Sydney Harbour’s corals are bleaching

19 April 2016

Coral bleaching has been brought closer to home for Sydneysiders with the discovery that bleaching is also occurring in the unique coral populations of Sydney Harbour.

Initial analysis of the monitoring data, part of a joint research project between marine biologists from the University of Technology Sydney (UTS) and Macquarie University, indicates that at specific locations up to 45 per cent of the corals are currently bleached. The scientists believe this is the first time this phenomenon has been reported for Sydney Harbour.

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,” Madin says.

The Great Barrier Reef is currently undergoing a mass bleaching event surpassing previous records and now PhD candidate Samantha Goyen and Dr Matthew Nitschke from the UTS Climate Change Cluster (C3) have discovered 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 temperate waters of Sydney Harbour. She said there were expectations 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.

“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,’ 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. Dr Matthew Nitschke, from UTS Future Reefs research program notes that the corals on the upper surfaces of boulders were the most severely impacted.

“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. 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 Prof David Suggett, who leads the UTS Future Reefs research emphasises that despite these concerning observations, it is yet unclear how this will impact the Sydney Harbour coral populations in the future. A/Prof Suggett says, “We are optimistic the corals can recover as conditions return to normal.

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

Filed under: Science & nature

Red coral variety bleaching on upper sides of the colony. Credit: Dr Matthew Nitschke

Red coral variety bleaching on upper sides of the colony. Credit: Dr Matthew Nitschke

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