Marine heatwaves affect ecosystems far below the surface

Photo: Pexels/Jeremy Bishop

Marine heatwaves can take a great toll on life in the oceans, which absorb 90% of the excess heat produced by climate change. As these spikes in water temperatures become more frequent, their effects will have far-reaching consequences and not only on the surface.

In fact, greater spikes in temperatures last longer in deeper water, thereby posing a threat not only to fragile coral reefs but other ecosystems deeper down, scientists say.

“Marine heat waves and their effects have been studied mostly at the ocean surface and we did not know much about their characteristics in the deep ocean,” explains Eliza Fragkopoulou, a marine scientist at the Centre of Marine Sciences at Portugal’s University of Algarve.

Fragkopoulou was a key part of new research, which examined marine heatwaves globally from 1993 to 2019, measuring their effects all the way down to 2,000 meters below the surface.

The intensity of heatwaves is highest at between 50 meters and 200 meters below the surface. At times these heatwaves are 19% stronger deper down than at the surface. The greatest impact on biodiversity is likely the most severe in the area between the surface to a depth of 250 meters, according to the study.

“The duration also increases with depth, with warming persisting up to two years after temperatures returned to normal on the surface,” the scientists report, adding that high-stress conditions overlapped in up to 22% of the oceans worldwide.

“Regional variability of marine heat waves makes measuring biodiversity exposure complex, and their duration varied by location due to different oceanic conditions,” they explain. “The largest portions of the oceans categorized as highly exposed were found in the North Atlantic and Indian oceans, at depths between 1,000 and 2,000 meters.”

Further research is urgently needed to better understand the impacts of heatwaves at greater depths, Fragkopoulou says, noting that fisheries globally could be adversely affected.

“Considering that marine heat wave impacts on deep-sea biodiversity are still largely unknown, there is an urgent need for more and better monitoring of the global ocean to understand their effects,” she said.