[Coral-List] Acidification & Boring Erosion

[Dennis Hubbard]

I'm not sure that there is a definitive vote on bioerosion vs
acidification, nutrient levels or a host of other things associated with
anthropogenic stress. However, wherever one comes down in those arguments,
it seems obvious that the more open space you have - the more bioerosion
will occur - unless someone has come up with some sort of systems-level
limit on this process. Alvarez-Filip has already documented "flattening" of
Caribbean reefs. If we look at the coral-cover data from Gardner, Hughes
and other folks who have synthesized such things in the Caribbean, it is
surprising how the high coral cover was in the years over which this change
was calculated. It's hard to put your finger on a point in those long-term
record when the carbonate budget went negative, but I wouldn't be surprised
if that happened when coral cover was 20% or even a little higher. This
seems a bit counter-intuitive when we think about reefs two-dimensionally
as we tend to do in quadrats and photo/video surveys.

One factor we've been ignoring to varying degrees is rugosity. There are
lots of cryptic spaces that we don't count when we do reef surveys - yeah,
we all try to drape the tape down into the holes, but....... Since Caroline
Rogers started advocating chain transects, "typical" values of 1.5 - 2.5
have been common. However, we recently did a study in the USVI where the
reefs are dominated by *M. annularis* (the "Louisville Slugger" morphology)
and the total bioerosion (per sq-m) was an order of magnitude higher down
in the crypts than it was up on top where we all count things. We have some
pretty good guestimates of rates because we know when these colonies died
and have growth data for them before that and it looks like the rates are
also higher - until perhaps they hit some threshold density of boring.

Anyway, my point is that rugosity rates of 5-10 don't seem too unrealistic
for these really complex colonies and the reefs they form. If we take that
into consideration, reefs will go negative a LOT earlier than we might
think as coral cover drops - more there after we've crunched the numbers.
And, this weakens colonies and encourages storm damage - flattening reefs
further.

Those of us who try to actually put numbers on some of these things on a
systems level (and at geological time scales) often find disturbing
disconnects between survey-based budgets and what cores through those same
reefs tell us. Our data from St. John indicate that the reef should be a
lot thicker than it actually is. The production values we're getting today
with 10-15 percent cover are probably a LOT higher than what was there
throughout the late Holocene. Nevertheless, the reef isn't as thick as the
budget numbers tell us they should be. Short message.... we're pretty
clever in how we measure things but not particularly smart about how we
think about them.

So, while I can't say much that is definitive about this, the topic really
excites me. I think we understand less than we think we do about carbonate
cycling and going back to the basics will be a humbling but useful
exercise. If bioerosion is the elephant, scaling from what you see on the
surface  to what's really going on inside (and through time) is the blue
whale.

Dennis


On Fri, Dec 7, 2012 at 11:21 AM, andrew ross <andyroo_of72 at yahoo.com> wrote:

> List,
> With discussion about acidification related reef erosion is there an
> elephant in the room in boring organisms? Admittedly my experience is
> primarily in very broken systems, but for me it's common to be able to tear
> chunks of hard reef away with my hands. When I look inside the hole I've
> made I see little but sponge, plus scrambling cryptic beasties. This has
> been my observation in obviously "enriched" locations like Montego Bay
> (Jamaica) and on less obvious sites such as St. Mary to the East, where our
> recent visitor Hurricane Sandy demonstrated the issue on a much larger and
> messier scale. I found this condition particularly notable when I could not
> break bits off at the more remote Pedro Banks this past spring.
>
> Can we expect that boring erosion be increased under a reduced ocean pH?
> Can we expect this to occur additively, or might these processes work in
> synergy?
>
> Should this be part of our climate change adaptation planning?
>
> Andrew Ross
> UWI (Mona)
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> Coral-List at coral.aoml.noaa.gov
> http://coral.aoml.noaa.gov/mailman/listinfo/coral-list
>



-- 
Dennis Hubbard
Dept of Geology-Oberlin College Oberlin OH 44074
(440) 775-8346

* "When you get on the wrong train.... every stop is the wrong stop"*
 Benjamin Stein: "*Ludes, A Ballad of the Drug and the Dream*"