et al
2001, Sinauer-http://www.amazon.com/Marine-Community-Ecology-Mark-Bertness/dp/0
.  And look at the recent review by Heck and Valentine (2007 Estuaries
and Coasts) which outlines all the evidence for top down control in
estuaries around the world.

If you stick to the hard science, the answer to your question is
fairly clear - at least as clear as anything gets in ecology.  Let me
know if you have any trouble getting any of these papers.



John Bruno
Associate Professor
Depts. of Biology and Marine Sciences
UNC Chapel Hill

Dear Imam,
 The relationship between nutrients and algal growth is well
established, and the influences of herbivory on algal growth and
cover have also been demonstrated.  However, as Dr. Goreau stated,
the lack of successful integration of nutrients and herbivory in any
of those studies has contributed to a disconnect.  In Florida, where
a large coastal population has resulted in mesotrophic and more
typically eutrophic coastal waters, we have a high % of algal cover
and biomass.  Add the facts that we do not fish for herbivorous
fishes, and that we have removed a fair % of their predators; it
follows that Florida would have an increase in herbivorous fishes,
and therefore our relative herbivory.  But we are still plagued by
macroalgal dominance, losses of coral and Harmful Algal Blooms. This
suggests that nutrients are indeed an important factor shaping the
algal community.  Hatcher and Larkum (1983, JEMBE 69, pp61-84)
compared grazing and nitrogen concentrations on One Tree Reef in
 Australia and found both grazing and nitrogen were important in
limiting algal growth.


Rex "Chip" Baumberger
Biological Scientist, FAU
Marine Nutrient Dynamics Dept.
Marine Science Division
Harbor Branch Oceanographic Inst.
5600 US1 North
Fort Pierce, FL 34946
772-465-2400 x398

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