Coral pales in shifting climate
Coral bleaching has been discovered in Sydney Harbour, while the Great Barrier Reef worsens.
Initial analysis of a large scale marine research project has found that up to 45 per cent of the corals at specific sites have been bleached.
“To our knowledge, bleaching like this has never been observed in Sydney Harbour corals,” says Associate Professor Joshua Madin from Macquarie University.
“Where we normally see corals here with vibrant hues ranging from iridescent green to a reddish-bronze, many of them are now showing clear signs of bleaching.”
Authorities first discovered the paled coral colonies during routine monitoring of the temperate waters of Sydney Harbour.
PhD candidate Samantha Goyen says she expects that “the El Nino event occurring across the Pacific could result in an unusually warm summer and consequently warmer waters.”
“Although these corals are specialists in cooler waters, what we didn’t expect was to see such a rapid change in their physiology. They appear to have bleached in a matter of weeks,” she said.
“In comparison to those of the Great Barrier Reef these coral populations are little studied. Scientifically there is still so much we don’t know about these corals considered to be living in an already ‘extreme environment’.”
Analysis of the Sydney Harbour data continues.
Meanwhile, in Queensland, scientists have gained a greater view of coral bleaching that is unfolding on the Great Barrier Reef.
“We have now flown over 911 individual reefs in a helicopter and light plane, to map out the extent and severity of bleaching along the full 2300km length of the Great Barrier Reef,” says Professor Terry Hughes, convenor of the National Coral Bleaching Taskforce that is documenting and studying the event.
“Of all the reefs we surveyed, only 7 per cent (68 reefs) have escaped bleaching entirely. At the other end of the spectrum, between 60 and 100 per cent of corals are severely bleached on 316 reefs, nearly all in the northern half of the Reef.”
“We’ve never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once.”
Underwater divers have confirmed the accuracy of the aerial surveys, and are continuing to measure the ongoing impact of the bleaching.
“The bleaching is extreme in the 1000km region north of Port Douglas all the way up to the northern Torres Strait between Australia and Papua New Guinea,” says Prof Andrew Baird from the ARC Centre of Excellence for Coral Reef Studies, who has spent the past 17 days at sea.
“Tragically, this is the most remote part of the Reef, and its remoteness has protected it from most human pressures; but not climate change.
“North of Port Douglas, we’re already measuring an average of close to 50 per cent mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90 per cent.
“When bleaching is this severe it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return.”
More modest bleaching is now reaching its peak in a 600km central band of the Great Barrier Reef, between Cairns and Mackay.
Reefs further south appear to have escaped damaging levels of bleaching because water temperatures there were closer to the normal summer conditions over recent months.
“The severity of bleaching in the central section is less, and closer to the intensity of the first two mass bleaching events on the Barrier Reef, in 1998 and 2002,” says Prof Hughes.
“Thankfully, many of the corals there are more moderately bleached, so we expect that most of them will survive and regain their normal colour as temperatures drop over the coming months.”