[Coral-List] reassessing coral reefs

Douglas Fenner douglasfennertassi at gmail.com
Tue Apr 21 17:02:52 EDT 2015

     I completely agree that coral growth and reef growth are two very
different things.  I've always tried to make that clear, citing different
rates for the two.  Both are highly variable.  Corals can grow up to at
least 100 mm a year (not 30 mm as some have suggested in the past), but
that applies only to the tips of branches of staghorn corals and the edges
of table corals.  The thickness of the staghorn branches grow much much
slower, as do the thickness of tables (if they increase in thickness at
all).  Massive corals grow much slower, around 5-10 mm a year I believe,
correct me if I'm wrong (still more than current rates of sea level rise),
and encrusting corals hardly add any thickness at all, well less than sea
level rise.
     Thanks for reminding us of the important role of coral rubble in reef
building.  Coral rubble comes from live coral of course, and while the reef
doesn't grow by adding primarily live coral skeletons that die in place and
stay that way, live coral is what furnished the rubble for reef growth (and
calcareous algae added some as well, I'd think).  While reef doesn't grow
as fast as coral, coral is one of the two major contributors along with
calcareous algae, though the relative importance of those two varies
between reefs, reef zones, etc. as far as I understand.  Corals are often
said to be critical for reef development in that they can act as baffles to
keep waves from washing rubble and sand away.  Coralline algae can also be
critical at the reef crest, keeping waves that can smash coral from ripping
the reef apart.
     My reading of Montaggione's article is indeed that there is a wide
range of rate of reef growths, and 1-15 mm is the most common,
Montaggione's abstract says the maximum reached is 20 mm/yr.  I quote from
his abstract, "In the reef zones driven by a "keep-up" mode, mean vertical
accretion rates range at around 6 mm year -1."  Reefs that are at the
surface now and have reef flats are in "keep-up" mode, I would think, since
they have in fact kept up.  He writes that "The reef zones developed
through a "catch-up" mode at
rates of 3–4 mm year- 1."  So to base it on his article, the most common
range is for 3-6 mm reef growth a year, and reefs that are at the surface
and so have been in "keep-up" mode have growth that 'ranges around 6 mm a
year,' not 2-6 mm a year.  That 6 mm is double the rate of current average
sea level rise, and as far as I can tell that applies most directly to
reefs that have shallow reef flats.  The growth rate of individual reefs
varies around the most common, so some will grow slower than 6 mm and some
grow faster.  A minority would be predicted from this to grow slower than 3
mm a year, but a majority would be predicted to grow faster than 3 mm a
year.  What the proportions are will depend on the shape of the
distribution of reef growth rates, I would think.
      I haven't kept close track of it, but I thought that predictions for
sea level rise by 2100 were around 30-60 cm total rise.  That would be
300-600 mm.  But that may be an old prediction that is now out of date.  I
have read exactly as you write, that the understanding of ice melting
processes is quite uncertain, and completely agree that almost on a daily
basis we are hearing of more evidence that ice is melting faster than was
thought and is likely to do more and more melting as time goes on.
      I completely agree that the rate of sea level rise is predicted to
increase in the future.  As long as we continue to add CO2 to the
atmosphere faster than it is removed and CO2 concentrations in the
atmosphere continue to rise (and as far as I know it is, we would have to
completely stop emitting CO2 for it to stop rising, I believe that this
past year was the first in a long time that the *rate* of CO2 emissions
didn't increase other than in a recession), the atmosphere will continue to
trap more and more heat, which will melt ice and warm ocean water that
expands and causes sea level rise.  Lots of complexities in that, I know.
Further, I think that if sea level rise between now and 2100 will average
more than 8-14 mm a year instead of 3 mm a year, it is very unlikely that
next year it will suddenly jump from 3 mm to 8-14 mm increase a year, and
stay that way until 2100.  Rather, the rate of sea level rise will slowly
(in a human lifetime scale) increase from 3 mm a year.  In order to reach
an average of 8-14 mm a year between now and 2100, I think that means it
will have to be well over 8-14 mm a year by the time it gets close to
2100.  By the time it gets close to 2100, it seems highly likely that very
few reefs if any will be able to grow upward as fast as sea level rise,
