coral reefs - calcification and bioerosion

[christine.schoenberg]

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
coral.

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
GERMANY
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


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