[Coral-List] reassessing coral reefs

Mon Apr 13 15:12:32 EDT 2015

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/>*