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Warming seas causing massive die-off of Australia’s reef forests

  • Before 2011, kelp forests covered more than 70 percent of the rocky southern reefs.
  • In just two years, the 2011 heatwave killed around 43 percent of kelp forests on the west coast.
  • The worst hit, according to the study, are the kelp forests in the reef’s north-western tip that have still not shown any signs of recovery.

Rising sea temperatures are destroying corals in Australia’s famous Great Barrier Reef. But warmer-than-normal waters are also damaging the Barrier Reef’s lesser known cousin, the Great Southern Reef, a new study has found.

The massive Great Southern Reef, a temperate rocky reef straddling the southern half of the continent, is dominated by kelp forests (Ecklonia radiata) — highly productive communities of large brown seaweed — just as corals dominate the Great Barrier Reef.

A marine heatwave that hit the region in 2011 has wiped out hundreds of kilometers of kelp forests in the Great Southern Reef, researchers have discovered. And five years after the heatwave, the kelps show no signs of recovery, according to the study published last week in Science. This has conservationists worried.

“This is a strong reminder that temperate marine ecosystems are also under pressure from ocean warming and marine heatwaves, and that we are likely to see substantial changes in these valuable ecosystems in the future,” lead author Thomas Wernberg of the University of Western Australia in Perth, told Mongabay.

Rocky reefs in Kalbarri before (left) and after (right) the 2011 marine heatwave, which wiped out extensive underwater forests of Australian kelps (Ecklonia radiata). Photo by Thomas Wernberg.

Before 2011, kelp forests covered more than 70 percent of the rocky southern reefs. But the 2011 heatwave, which raised ocean temperatures by more than two degrees Celsius above normal, exposed the kelps to the highest temperature in recorded history, far above their tolerance limit. In just two years, the heatwave had killed around 43 percent of kelp forests on the west coast, the team found.

The worst hit, according to the study, are the kelp forests in the reef’s north-western tip.

“We did not observe any signs of recovery in the northern kelp forests, a stretch of around 100 kilometers from Kalbarri even though the high temperatures have now abated,” Wernberg said.

Instead, Wernberg’s team observed that the cool water kelps and fishes are being replaced by seaweed turfs and herbivorous fish typically found in tropical and subtropical waters.

This is troubling, Wernberg said, because the turf and the grazing herbivores now prevent recovery of the kelp forest. “New kelp cannot recruit onto the turf and any kelp that might find a place to settle will get eaten immediately.”

Kelp forests are highly productive communities of large brown seaweed that support rich biodiversity. Photo by Thhomas Wernberg.

The north-western part of the reef is also the most severely affected because it lies closer to tropical regions like Ningaloo Reef, which means that tropical species can move in more easily, the researchers say. Moreover, a strong southward-flowing current prevents kelps from recolonizing the worst-affected northern tip of the reef, they add.

Wernberg’s team believes that the destruction of the northern kelp forests — first initiated by the 2011 marine heatwave and now maintained by a combination of physical and ecological barriers — is unlikely to reverse any time soon without intervention.  The problem, he said, is that the ocean will continue to get warmer, and marine heatwaves will become more frequent in the coming years.

The study provides “alarming and detailed evidence for one of the most dramatic climate-driven ecosystem shifts ever recorded,” Adriana Verges from the University of New South Wales, who was not involved in the study, told the Atlantic.

However, despite the threats to kelp forests, the Great Southern reef has received very little funding and public attention. This could be because kelp forests are not as exotic as corals.

“Generally the water is colder, not as clear and kelp forests are often ‘in peoples back yard’ and therefore perhaps less exotic,” Wernberg said. “In Australia, for example, 70 percent of the population live within 50km of the Great Southern Reef. More than anything, I think there is a general lack of understanding of what these magnificent temperate marine ecosystems do for us – the ecosystem services they provide and how valuable these are.”

Harlequin fish are endemic to the kelp forests of southwestern Australia. Photo by Thomas Wernberg.

Kelp forests are extremely valuable. Wernberg’s team calls them the “biological engine” of Australia’s Great Southern Reef, supporting rich, complex diversity of species, many of which are found nowhere else on earth. The region also supports recreational and commercial fishing, as well as biodiversity and reef-related tourism, worth billions of Australian dollars.

“In Australia, the Great Southern Reef supports our two most valuable single-species fisheries, rock lobster and abalone, and between 30-80 percent of all species are only found on this reef and nowhere else on Earth,” Wernberg said. “These values add up to at least $10 billion per year which is much more than the Great Barrier Reef.”

So can kelp forests be saved?

Not all kelp forests are at the risk of extinction, Wernberg said. Kelps off the coast of South Africa and in the Arctic, for example, are doing fine. But to save kelps in places like south-western Australia, where they could soon become extinct, kelps need immediate help, he added.

The obvious first step, according to Wernberg’s team, is to curb carbon emissions, which this is the root cause of the rise in sea temperatures. Reducing disturbances like sewage run off can also help increase the resilience of the kelp forests.

In some places though, active intervention, such as replanting kelps, or boosting resilience through assisted breeding of tolerant genotypes, might be necessary to help restore forests.  “These options are obviously in the extreme end of the spectrum but we need to consider the full range of the toolbox as the future will be extreme,” Wernberg said.

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