A frenzy of rabbitfish feeding on kelp transplanted by a University of New South Wales-led team of researchers off the coast of New South Wales in eastern Australia. (Image: Adriana Vergés)
We’ve heard a lot lately about the destruction of tropical coral reefs brought on by a warming climate. And that’s a big deal — corals are the lynchpin of entire undersea ecosystems. When they go, the damage reverberates widely and ultimately, even people pay the price.
But something similar has been happening to corals’ more temperate cousins in many locations: Forests of kelp. These swaying seaweeds, too, anchor communities of diverse types of fish and other living organisms and, in turn, provide great value to humans through their contribution to fisheries. And recently, there has been some very troubling news for kelp. One forest was wiped out off the southwestern coast of Australia by extremely warm temperatures in 2011, an event that scientists called a “rapid climate-driven regime shift.” More recently, there has been massive death of giant kelp around Tasmania as well.
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And now, new research in the Proceedings of the National Academy of Sciences demonstrates another way that a changing climate can devastate kelp. Adriana Vergés of the University of New South Wales in Sydney and her colleagues at institutions in Australia, Spain, and Singapore used underwater cameras to study kelp forests around the Solitary Islands, along the eastern Australian coast between Sydney and Brisbane.
This region is what Vergés calls “a famous place for being a tropical-temperate transition zone. We went there because that’s where we thought, if there’s something that is happening to the kelp, that is climate mediated, this is where it is going to be happening,” she said.
And it was. Between 2002 and 2011, all of the kelp in the 25 kilometer study area died. And the cameras showed why: They were being devoured by several species of fish, including tropical rabbitfish, that had been thriving as the temperate waters warmed up just slightly, by 0.6 degrees Celsius, during the time period. That wasn’t much, but it was apparently enough to support herbivorous fish from tropical climes that could wipe out the kelp.
“I think what’s interesting is this realization that the greatest impacts of climate change may not be the direct effects of warming on one species, but it’s more the effects of warming on the way species interact with each other,” said Vergés. “The effect of warming on how fish eat the kelp. And it’s kind of the same with coral reef systems. Warming doesn’t kill the coral itself, it actually breaks the relationship between the coral and the symbiotic algae. It’s a case of warming impacting the way species interact.”
It’s not just the fish invading from tropical latitudes (which in the case of Australia, would be from the north). It’s also the temperature itself, which increases fish metabolism and may make them consume more, Vergés said.
What’s particularly striking about this study is that, unlike in other cases involving coral reefs or kelp forests, there was no sudden surge of warm water, sometimes called an “underwater heat wave,” that proved deadly. Instead the warm-up was slow, steady, and not even all that large in magnitude. Yet nevertheless, it seems to have shifted the conditions in which tropical fish who are herbivores and kelp forests interact, and the consequence was a total loss of kelp, at least in this particular region.