The Indian Ocean off Western Australia experienced record summer temperatures between 2011 and 2013 caused by a double whammy of global warming and a La Niña weather phase. At their peak, in 2011, sea surface temperatures reached more than 6 °C above average in some areas.
By the end of the heatwave, declines in kelp cover were observed along more than 500 kilometres of the south coast, with complete extinction in the northernmost 100 kilometres.
This rate of kelp loss is the most rapid and extensive ever documented in the world, says Thomas Wernberg at the University of Western Australia in Perth, who led the survey.
“It was quite a shock to come back to these diving locations and all of a sudden realise: ‘Wow – this is completely different’,” he says. “When we went up to the northern regions and saw that everything was gone, it was devastating.”
The most recent surveying expedition in 2015 found no signs of kelp recovery. Instead of lush temperate-water forests, tropical turf-forming seaweed carpeted the ocean floor, and tropical fish, corals and invertebrates from the north coast of Western Australia had spread south into the newly warmed waters (move the slider below to see the difference).
As sea temperatures continue to rise, kelp forests will retreat further south until they have no more coastline to cling to, while tropical species will take over the old kelp heartlands, says Wernberg.
However, this does not mean that the heat-stressed tropical reefs of the north, such as the Great Barrier Reef, will be able to relocate to cooler climes in the south, he says. Only some species will be able to make the leap.
A kelpless future?
Unpublished research by Alexandra Campbell at the University of New South Wales, Sydney, and her colleagues shows that kelp forests are dying off at similar latitudes on the east coast of Australia.
The parallel findings provide robust evidence that rising sea temperatures are to blame, says Campbell: “Warming waters may have a direct physiological effect on kelp or increase their natural enemies, like pathogens and herbivores.”
The loss of kelp forests is as catastrophic as the demise of the Great Barrier Reef, she adds.
“Underwater kelp forests are like forests on land: they produce oxygen, they capture and sequester atmospheric carbon dioxide, and they provide food and shelter to a huge diversity of fish and other marine organisms,” she says. “Kelp forests receive far less attention than coral reefs, but that needs to change because they are extremely important – we all rely on them.”
Journal Reference: Science, DOI: 10.1126/science.aad8745