bleaching on GBR

Anya Salih anya at
Wed Apr 1 09:04:22 EST 1998

Re: More on bleaching on Great Barrier Reef

Between 1-14 March, I was able to join one of the GBRMPA crown-of-thorn
starfish survey cruises to GBR mid-shelf reefs, between Cairns and
Innisfail. At each visited reef I made video and coral spp
cover/bleaching-condition transects, which I'm now analysing. Briefly, the
coral bleaching situation on these reefs was as follows:

- Seawater temperatures (taken in 7-15 m of water at backreef moorings) was
27C to 30C, and mostly 30C throughout the day.

- A variable degree of bleaching was recorded on all reefs; was most
pronounced in shallower water, on reef flats and reef tops (1-3m depth);
and extended to 15m and possibly deeper. The degree of bleaching (complete
vs partial) and the numbers of spp affected, sharply declined with depth.
At shallow depths almost all corals had some degree of bleaching and many
were completely bleached. Deeper (below 2m depth), bleaching was more
pronounced on unshaded coral surfaces while undersides of branches and
other shaded colony parts were still pigmented. This indicates that solar
radiation such as high PAR (and possibly PAR+UV or UV) played a role in
this mass bleaching episode.

- Bleaching was generally less severe in front-reef rather than back-reef
- The degree of bleaching on the 5 surveyed reefs (in a line approx 100 km)
was variable:
1.The northern-most reef surveyed, Elford Reef (at approx. 35km from
shore), was moderately bleached, with most corals showing partial rather
than full bleaching;
2.Coates Reef,in the middle of that line of reefs (and about 43 km from
Mulgrave and Russel Rivers) was the most affected, with approximately 90%
bleaching on reef tops and reef edges; and 60-70% on upper slope in
backreef areas and approx. 70% bleaching of reef top corals in reef-front
 3. Hedley Reef, close to Coates reef (and approx 50km from mainland), less
than 60% bleaching.
4. About 30 km south and approx. 42km from the mainland, Cayley Reef was
severely bleached on reef tops to 3m depth (approx. 80% of corals partially
or fully bleached) and less severely bleached on slopes, with many
Pocillopora damicornis and Porites bommies remaining unbleached.
5. Feather Reef, (about 33 km from Johnstone River) - mostly only partial
bleaching of corals in shallower depths and only a few spp with complete
bleaching (P. damicornis and S. hystrix); and relatively little bleaching
on slopes.

- Some rough estimates of bleaching susceptibilities of some spp:
P. damicornis, S. hystrix and S. pistillata were the mOst affected species,
(70-100% fully bleached); less so, but also strongly bleached (60-90%) were
tabular (A. hyacinthus group) and digital (A. humulis group) acroporiids;
faviids were 50-90% bleached in shallow reef areas and less than 40% on
slopes; massive Porites were only partially bleached, with many unbleached,
while approx. 70% of encrusting poritiids (P.lichen, P. annae) were fully
bleached; very fleshy corals were least affected, e.g. Symphyllia spp less
than 5% bleached.

- Bleaching appeared to be induced by combined effects of elevated
temperature and solar radiation. Low salinity, caused by heavy rainfalls a
few weeks prior to the onset of bleaching, may have contributed to stress,
particularly on inner-shore reefs, but seems unlikely to be the main cause
of bleaching on mid-shelf reefs.



Anya Salih                      Internet:  anya at
Marine Physiology Lab           Telephone:02-93513006 (Zool)
Zoology AO8                               02-93517540 (EMU)
School of Biological Sciences   Facsimile:02-93517682
The University of Sydney
Sydney, 2006, AUSTRALIA

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