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A team of climate scientists, led by Dr Iestyn Woolway from �ᰮ�Թ�, conducted a study that has been published in the scientific journal . 

It found that over the past 40 years deep-water heatwaves have increased in frequency, duration and intensity.  

In addition to that, what are known as vertically compounding heatwaves, where extreme heat occurs simultaneously at the surface and bottom, have risen.  

To capture changes in some of the largest lakes of the world, and to investigate within-lake variations in heatwaves, the outputs from a three-dimensional (3D) model of the Laurentian Great Lakes of North America was investigated. 

Extreme hot water events in lakes can substantially disrupt aquatic ecosystems, and although surface heatwaves are well studied, their vertical structures within lakes remain largely unexplored.  

The findings of the study, in which �ᰮ�Թ�’s School of Ocean sciences collaborated with scientists from Michigan Technological University and Southern University of Science and Technology reveal that subsurface heatwaves are frequent, often longer lasting but less intense than surface events.  

It also warns that by the end of the century, changes in heatwave patterns, particularly under high emissions, are projected to intensify.  

Though the rise of unprecedented temperatures during a lake heatwave can benefit some aquatic species by expanding their habitat, it can be detrimental for others. Because of this quantifying changes in lake heatwaves is critically important to anticipate the likely impact of climatic warming on lakes. 

Mobile aquatic species can respond to environmental disruptions, such as extreme surface temperatures, by relocating to more favourable habitats.  

In stratifying systems, bottom waters are often cooler than the lake surface. If other environmental factors are favourable, many aquatic species could migrate to these deeper layers to escape surface thermal stress.  

Cooler water at depth could provide a potential thermal refuge for aquatic species as surface heatwaves become more common and intense. However, the study found that the potential for this is decreasing.  

Dr Iestyn Woolway, of �ᰮ�Թ�, said,  “Our investigation identified several findings relating to the decreasing potential of a vertical thermal escape from surface heatwaves, and an increase in the vertical distance that species should travel to escape a surface heatwave when a thermal refuge exists. 

“It found considerable variability in the vertical structure of subsurface heatwaves across lakes, and the increased occurrence of bottom heatwaves with and without extreme surface conditions and an increase in the frequency of vertically compounding heatwaves.” 

Previous studies have reported that lake surface heatwaves have increased in intensity and duration in recent decades.  

The extreme water temperatures that species must endure during a surface heatwave can often lead to severe consequences.  

Dr Iestyn Woolway added,  “Previous studies have suggested that many aquatic species will need to migrate to cooler water at higher elevation or latitude this century to maintain a preferred thermal habitat. Aquatic species could also escape the thermal stress of surface heatwaves by migrating to deeper regions within a lake.  

“However, our investigation demonstrates that, while there is often the potential for aquatic species to travel vertically within a lake to reach cooler water, the proportion of lake surface heatwaves without a thermal refuge in deeper water has increased.  

“This vertical expansion of lake surface heatwaves highlights the dynamic nature of these extreme heat events, prompting aquatic organisms to adjust their distribution patterns. As the effects of lake surface heatwaves reach increasingly deeper water, a reduction in sufficient habitat can result in changes to species abundance and range. 

“These findings highlight the need for more subsurface monitoring to fully understand and predict the ecological impacts of lake heatwaves.”