Super-warm seas wiped out an entire underwater forest, scientists report
7 July, 2016
This year, the tragic die-off of large volumes of coral at the treasured Great Barrier Reef has provided a climate change shock like few others. The cause was too much warm water — which seems to have pushed the corals past a thermal survival threshold. And that warm water, in turn, is tied to climate change.
Now, however, a team of researchers has revealed that another Australian coastal ecosystem that gets less attention — Australia’s kelp-dominated Great Southern Reef, which covers a huge expanse along its more temperate southern and southwestern coasts — saw an equally dramatic ecosystem upheaval five years ago. And the cause was the same — what the scientists call an “extreme marine heat wave.”
“Some of the same types of drivers are behind all of this, and I think this really emphasizes the manifestation of these climate-driven events; it’s more than just the coral reefs that are being affected by this,” says Thomas Wernberg, a marine biologist at the University of Western Australia who led the new research, which appeared Thursday in the journal Science. Wernberg conducted the research with 21 other authors from a variety of institutions in Australia and abroad.
Many Americans will be familiar with the towering kelp forests off the coast of California, where these enormous organisms can grow to be more than 100 feet tall. The kelp off Australia’s southern and southwestern coasts aren’t nearly as tall, but they are still the backbone of a rich ecosystem, and the researchers refer to the organisms as creating a “kelp forest.” Wernberg and his colleagues estimate the Southern Reef as a whole to be worth $10 billion each year in economic terms due to tourism, fisheries and other benefits.
But in 2011, a surge of ocean temperatures between 3 and 5 degrees Celsius above normal — conditions that for the kelp represented “the hottest in recorded history, and that’s going back 215 years,” Wernberg says — took a devastating toll on a major part of this ecosystem.
In the most northern (in the Southern Hemisphere, the warmest) latitudes, along the southwest coast of Australia south of Kalbarri, kelp forests died in dramatic numbers, suggesting the temperatures had “exceeded a physiological tipping point for kelp forests,” the study reports.
“When the heat wave happened, all of those northern kelp forests were basically wiped out in a couple of months,” says Wernberg. “And southwards, at least a couple-hundred kilometers, there were quite substantial impacts, but you gradually got more and more kelps as you went farther south.”
Here’s a pair of before and after photos, taken by Wernberg, that show the dramatic change:
Rocky reefs in Kalbarri before, left, and after, right, a 2011 marine heatwave wiped out underwater forests of Australian kelps. (T. Wernberg)
The study, based on years of underwater surveys, finds that before the marine heat wave in late 2010, kelp covered around 70 percent of reefs in the midwestern Australia coastal area — but afterward, in 2013, their area had declined by 43 percent, or 963 square kilometers (371 square miles).
Moreover, north of 29 degrees North latitude, kelp forests were gone entirely or 90 percent decimated. This represented “a 100-kilometer range contraction” as well as “functional extinction from 370 [square kilometers] of reef,” the scientists report. (That’s 142 square miles.)
And it is not like the kelp then came back afterward or recovered. Rather, the study reports that more tropical species of fish seized the opportunity to colonize the warmer waters, including grazing fish that ate kelp as it started to regrow. As a result, the ecosystem, sparked by one extreme event, underwent what the scientists call a “rapid climate-driven regime shift,” from which it simply may not return unless the warming trend itself reverses.
“I can’t remember having seen or heard about anything similar,” Wernberg says.
Although it does not perform a formal statistical attribution for the marine heat wave, the paper clearly suggests this is all tied to climate change, noting that the Indian Ocean, off Australia’s western coast, is “a ‘hot spot’ where the rate of ocean warming is in the top-10 percent globally.” Another such hot spot, incidentally, has been off the U.S. East Coast in the Gulf of Maine, where similarly dramatic effects have been observed.
“It was extreme, so very rare by all accounts — I don’t believe it was ‘natural,’ ” Wernberg wrote by email.
In sum, we are starting to see that with certain very sensitive ecosystems, temperature surges can have devastating and perhaps irreversible effects. And one can argue this fits a key definition of “dangerous climate change,” which turns on whether we are able to stop the increase of greenhouse gases at a rate “sufficient to allow ecosystems to adapt naturally.”
That certainly doesn’t seem to be what happened in the kelp forests off Australia’s western coast. “The future of kelp forest