While the GBR has been identified as the best-protected and
best-managed coral reef ecosystem in the world, it is still vulnerable
to the global ‘enhanced greenhouse’ phenomenon. Although average water
temperatures of the GBR have increased only 0.5°C since the late
1800s, the GBR suffered major bleaching events in 1998 and 2002 linked
to unusually warm waters in summer.
Bleaching thresholds vary along the GBR but the threshold for coral
mortality is approximately 1°C higher than the bleaching point.
| Hydrodynamic
processes play an important role and have been shown to make some
areas consistently cooler and therefore less susceptible to
bleaching. An understanding of this spatial variation using
satellite sea surface temperature (SST) data (in collaboration
with NOAA) enables scientists to predict areas most at risk of
warming and possible bleaching.
Studies on massive coral cores from inshore and mid-shelf reefs
revealed growth hiatuses coinciding with major bleaching events in
1998 and 2002. These appear unprecedented in coral growth
histories over the past several centuries. Some coral species may
be able to respond to warmer waters by changing their algal
symbionts to more thermally tolerant species. This is being tested
as the possible cause for this change in growth associated with
the bleaching events. |
Investigations are being conducted
into the capacity of corals to acclimatise to warmer water. Some
colonies possess the ability to change the balance of
zooxanthellae type in their tissues, leading to inceased thermal
tolerance.
Photo: R. Berkelmans
|
Modelling of future impacts suggest that a 1-3°C increase in GBR
water temperatures, predicted for the end of this century, would
result in ~80-100% bleaching of the GBR, compared with ~50% in 1998
and 2002. To maintain current levels of hard coral cover on the GBR
corals would need to increase their heat tolerance limits by 0.1°C per
decade. Even with adaptation, this may not help them acclimatise fast
enough to match the most conservative projected temperature rise.
