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Marine heatwaves may expose coastal ecosystems to substantially more cumulative heat than conventional measurements suggest, according to a new study led by researchers at William & Mary’s Virginia Institute of Marine Science (VIMS) and Batten School.
The study, published in Communications Earth & Environment, found that unusually warm conditions often extend for weeks or months before and after a defined marine heatwave (MHW). Including these surrounding periods in assessments could provide a more accurate picture of the thermal stress experienced by marine organisms.
Researchers analysed more than 2,580 marine heatwaves recorded over two decades across 20 US estuaries. They found that conventional assessments underestimated cumulative heat exposure by more than 150% on average when the broader periods of unusually warm water were taken into account.
“Studies usually focus only on the marine heatwave window and that’s not an accurate representation of real-world conditions,” said lead author Ricardo Utzig Nardi, a research specialist at VIMS and the Batten School.
The researchers developed a framework that treats marine heatwaves as part of longer periods of elevated temperatures rather than as isolated events. They found that warming before and after a heatwave could contribute as much as, or even more than, the heat exposure during the formally defined MHW.
Lead researcher Piero Mazzini, an assistant professor at VIMS and the Batten School, said marine heatwaves should be considered within the broader periods of warming in which they occur when assessing their effects on coastal and ocean ecosystems.
The study classified the heatwaves into two broad groups. About two-thirds were “individual” events, occurring within roughly 60 days of elevated temperatures outside the MHW period.
The remaining one-third were classified as “compound” events, which were associated with much longer periods of warming, extending for about 90 days before and after the heatwave. These compound events produced more than three times the cumulative heat exposure of the MHW itself.
The researchers said the findings could have implications for laboratory experiments designed to study the biological effects of marine heatwaves. Such experiments often expose organisms to elevated temperatures for only a few days or weeks, potentially failing to reproduce the prolonged exposure experienced in natural environments.
“Biological responses depend not only on how warm the water becomes, but also on how long that warming persists,” Nardi said.
The researchers noted that prolonged heat exposure can become particularly stressful when combined with other environmental pressures, potentially affecting the survival and health of marine species.
The study was made possible by long-term, high-frequency temperature observations from NOAA’s National Estuarine Research Reserve System. The researchers said such monitoring programmes provide the data needed to identify longer-term warming patterns that may otherwise remain hidden.
The findings add to recent research by VIMS and Batten School scientists on marine heatwaves, including work examining vertical heatwaves in the Chesapeake Bay and forecasting an increase in marine heatwaves along the US East Coast.