Unusually cold weather on the southern Great Barrier Reef has triggered localized bleaching on the reef crest at Heron Island. Low tide, strong winds and unusually cold wintry weather in July 2007 has caused the top few centimeters of coral in the intertidal sections of the reef to bleach. Cold bleaching has been reported before at Heron Island by others during 2003, and is very similar to bleaching that occurs when water gets too warm.
So, is this event a sign of global climate change?
I don’t think we have enough evidence to say this right now. Some models, however, suggest that the southern Great Barrier Reef may experience colder winters with a weakening if the south Pacific gyre, which runs down the east coast of Australia and normally pushes warm water southward. Certainly, colder years tend to follow strong El Nino (warm) years. So far we have seen winter bleaching on the southern Great Barrier Reef in 1999, 2003 and now in 2007. In the three cases, the preceding years 1998, 2002 and 2006 were very warm years and saw extensive coral bleaching on the southern Great Barrier Reef.
Perhaps a passing meteorologist could work this one out for us. If there is a connection to climate change, then we might need to consider the effect of corals stressed out in summer that are then stressed again in the following winter.
I would like to inform you and your readers about a cold water bleaching event that occurred at Ningaloo Reef, Western Australia in July 2006: During mid July 2006, at Ningaloo Reef, the combination of unusually cold air temperatures and aerial exposure of corals due to a low spring tide and a high pressure system appeared to cause bleaching of shallow-water corals. Observations made during an aerial survey indicated that bleaching had occurred along the entire length of the Ningaloo Reef. This is the first major coral bleaching event to be observed at Ningaloo Reef to date. The most severe bleaching was recorded at a southern back-reef site (Pelican Point) where approximately 83% of live hard coral was bleached. Bleaching was mainly restricted to shallow-water corals of back-reef and patch reef environments which were dominated by plate and corymbose acroporids. To investigate the recovery of bleached corals two surveys were undertaken at Coral Bay (mid Ningaloo)and Pelican Point (southern Ningaloo). At Coral Bay 95% of coral had recovered by 14 weeks after the event and 100% of coral had recovered at Pelican Point 30 weeks after the event.
We are keeping a close eye on the reef, should a similar event occur this winter. As you mention in your 2004 Coral Reefs Reef Site, the combination of both winter and summer bleaching occurring in one year at the same reef may result in an increased threat to corals and should be considered among the climate change projections for coral reefs at some locations.
Marine Science Program
Department of Environment and Conservation, Western Australia
Very interesting. Thank you for the report Shannon. I am interested in the fact that “this is the first major coral bleaching event to be observed at Ningaloo Reef to date”. Do you have any thoughts on why that might be (i.e. the lack of bleaching in this region)?
Yes, very interesting. Perhaps the Ningaloo region experiences higher variations in seawater temperature than reefs elsewhere in the world and Ningaloo corals are adapted to cope with these variations. Sea temperature appears to vary considerably at Ningaloo Reef between seasons and even between days. For example between March 2001 and May 2001 sea temperatures at Ningaloo ranged from approximately 31.5 to 18.3*C respectively. I have not compared these variations with records from other fringing reefs from similar latitudes, perhaps someone can comment. Presumably the warm pole-ward flowing Leeuwin current, strongest in autumn and winter, would help to prevent cold water bleaching of corals during winter. During summer, the northward flowing Ningaloo Current transports relatively cool water along the coast, thereby forcing the Leeuwin Current to flow further offshore than it normally does. Perhaps the Ningaloo Current helps to prevent warm water bleaching of corals during summer. The possibility that Ningaloo Reef may be more resistant to bleaching than reefs elsewhere in the world highlights the need for careful conservation management to maintain Ningaloo Reef as a coral reef refugia and reference site of local, regional and international significance.
Interesting possibilities. The fact that the reefs off Ningaloo are well-flushed might also be a significant factor. The areas of the Great Barrier Reef that show the repeatedly show impacts are mostly inshore where waters can pond and can remain very still for days, heating very rapidly beyond thresholds for bleaching. Corals on these reefs should be like Ningaloo though – having adapted over long periods of time to the inherent variability in the temperature regime. Intriguing.
Ove, I am interested in your larger vision for projects on the Great Barrier Reef. I understand time is short.