Deep-sea marine heatwaves could outpace surface extremes, study finds 

Marine heatwaves on the seafloor are expected to become more frequent than those at the ocean surface, a new study warns, raising concerns for bottom-dwelling species and the wider ecosystems and communities that rely on them.

The study, led by Dr Robert J. Wilson at Plymouth Marine Laboratory (PML), examined future trends in marine heatwaves on the northwestern European continental shelf – a region spanning the North Sea, English Channel, Celtic Sea, Irish Sea, Bay of Biscay and parts of the Norwegian Sea. The area is a critical hub for fisheries and Europe’s rapidly growing offshore wind industry.

A marine heatwave is defined as a period of at least five days during which sea temperatures exceed the 90th percentile of historical levels for that time of year. While surface heatwaves have been increasingly documented, little was previously known about how such events could affect the seafloor, where temperature fluctuations are normally smaller.

Using high-resolution regional climate models, the researchers found that seafloor heatwaves could become more than 50 per cent more frequent than surface events during summer months. 

Unlike pelagic species that can move to cooler waters, bottom-dwelling organisms such as shellfish, seagrasses and certain fish have limited mobility, making them particularly vulnerable to rising temperatures.

The study involved a multidisciplinary team from PML, including Dr Yuri Artioli, Prof Ana M. Queirós and Dr Giovanni Galli, and colleagues from the National Oceanography Centre, including Dr James Harle, Prof Jason Holt and Dr Sarah Wakelin.

Lead scientist Dr Wilson says: “Until now, we have largely predicted future marine heatwave rates based on what will happen on the sea surface. We have shown that this can be highly misleading for critical European seafloor ecosystems, where climate change will cause more frequent extreme temperatures than at the sea surface. It is therefore critical that marine heatwave monitoring and assessment move beyond the surface and shift to a 3D view of the ocean.” 

The full paper on deep-sea marine heatwaves is available online.

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