Ocean heatwaves more frequent, intense

While scientists have long warned of the negative effects marine heatwaves have at the ocean’s surface, new research suggests these temperature spikes are also having prolonged impacts deeper down.

It risks species living at around 250m beneath the ocean’s surface shifting their habitat and “consequent effects on ecological interactions and ecosystem processes” if subsurface marine heatwaves become more frequent.

The research, published today in Nature Climate Change – led by the University of Algarve in Portugal and drawing contributions from marine scientists in Australia, Norway, Spain and Belgium – found marine heatwaves were at their most intense in the “upper subsurface”, particularly in the equatorial Pacific and Indian Oceans, becoming more prolonged and frequent at lower levels of the water column closer to the poles.

Marine heatwaves aren’t anything new and have been part of the ocean’s story throughout human history. But the frequency of such events is on the rise, especially at depths below the water column’s ‘surface zone’, potentially extending hundreds of metres.

If ocean warming caused by anthropogenic climate change continues, the knock-on impact will be on ecosystems and marine organisms.

That, says one of the study’s contributors Professor Thomas Wernberg, a marine botanist at the University of Western Australia’s Oceans Institute, could also wreak havoc with ocean food supplies.

“Various ecosystem services for people depend on the species at those depths… some fisheries that extend quite deep,” Wernberg says.

“It obviously has consequences whether or not those species persist, or whether they succumb to the heatwave in various ways, or they move.”

Species that exist in marine ecosystems in surface and subsurface water are quite mobile. At greater depths, they might also be sensitive to even small fluctuations in temperature. Just as land animals (including humans) seek refuge from the heat, so too might those in the ocean.

But when the temperature spikes in your marine neighbourhood, retreat means descending further.

Should species resort to deep-diving to new, cooler habitats, the composition of fisheries and wider ecosystems could change.

“Many of these organisms are mobile, they can either go deeper or they can move to another geographical location,” Wernberg says. “The extent to which they do that, and therefore, knowing where you can catch them if that’s what you’re doing, becomes important.”

Cosmos recently reported on yellow-fin tuna and changing oxygen levels at depth.

Shallow water heatwaves have been recorded for decades, but with improvements in research methods, scientists are expanding their knowledge to temperature spikes across greater ranges and depths.

“Every summer there are new examples of something that’s happening in various places, with massive impacts on ocean species and all sorts of ecosystem services that people derive from the oceans,” Wernberg says.

“The stuff at depth, there’s absolutely no reason to expect that you don’t have similar effects down there, but because [these regions are] out of sight, we won’t see the effects as quickly as what we experienced in the shallow waters.”