even if live coral cover were able to stay as high as it is now.
      But I just wrote a very very big "IF".  "IF live coral cover were
able to stay as high as it is now."  All signs are that within the next 3-4
decades, higher water temperatures will be killing a lot of coral during
the high temperature events like the 1998 El Nino.  The big wild card is
the ability of corals to acclimate and/or adapt.  We know that they can do
some acclimation and/or adaptation, there are several studies documenting
that, like the recent Palumbi paper in Science.  It also makes sense, many
characteristics of organisms have variation that is genetically based, and
can be selected for, and organisms can acclimate to a variety of things.
But there are limits to both these processes, in how much the organism can
do, and how fast they can do it.  We don't know what those limits are.
Some of us are hopeful that at least some corals will survive, but it seems
pretty likely that there will be lots of mass mortality in the future of
the type seen in 1998, when around 16% of the world's corals are estimated
to have died.
      If there is a significant reduction in live coral cover on reefs, the
rate at which reefs can grow will surely be decreased, since live corals
are one of the two primary sources of the rubble and sand that builds
reefs.  I think it also means that there will be a lot less corals on reef
flats, no matter what the water depth is on the reef flats.  However, it is
well known that the fastest growing corals are the ones most sensitive to
high water temperatures and other stresses.  So they are most likely to be
killed by high temperature events.  Slower growing corals like massive
Porites, have a better chance of surviving mass bleaching events.  If they
survive, they might increase in cover on reef flats, in the deeper water
there as water levels rise.  They will grow slower than the staghorns.  But
notice the implications of the paper that documented increasing coral
growth rates in Western Australia, and the one by Scott Wooldridge.  As the
baseline temperatures increase, the rate of coral growth increases, until
suddenly it gets over the maximum they can stand, and they bleach and may
die.  So the corals that survive are likely to be growing faster, though
only incrementally faster.
      So while I completely agree about "homo stupidus" damaging reefs, the
increases in baseline temperature are highly likely to INCREASE the rate of
coral growth in the near future (except where the maximum rate has already
been exceeded, as in Thailand), and since the corals are a major
contributor to the growth of reefs, likely increase the growth of reefs as
well.  That faster growth will be a bad sign for coral health, but none the
less, it is faster growth.  So reefs near humans that have all the damaging
effects of sedimentation, nutrients, etc etc will probably loose coral and
reef growth will slow, but reefs that are far enough from humans to have
little such impacts will probably grow faster, except in the areas which
are already hottest.  My understanding is that fishing can easily extend
200 km or more from a coast, since people can easily go in boats, and while
runoff from a continent like Australia can go a long ways out (maybe 100 km
on the GBR), it can't on most small islands, it would usually go less than
1 km I would think.  Of course rising sea temperatures, rising seas, and
acidification affect the whole world oceans.   And there are quite a few of
those reefs that are far enough from humans not to get runoff effects, like
unpopulated atolls and reefs along unpopulated desert coasts, like the west
coat of Australia and perhaps the Red Sea.  All of which is likely to be
temporary, because when the bleaching temperatures are exceeded, then fast
coral growth gets replaced by dead coral.
      I hold out NO hope that any slight increase in coral cover on reef
flats will save coral populations, coral reef ecosystems, or coral reef
growth as a geological phenomenon.  But, we could end up near the end of
the century with a few very tough coral species with high living cover on
reef flats (rather like the massive Porites that dominate the high CO2
reefs Fabricius et al. documented in New Guinea, or the tough massive
corals that survive on Thailand reef flats, pointed out by Richard Dunne),
with lots of water above them, water level rising more rapidly than the
coral can grow and much faster than the reef can grow.  Or, we could end up
with just a few scattered corals on reef flats and on slopes, with reefs
looking pretty similar to what the reefs in Florida look like today.  With
our luck it seems more likely it will be the latter.
      Sorry to be such a pessimist.   Cheers,  Doug

