Ocean Heatwaves: Longer Than We Thought 

A recent study of 20 estuaries has found that ocean heatwaves start earlier and end later than previously thought. Led by the Virginia Institute of Marine Science (VIMS), the research team used data from National Estuarine Research Reserve System (NERRS) to determine that warm-water periods immediately before and after a heatwave often outlast the heatwave itself. The study, published in Nature’s Communications Earth & Environment, could reshape how scientists design experiments and how managers assess ecological risk in a warming climate.

“Standard approaches to measuring ocean heatwaves tell us whether the water crossed a certain temperature threshold for five days or more, but they don’t tell us the full story of how an ecosystem experienced that heat,” says Ricardo Nardi, the study’s lead author and a research specialist at VIMS, which is also the host for Virginia’s Chesapeake Bay Reserve.

“We wanted to know just how much warmth was being missed on either side of these events, and whether it was significant enough to matter.”

To answer that question, the team turned to the NERRS System-Wide Monitoring Program (SWMP), a long-running, standardized water-quality monitoring effort spanning the coasts. Since the 1990s, NERRS SWMP has generated one of the most comprehensive estuary water temperature datasets in the country. Scientists leveraged two decades of high-frequency temperature readings from 54 NERRS SWMP monitoring stations to identify and characterize more than 2,580 marine heatwave events.

With 300 monitoring stations collecting more than 48 million publicly available readings each year; the NERRS System-Wide Monitoring Program has generated an irreplaceable dataset on water quality for U.S. estuaries.

“What surprised us most was how independent the before-and-after periods were from the heatwave itself,” says co-author Piero Mazzini, an assistant professor of oceanography at VIMS.  “A short, intense heatwave could still be preceded or followed by weeks of lingering warmth, and you’d have no way of knowing that just by looking at the standard heatwave statistics.”

Marine heatwaves are linked to degraded habitats, harmful algal blooms, and low-oxygen “dead zones” that threaten fisheries and aquaculture. Because warm water lowers oxygen solubility and speeds up the metabolic processes of marine life that consumes it, the extended warm periods identified in this study likely widen the window during which estuaries are vulnerable to these compounding problems. The team also found that closely spaced heatwaves can create extended periods of elevated warmth lasting up to roughly 90 days beyond the officially recorded event.

The study revealed that marine heatwaves are often preceded or followed by warmer periods lasting months.

“The researchers also found that not all heatwaves behave the same way. About a third of the events studied were “compound” heatwaves, or closely spaced events that blend into extended stretches of warm water, while the remaining two-thirds were more contained, isolated events.

“Science like this is a perfect example of why the NERRS has invested in long-term, standardized water quality monitoring,” says Dwayne Porter, the director of the NERRS Centralized Data Management Office at the University of South Carolina. “Being able to access decades of high quality data from around the coasts helps researchers distinguish between short-term change and long-term trends and supports them in asking questions we couldn’t have anticipated when the sensors first went in the water.”

To help other scientists build on these findings, the team has published a foundational summary dataset, along with the code used to detect and characterize pre- and post-event phases. The study received no external funding and was carried out by researchers at the Virginia Institute of Marine Science, William & Mary, and California Polytechnic State University, using publicly available NERRS data.