coral reefs - calcification and bioerosion
buddrw at kgs.ukans.edu
Mon Oct 1 10:36:40 EDT 2001
Christine's comment raises some points that relate back to Mike's comments and
the whole issue of CO2 and carbonate balance. It is important to distinguish
between net and gross bioerosion and among the various functional components of
1. chemical erosion, which returns solid carbonate to dissolved inorganic
carbon and is the only kind that is directly involved in CO2 and acid-base
2. mechanical/physical erosion, which reduces the integrity and grain size of
solid features (of greatest concern, reef plates and lithified substrate), and
which can have two different outcomes:
a. change in the structure, relief, and distribution of grain sizes on the
reef itself; or
b. loss of carbonate material from whatever we choose to define as the reef
The two forms are related -- a minor amount of chemical erosion can precipitate
physical breakup on a much larger scale, and smaller grains resulting from
mechanical (bio)erosion have a higher surface-to-mass ratio that facilitaties
dissolution, especially in porewater environments.
I assume that discussions of the balance between production and bioerosion are
referring to a gross balance that includes all forms of bioerosion -- if not,
straighten me out on the conventions in the field, please.
Note that I'm using 'grain' in the geographic sense of granularity, not in the
colloquial sense of 'something small.'
All of these, plus the related issue of import of carbonate from elsewhere to a
specific reef system, are aqddressed in conceptual models presented by Kleypas,
J.A., Buddemeier, R.W. and Gattuso, J.-P., 2001. Defining 'coral reef' for the
age of global change. International Journal of Earth Sciences, 90: 426-437.
I hope this clears up the point Mike addressed about carbonate models that do
or do not include bioerosion. A carbonate budget model of a reef system has to
include bioerosion, but a calcium carbonate production or calcification model
addresses the gross input to that system. The CO2-caclification models are
production models, not total budget models, which require local/regional inpout
and calibration, as suggested in the reference given above.
> Dear list,
> just a few comments on Mike Risk's latest letter, from a bioeroding sponge
> worker's point of view:
> >they have all come up with the same answer: on "normal" reefs,
> >bioerosion and calcification are in approximate balance. On most fringing
> >reefs, subject to increasing terrestrial nutrient input, therefore, the
> >balance has already been shifted towards destructive processes.
> This matches my own experiences when working on the Central Great Barrier
> Reef, where the balance may still be better than most other places. We
> still need to keep an eye on it though.
> The common sponge Cliona orientalis reacts to elevated nutrient conditions.
> _Extreme_ situations may have negative effects, however, so that the
> sponge's growth is slowed. Bioerosion of this sponge appears to be enhanced
> by a higher concentration of nutrients. This is a sponge, which is just
> everywhere on Australian (and other Pacific) inshore reefs, which grows
> over large surfaces, several m in diameter and which is able to invade live
> Another thing I would like to mention: this sponge also contains
> zooxanthellae, as do some other successful, competitive bioeroding sponges.
> Cliona orientalis bleaches under extreme conditions (evidence from the
> aquarium), but during the 97/98 bleaching on the GBR all sponge colonies I
> knew survived just nicely (in contrast to most corals on my sample site).
> Revisiting my site at Orpheus Island end of 2000 showed me a reef much
> reduced in live coral cover and coral diversity, but the bioeroding sponges
> did very well and seemed much increased in their abundance (no
> quantification done).
> Just some food for thought...
> Cheers, Christine
> Dr. Christine Schönberg, PhD
> Dept. of Zoosystematics & Morphology
> Fachbereich 7 - Biology, Geo- & Environmental Sciences
> Carl von Ossietzky University Oldenburg
> 26111 OLDENBURG
> ph +49-441-7983373
> fax +49-441-7983162
> email christine.schoenberg at mail.uni-oldenburg.de
> internet http://www.uni-oldenburg.de/zoomorphology/Whoiswho.html
> For directions on subscribing and unsubscribing to coral-list or the
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Dr. Robert W. Buddemeier
Kansas Geological Survey
University of Kansas
1930 Constant Avenue
Lawrence, KS 66047 USA
Ph (1) (785) 864-2112
Fax (1) (785) 864-5317
e-mail: buddrw at kgs.ukans.edu
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