(Trivial point:  8-14 mm a year is not an order of magnitude greater than
2-6 mm a year, 20-60 mm a year is an order of magnitude greater than 2-6
mm.  That is, if my understanding of "order of magnitude" to mean by a
factor of 10 is correct.)

On Mon, Apr 13, 2015 at 8:12 AM, Storlazzi, Curt <cstorlazzi at usgs.gov>

> Dear colleagues,
> I would like to point out two very important things that seem to get lost
> in these discussions about coral reefs and sea level change.....of which
> I'm not an expert....but I see the same confusion happen time and time
> again.
> Simply, individual coral growth rates do NOT equal vertical coral reef
> accretion rates.
> Most coral reefs accrete vertically not by intact coral growth, but rather
> by binding of coral rubble (I've seen numbers on coral cores - and
> colleagues, please correct me here - of 70% to >90% bound coral rubble) by
> a host of different mechanisms such as algae, etc. So high coral growth
> rates (e.g., 100 mm/yr) do not directly translate to reefs accreting at
> such rates. Montaggioni's beautiful review article, based on decades of
> wonderful research by many of our colleagues, show that vertical accretion
> rates (those needed to keep up with sea level) varied during the Holocene
> on the order of 1-15 mm/yr, with averages on the order of 2-6 mm/yr, and
> higher-energy reefs generally having lower rates than those in more
> quiescent locations.
> Now if we're going to discuss this in terms of reefs keeping up with sea
> level, here is my train of thought:
> The IPCC-AR5 report and the 2013 US National Climate Assessment put likely
> bounds on sea-level rise of 80-140 mm by 2100....and please remember, these
> reports clearly acknowledge their lack of fundamental understanding of the
> grounding mechanisms and potential loses of large ice sheets that we're
> starting to see which might push us closer to, if not beyond, those upper
> limits of 140 mm or so. Nevertheless, if we're going to reach 80-140 mm by
> 2100, that equates to sea-level rise rates of more than 8-14 mm/year, which
> are at or above the average values (if not limits) published by
> Montaggioni and
> others. Again, this does not take into the possible (and recently argued,
> likely) loss of large ice sheets.
> Key point, as noted by Denny Hubbard - most all of the those data on reef
> vertical accretion rates were during a period of optimal growth in the
> Holocene when "Homo stupidus" (I'm definitely stealing that term) wasn't
> driving elevated nutrification, sedimentation, contamination,
> acidification, and temperatures that generally tend to reduce coral health
> and both calcification by corals and algae....that play into net reef
> accretion.
> So if the optimal values of net vertical reef accretion before human
> impacts (average 2-6 mm/yr) were at or up to an order of magnitude below
> those proposed for sea level rise during this century (8-14 mm/yr), it is
> not clear to me how reefs will keep up with projected sea-level rise during
> the Anthropocene (which probably is resulting in depressed net accretion
> rates).
> If reefs were able to keep up, one would assume that we would have seen
> evidence of this in places like the Republic of the Marshall Islands, the
> Federated States of Micronesia, Kiribati, Vanuatu, Tuvalu, etc where sea
> level is up to 300 mm higher today than it was 20 years ago (relative
> sea-level rise rates of ~15 mm/yr). Have all of those reefs accreted on the
> order of 120 mm if the average growth of keep-up reefs is 6 mm/yr? I think
> this would be a worthy goal for our colleagues to determine via focused
> high-resolution coring studies.....but I'll openly bet one (1) cold, frosty
> libation at the next ICRS that they have not, likely due to effects related
> to humans.
> Curt
> On Mon, Apr 13, 2015 at 10:27 AM, <coral-list-request at coral.aoml.noaa.gov>
> wrote:
> > ------------------------------
> >
> > Message: 4
> > Date: Fri, 10 Apr 2015 10:41:29 -1100
> > From: Douglas Fenner <douglasfennertassi at gmail.com>
> > Subject: Re: [Coral-List] Reassessing Coral Reef Scientists
> > To: Dennis Hubbard <dennis.hubbard at oberlin.edu>
> > Cc: "<coral-list at coral.aoml.noaa.gov> list"
> >         <coral-list at coral.aoml.noaa.gov>
> > Message-ID:
> >         <
> > CAOEmEkGYX1J+soq5KthJ2mJXfcgmFLCSS1pQwSnE7psmjhujPQ at mail.gmail.com>
> > Content-Type: text/plain; charset=UTF-8
> >
> > Dennis,
> >      I agree with much of what you say.  I was very unspecific about
> what I
