Marine subsurface heatwaves threaten deep-sea organisms

Deep marine ecosystems subject to severe temperature fluctuations at depth during the coming months may be put under increasing stress, say researchers.

“If you’re a whale or a tuna, you’re able to move faster away [from a heatwave] than if you’re a very small type of plankton or small snail floating in the water column,” says Dr Karen Filbee-Dexter, a marine ecologist from the University of Western Australia.

Filbee-Dexter was responding to reports in Cosmos earlier this week from scientists aboard the CSIRO’s research vessel RV Investigator, off the Sydney coast who told Cosmos they were tracking a severe subsurface ocean heatwave.

That heatwave, extending deep beneath the surface is, according to voyage leader Professor Moninya Roughan, “enormous and hot” and more than 3°C above average for the area.

Compared to surface temperatures, it’s hard for the public to keep track of deep ocean heatwaves.

Right now, satellite imagery shows a moderate sea surface heatwave off the east coast of Australia although six-month forecasting by the Bureau of Meteorology has scaled back predictions of an “off the chart” heatwave off the east coast of Tasmania.

“It was a tiny bit higher a couple of months ago, it’s still very much present in all the months in the six months to April,” says Bureau oceanographer Grant Smith.

The Bureau uses several atmospheric and ocean parameters combined with rainfall and maximum temperature predictions to chart expected marine temperatures. Nearly 100 different variable simulations are run to produce its forecasts.

But beneath 50m of the surface, where the ocean transitions into the water column’s ‘subsurface’, tools are harder to come by.

Marine heatwaves are expected to be more frequent due to climate phenomena like El Nino, as well as the overall increase in global average temperatures arising from record levels of carbon emissions. The Intergovernmental Panel on Climate Change expects that marine heatwaves have become more frequent over recent years and will become increasingly likely as global warming is amplified in the future.

The IPCC’s special report on oceans and the cryosphere published in 2019 also indicates that some ecosystems will reach survival thresholds with even modest warming.

Such events are particularly problematic deeper in the water column, where some species are less mobile and potentially exposed to greater extremes.

A recent international collaboration found marine heatwaves are more intense 50-200m beneath the surface and can be twice as long at greater depths than shallow subsurface regions.

So for species living in the area with water column temperatures exceeding 3 degrees above the average, the risk of experiencing deadly conditions is potentially more pronounced.

“If you live in the intertidal [zone], the very shallow part [of the ocean] that’s exposed to the air, you tend to be able to tolerate a crazy range of temperature,” says Filbee-Dexter, who is not involved in the RV Investigator research.

However, her own research studies the impact of climate change on marine ecosystems. Fluctuations in temperature, such as during ocean heatwaves, and how species tolerate these is a particular area of interest to her.

“The deeper you go… organisms tend to be adapted to a narrower range of temperatures,” she says.

“A change in 3 degrees for a species living in the deeper subtidal can be stressful because it might not be used to those fluctuations.

“If it’s a change in the maximum temperature that species has ever experienced, that’s when you start seeing whole ecosystems transform.”

Some species might be able to navigate out of a suddenly hot segment of the water column either by descending in the water column or by lateral movement.

This is certainly true for larger animals, Filbee-Dexter says, for smaller species like plankton, or plants and animals clamped to the ocean floor, evading the heat is less likely.

“There’s all kinds of species that will be attached to the seafloor,” she says. “These species can go down to the hundreds of meters and would not be able to move away.

“If you’re a whale or a tuna, you’re able to move faster away [from a heatwave] than if you’re a very small type of plankton or small snail floating in the water column – you don’t really have the ability to jump a couple of kilometres away.”

In a comment published in Nature in September, Filbee-Dexter and her peers warned strong marine heatwaves should be expected as the world enters “uncharted waters”. She told Cosmos there’s “high certainty in models, that marine heatwaves are going to become more frequent, more common across all of our oceans, and that they’re going to become more severe, and that’s with climate change”.

Along with her peers, led by the CSIRO’s Research Director for Sustainable Marine Futures Dr Alistair Hobday, they called on decision-makers to reduce risks emerging from such events. Hobday’s team at the CSIRO collaborates on providing accurate marine heatwave forecasting, in collaboration with the BOM.

“The forecasting we can now do means that we take away some of your ability to be surprised,” Hobday says.

“Having access to that information so far in advance means there’s a whole spectrum of things that can be done – starting when the risk is likely but not certain, and moving all the way through to when the risk is emerging.”

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