Coral bleaching discovered in Sydney Harbour

Red coral variety shows bleaching on the upper sides of the colony. Photo: Matthew Nitschke

Red coral variety shows bleaching on the upper sides of the colony. Photo: Matthew Nitschke

In summary: 
  • Scientists monitoring the temperate corals of Sydney Harbour have observed bleaching effects for the first time: above-average sea-surface temperatures are a major factor
  • At some locations almost half the corals are bleached, mirroring the impact of El Nino on the Great Barrier Reef

The unprecedented coral bleaching recorded on the Great Barrier Reef has now been found in Sydney Harbour. Marine biologists from the University of Technology Sydney (UTS), in a joint research project with Macquarie University, say initial analysis of the monitoring data indicates up to 45 per cent of the corals at certain locations are bleached.

The scientists believe this is the first time the phenomenon has been reported for the unique corals of Sydney Harbour. At the same time, the Great Barrier Reef’s mass bleaching event is surpassing previous records.

PhD candidate Samantha Goyen and Dr Matthew Nitschke from the UTS Climate Change Cluster (C3) discovered the paled coral colonies during routine monitoring at a number of locations in Sydney Harbour.

Goyen’s research investigates coral survival in extreme environments, including the harbour’s temperate waters. She says there have been expectations “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,” Goyen says.

Bleaching occurs when the coral experiences physiological stress for extended periods. Sea-surface temperatures that rise above the norm for the season are widely recognised as a major contributing factor to the bleaching response.

In comparison to the corals of the Great Barrier Reef, Sydney Harbour’s 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’,” Goyen says.

Dr Nitschke, from the UTS Future Reefs research program, says the corals on the upper surfaces of boulders have been most severely affected.

Plesiastrea versipora bed with both colour varieties in the process of bleaching. Photo: Matthew Nitschke Plesiastrea versipora bed with both colour varieties in the process of bleaching. Photo: Matthew Nitschke

“Often we see that bleaching is the result of a synergistic effect of extreme water temperatures combined with high light levels, which further intensifies the stress,” Dr Nitschke says.

“While some corals will bleach when either of these two conditions happen, some resilient species only bleach when the interplay of these two stressors tips them over the edge.”

Associate Professor Joshua Madin, head of the Quantitative Ecology and Evolution group at Macquarie University, has been monitoring these corals since 2010 as part of a project aimed at understanding the migration of tropical corals down the NSW coastline.

“To our knowledge, bleaching like this has never been observed in Sydney Harbour corals. 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,” A/Prof Madin says.

Associate Professor David Suggett, who leads the UTS Future Reefs research program, says that despite these concerning observations, the longer term impact on Sydney Harbour’s coral populations is unclear.

“We are optimistic the corals can recover as conditions return to normal,” Associate Professor Suggett says.

“However, we believe this serves as a good example of why we need to understand how organisms like corals, which are at the frontline of climate change, survive extremes and persist at the edge of their ranges.”

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