> > meant by reef flat, sorry.  I was thinking of reef flats that are very
> > close to the lowest tide levels of the year.  I agree that corals 2-3 m
> > deep can grow without being limited by exposure to air at low tides.
> >      I was trying to make the point that on reef flats that shallow, any
> > corals that grow above the level of the lowest tides of the year will be
> > killed by exposure during those low tides.  They can grow higher than
> that
> > the rest of the year, but will be killed by exposure to air by the lowest
> > tides.  Low tides limit the growth of corals on reef flats, that is for
> > sure, corals die in air (though they can last for perhaps a few hours at
> > most in air).
> >      The two papers I referred to document that when there are longer
> > periods without the lowest tides, coral cover increases on these shallow
> > reef flats.  That makes sense to me, because when the lowest tide occur,
> > they kill corals that have grown too high.  Most of us would see
> increased
> > coral cover as a good thing, and these two papers plus the observation of
> > low tides killing corals on the reef flat, indicate that sea level rise
> > will lead to increased live coral cover on shallow reef flats.  Actually,
> > if the reef flats don't grow upward at all, the effect should be larger
> > than if the reef flats do grow upward, since the water will be deeper as
> > sea level rises, and thus there will be more room for corals to grow.
> That
> > might well lead to more coral cover on shallow reef flats than would be
> the
> > case if the reef flats grow upward.  But the more live coral there is on
> > the reef flat, the faster the reef flat should be able to grow upward I
> > would think, since the corals are the fastest growing calcifiers on the
> > reef flat, the more corals the faster the reef flat would grow upward, I
> > would think.  I don't know if that's been documented.
> >      I don't know how many reefs will manage to keep up with sea level
> > rise.  The article I read on reef growth rates, Montaggioni, 2005, says
> > that average growth of catch-up reefs is 3-4 mm a year, and the average
> > growth of keep-up reefs is 6 mm a year.  Reefs with flats near the water
> > surface presently would be considered keep-up reefs I would think, and
> thus
> > reefs with shallow reef flats would be predicted to grow upward at an
> > average of 6 mm a year, twice the present rate of sea level rise.  That
> is,
> > if Montaggioni's review of the evidence is correct.  Those figures can't
> > apply to reef flats, or else during periods of stable sea level, the reef
> > flat would grow up into the air.  Surely they apply only to reef areas
> > below low tide level.  I believe that he says that there is considerable
> > variation between reefs in the rate that they grow, the 6 mm is only an
> > average.  So a minority of keep-up reefs would grow at less than 3 mm a
> > year.  Corals can clearly grow much faster than both sea level rise and
> the
> > average rate of growth of a reef. Staghorns can grow 100 mm or more a
> year
> > on their branch tips, massive Porites grow around 5-10 mm a year, but
> > encrusting corals probably add very little to their thickness in a year..
> > So it does make a big difference which kind of coral.  If most corals on
> > reef flats can grow faster than present sea level rise, my guess is that
> > higher coral cover with deepening water will lead to a faster reef growth
> > rate.  But if such an effect exists, I don't know how big it might be, I
> > don't know how much faster the reef flat could grow upward, though I'd
> > predict that would depend on how much coral cover there is, and how fast
> > that coral grows.  Local human impacts are indeed likely to slow coral
> and
> > reef growth I would think, but there are lots of atolls with no people,
> and
> > the longest fringing reef in the world, Ningaloo Reef, on the west coast
> of
> > Australia, has almost no human impacts and the coast is a desert so
> little
> > if any runoff.
> >     But my main point that rising sea levels will lead to more coral
> cover
> > on reef flats is supported by those two articles.
> >     Coral growth is encouraged by water motion, up to the point at which
> > skeletons start to break.  Most of the energy of a wave is dissipated
> where
> > the wave breaks, which is usually on the crest.  Thus, reef flats receive
> > much less wave energy than the crest, and don't have the concussion from
> > the falling wave, which I've read is the strongest force breaking
