Great swathes of the temperate kelp forests on Western Australia's reefs that underpin tourism and fisheries industries worth $10 billion annually are gone.

Key points: The ocean off Western Australia is warming twice as fast as the global average

The ocean off Western Australia is warming twice as fast as the global average Since 2000, nearly 1000 square kilometres of kelp forest have been lost from the area

Since 2000, nearly 1000 square kilometres of kelp forest have been lost from the area These "forests of the sea" underpin tourism and fisheries worth $10 billion per year



And the demise of these remarkable "forests of the sea" is likely permanent, researchers say in a study published today.

"Off the coast around Kalbarri to Geraldton, where these reefs used to be dominated by kelp forests, those forests have completely disappeared," researcher Dr Scott Bennett, now based at the Spanish National Research Council, said.

"A lot of the [temperate] fish and invertebrates have disappeared and we've seen these communities shift to something that resembles the tropical fish and seaweed communities we would find at Ningaloo."

Associate Professor Thomas Wernberg, from the University of Western Australia, who helped lead the research with Dr Bennett, described the kelp forests as the "biological engine" of the Great Southern Reef.

"They are as critical to the Great Southern Reef as corals are to the Great Barrier Reef. They are up to 16 times more productive than our most productive wheat fields and provide the foundations for the ecosystem."

The researchers said economically the kelp forests supported some of the most valuable fisheries in Australia and reef-related tourism worth more than $10 billion per year.

The loss of the kelp forests, outlined today in the journal Science, was discovered as part of a 15-year survey, which started in 2000, off Western Australia's reefs. The survey stretched 2,000 kilometres from Cape Leeuwin in the south to Ningaloo in the north.

Kelp forests support fishing and tourism industries. ( Supplied: Joan Costa 2013 )

2011 marine heatwave key trigger for kelp loss

Dr Bennett said the catalyst for the death of the kelp forests was a marine heatwave in 2011 that caused the primary loss of the forests followed by above-average ocean temperatures in 2012 and 2013 "that compounded the effects".

In December 2010, immediately before the extreme marine heatwave, kelp forests covered more than 70 per cent of shallow rocky reefs in mid-Western Australia between Geraldton and Kalbarri, 400-600 kilometres north of Perth.

Dr Bennett said by early 2013 their surveys showed the dense kelp forests in this area had disappeared representing a roughly 100-kilometre range contraction and effective extinction from 370 square kilometres of reef.

Across the entire surveyed area about 960 square kilometres of kelp forest had been lost.

Dr Bennett said the research team initially thought it had made an error when it dived the reefs off Kalbarri.

"We jumped into these waters at sites we've been going to for the past 10 years expecting to see large kelp forests and it was just a desert, it was barren," he said.

"We thought we'd made a mistake and got the location wrong. We searched all day and we searched for weeks, but the kelp was gone. It is just heartbreaking to see such a complex, beautiful, vibrant ecosystem decimated."

Dr Bennett said five years after the heatwave, the kelp was showing no signs of rejuvenation, and many cool water fish, seaweed and invertebrates had also disappeared.

"At this stage we are five years on from the initial kelp loss and from all the evidence we have is it is a permanent fixture," he said.

Kelp forests reached tipping point

Dr Bennett said turf algae had proliferated in the area and tropical fish communities had increased and were preventing the regrowth of the kelp by eating any that managed to re-establish.

He said the kelp forests were home to hundreds of unique species and commercially important to the wild abalone industry, which was worth about $200 million annually.

With the Indian Ocean off the mid-Western Australian coast warming at a rate twice the global average, the kelp loss in the northern reefs also sounded a warning for kelp forests further south.

Climate change was driving warming and more frequent heatwaves, Dr Bennett said, while the strong Leeuwin Current was helping the southward movement of warmer waters and tropical species,.

The survey had revealed that a rise in around 2.5 degrees C above long-term summer maximum temperatures was a "tipping point" for kelp forests.

"Kelp forests are under-recognised and under-appreciated but at severe risk right under our noses. There is the potential to see their wholesale collapse," Dr Wernberg said.

He said the loss of the kelp forests could mean the loss of marine animals such as abalone, rock lobster and several species of reef fishes used in commercial and recreational fishing.

"Some of the regional economies depend on these species, for example the abalone and rock lobster fisheries are the most valuable single species fisheries in Australia," he said.

The researchers had yet to document any extinctions — aside from those of local kelps.

However Dr Wernberg said it was clear many kelp-associated species were now under threat.

Dr Alistair Hobday is a Senior Principal Research Scientist at CSIRO Oceans and Atmosphere welcomed the research.

"These long-term studies are rare in Australia," he said, adding the changes seen in the reefs was equivalent to a change from a forest to a grassland.

"We can expect more of these dramatic changes around Australia's coasts in future."