Rising sea temperatures stress corals at Lord Howe Island

Above average sea temperatures throughout the early part of 2010 have led to the first recorded major coral bleaching event at Lord Howe Island, the world’s southern-most coral reef.

Water temperatures have exceeded 26-27 degrees over the last few months, which is a couple of degrees warmer than the usual summer sea temperature, leading to mild to moderate bleaching in some parts of the reef system and almost total coral bleaching in other areas.

The reef lies within the Lord Howe Island Marine Park and is part of the Lord Howe Island World Heritage site.

Southern Cross University researchers have mapped the extent of the bleaching and damage to the corals at the request of the Marine Parks Authority and will return again later this year to assess the rate of recovery.

Professor Peter Harrison, from the University’s School of Environmental Science and Management, has been monitoring the coral reefs off Lord Howe Island since 1993. He is leading a team of researchers including Steve Dalton and Andrew Carroll from the National Marine Science Centre.

“This is the southern-most coral reef in the world with a unique mixture of tropical, subtropical and temperate species,” Professor Harrison said. “This is a significant bleaching event, which is likely to lead to some mortality of these corals.

“This unusual bleaching event is further evidence that climate change is having a very real impact and that even the cooler water, sub-tropical reef systems are not immune to these changes.”

Professor Harrison said the danger for these reefs was that, unlike the Great Barrier Reef, they were geographically and genetically isolated making recovery a much longer process.

“This coral reef is globally significant but largely isolated from potential source reefs that might serve to replenish populations damaged by severe disturbances, and would take decades to recover from a severe disturbance,” he said.

Professor Harrison said two of the major sites affected by the bleaching were within sanctuary zones within the marine park, and noted that research from other tropical reefs showed that areas protected from fishing had better recovery rates from severe coral bleaching episodes.

This recent bleaching event, caused by warm seawater carried south on the East Australian Current, is much larger than the minor coral bleaching event that took place at Lord Howe Island in 1998. The 1998 mass coral bleaching event severely damaged many coral reefs around the world, whereas at Lord Howe Island relatively few coral species became bleached and most corals recovered after the 1998 bleaching period.

Lord Howe Island Marine Park manager Ian Kerr said while some mortality was expected he was hopeful of a good recovery with many of the most severely affected sites within sanctuary zones in the marine park. He said the Marine Parks Authority would continue to do everything to monitor and assess the situation.

“I’ve seen aerial photographs and I’m pleased to report the beauty and uniqueness of the reef is still intact, we remain very concerned about this event and will continue to facilitate the research and monitoring that needs to continue,” Mr Kerr said.

“We don’t expect a full picture to emerge for a while yet, and possibly not for another year. While there’s no doubt sea surface temperatures contributed to this bleaching, local weather in January was unprecedented. It was the hottest, driest, most cloudless January ever recorded. There was also very little ocean swell which led to poor mixing and thus hotter lagoon temperatures.

“Since 2004 no trawling, long-lining or any netting has been allowed in the marine park with nearly 30 per cent of the system protected in sanctuary zones.

“These events show how important marine protected areas are as climate change takes hold and affects the marine environment. The integrity of sanctuaries is very important in the recovery and resilience of these fragile and beautiful reef systems,” Mr Kerr said.

Professor Harrison said his team planned to return to Lord Howe in a few months to complete further surveys and assess the extent of coral and reef community recovery.

Photo: Above average sea temperatures throughout the early part of 2010 have led to severe coral bleaching in some areas of the Lord Howe Island Marine Park. Photo by: Professor Peter Harrison.