> > skeletons.  For reefs with coral on the crest, increasing wave action on
> > the reef flat should be good for corals, unless they are on unstable
> > substrate such as rubble.  For reefs with only coralline algae on the
> > crest, it might be that waves on the reef flat will break corals during
> the
> > heaviest wave periods, such as during storms.  So for those reefs, I
> don't
> > know what the net effect would be.  Could be that deeper water would
> allow
> > more coral growth between storms, but storms would break the coral.  But
> > for reefs with corals on the crest, increasing waves on the reef flat
> > should help coral growth there.  Unless it is along a coast with lots of
> > terrestrial sediment, which a good number of reefs have, but other reefs
> > like atolls and Ningaloo Reef don't have.
> >
> > Cheers,  Doug
> >
> > Montaggioni, L.F. 2005. History of Indo-Pacific coral reef systems since
> > the last glaciation: Development patterns and controlling factors.
> > Earth-Science Reviews 71: 1-75.
> >
> >
> > On Thu, Apr 9, 2015 at 1:45 AM, Dennis Hubbard <
> dennis.hubbard at oberlin.edu
> > >
> > wrote:
> >
> > > Hi Doug, Regarding your comments on SL rise, this conflates coral
> growth
> > > and reef building. The work of Peter Davies, David Hopley and others
> > > clearly showed that these reef flats broadened after reef caught up
> with
> > > slowed (actually stable or falling) sea level after 8 CalBP. The reefs
> > > built at their fastest rates after initial start-up, but it is unclear
> > > whether this was a response of faster sea-level rise or just the
> > background
> > > accretion rate. In the Caribbean, it is clear that reefs in 20+ m of
> > water
> > > build just as fast as those in 2-5 m of water. Our preliminary analyses
> > of
> > > other data suggest that this is  mimicked in other oceans. To me, the
> > fact
> > > that the depth-related patterns of coral growth is not mirrored by reef
> > > building suggests that coral growth is a very poor proxy for what will
> > > happen as accelerating sea level opens up accommodation space atop reef
> > > flats. Very careful and thoughtful studies have shown that even 20 cm
> of
> > > freeboard atop the reef crest can more than double the wave energy
> > normally
> > > filtered by the reef. Also, increased storm intensity will dramatically
> > > increase export from the ref proper (either across the reef flat in the
> > GBS
> > > and the Indo-Pacific or down-slope in the Caribbean. Reef building is a
> > > complex process and coral growth, while providing the building blocks,
> > is a
> > > very small part of the total budget. Existing data on reef building
> > > suggests that the present rate of sea-level rise is faster than the
> > > Holocene accretion rates of more than half of the reefs where coring
> has
> > > occurred (and this was with plenty of available accommodation space).
> > Also,
> > > we must remember that this was at a time before *Homo stupidus* was
> > > providing the myriad stresses that are common today. To me, it is not
> > > comforting to realize that so many reefs are already lagginf behind is
> > the
> > > most optimistic picture available.
> > >
> > > Best,
> > >
> > > Dennis
> >
> ------------------------------------------------------
> Curt Storlazzi, Ph.D.
> U.S. Geological Survey
> Pacific Coastal and Marine Science Center
> 400 Natural Bridges Drive
> Santa Cruz, CA 95060
> (831) 460-7521 phone
> (831) 427-4748 fax
> Staff web page:
> *http://walrus.wr.usgs.gov/staff/cstorlazzi/
> <http://walrus.wr.usgs.gov/staff/cstorlazzi/>*
> Pacific Coral Reefs:
> *http://coralreefs.wr.usgs.gov/ <http://coralreefs.wr.usgs.gov/>*
> Sea-level Rise and Pacific Atolls:
> *http://walrus.wr.usgs.gov/climate-change/atolls/
> <http://walrus.wr.usgs.gov/climate-change/atolls/>*
> _______________________________________________
> Coral-List mailing list
> Coral-List at coral.aoml.noaa.gov
> http://coral.aoml.noaa.gov/mailman/listinfo/coral-list

Douglas Fenner
Contractor with Ocean Associates, Inc.
PO Box 7390
Pago Pago, American Samoa 96799  USA

phone 1 684 622-7084

"belief in climate change is optional, participation is not."

Politics, science, and public attitudes: What we're learning, and why it
matters.  Science Insider, open access.


Homeopathy ineffective, study confirms.


website:  http://independent.academia.edu/DouglasFenner

blog: http://ocean.si.edu/blog/reefs-american-samoa-story-hope

More information about the Coral-List mailing list