Coral reefs are not the only marine ecosystem under threat from hot summers. A study of Western Australia's kelp forests has found nearly half died out during recent extreme summers. Other seaweeds moved in and are proving hard to displace.

North of Perth, the Western Australian coastline is one of the least populated on Earth, and as such free of many of the marine stresses seen in other parts of the world. It is also home to a rich array of ecosystems, including kelp forests that stretch for 800 kilometers (500 miles). At least it was in 2001, when a team began monitoring this coastline. The project gave researchers a rare ringside view of the ways ecosystems change.

“The Indian Ocean adjacent to western Australia is a 'hot spot' where the rate of ocean warming is in the top 10 percent globally,” University of Western Australia's Dr Scott Bennett and co-authors report in Science. Average temperatures at any point are the same as they were 20 to 50 kilometers (12 to 30 miles) north 10 years earlier.

^{Researchers explore the kelp forests that survive. (C) Joan Costa 2013}

While the slow warming set the stage, it has been the hot summers that leave a legacy. “In December 2010, immediately before an extreme marine heatwave, kelp forests covered over 70 percent of shallow rocky reefs in the midwest,” the paper reports. “By early 2013, only two years later, our extensive surveys found a 43 percent (963 kilometers squared/372 miles squared) loss of kelp forests on the west coast.”

Dividing the coastline into blocks 0.4 degrees of latitude, Bennett found near total extinction of kelp north of 29º, and almost complete survival south of Perth (32º). In between, a little more than half the kelp survived until 2015. Bennett told IFLScience that while most of the world experienced a record hot start to 2016, the Western Australian coast was relatively cool. “There are no signs of recovery but at least the damage was not compounded.”

Although there was a sixfold increase in “small hermatypic coral colonies” within the waters previously dominated by kelp, this still represented a small fraction of the affected area. “Instead, we found a dramatic increase in the cover of turf-forming seaweeds,” the authors report.

Bennett told IFLScience: “If we came back in 100 years we might have coral reefs here, but in order for coral to establish itself a number of other things have to kick into place, while turf seaweeds are fast growers, and were already in the area in low numbers.” The change has a devastating effect on the area's biological richness. Bennett noted that the kelp forests of the region contain more biodiversity than most coral reefs, with different species occupying niches from the point where the kelp attaches to rocks all the way up to the top of its fronds.

The branch of ecology known as Resilience Theory proposes that once an ecosystem has been stressed beyond a tipping point it will be replaced by another, and only a dramatic external shock will return things to the previous state. Bennett's observations are consistent with this. “Even though the acute climate stressor has abated, as of late 2015, almost five years after the heat wave, we have observed no signs of kelp forest recovery on the heavily affected reefs north of 29º S,” the authors report.

The paper notes a “400 percent increase in the biomass of scraping and grazing fishes”, which prevent kelp recovery, eating young kelps before they grow big enough to survive. And making it hard for the old system to ever restore itself.

^{When the kelp is gone, what replaces it is less biologically rich, and a lot less attractive. Scott Bennett.}

^{Infographic showing the distribution of kelp, fish and seaweed from north to south along Western Austrlia's coast. Awaroo}