The temperature of water in the Chesapeake Bay naturally fluctuates each year, tied to weather patterns and ecosystem changes across the bay’s massive, 64,000-square-mile watershed.
But a new study led by the University of Maryland is yet another confirmation of what scientists have reported for years: The bay is steadily warming and seeing a growing number of extreme heat incidents known as marine heat waves.
The frequency of underwater heat waves in the bay doubled over the past 20 years, researchers say in the paper published this week in a scientific journal called Estuaries and Coasts.
Marine heat waves refer to stretches of days or weeks with unusually hot water temperatures. On average, the bay annually experiences about 25 days of extreme heat, according to the new study.
Jacob Wenegrat, an assistant oceanography professor and part of the research team, said the increase is likely due to the bay’s slow warming over time.
That’s driven by changing climate conditions worldwide that show no signs of slowing down. The ocean absorbs 90% of the excess heat from global warming.
“If you project out to the end of the century, what we would call a heat wave now might just be sort of standard conditions,” Wenegrat said.
The group analyzed temperature data from 2003 to 2022 from a variety of satellites and validated it with long-term data collected by buoys that have sensors throughout the bay.
The Chesapeake is particularly ripe for research because it has a uniquely extensive dataset from decades of monitoring by federal and state agencies, Wenegrat said.
The researchers noticed significant differences between the upper and lower sections of the bay.
The area north of the Potomac River saw more frequent heat waves, but for shorter durations. In the lower part including Virginia, there were fewer heat waves but they lasted longer.
Wenegrat said it’s not yet clear what leads to the variability.
Officials are concerned about hotter water because it can kill off seagrass, which is a sort of “nursery” for other marine life, causing ripple effects including fewer rockfish and blue crabs for local fisheries.
Higher temperatures are also known to lessen the amount of oxygen in the water, which can suffocate animals and help spur the growth of harmful algae.
Satellite data only captures temperature at the top of the water, “a very surface snapshot,” Wenegrat said.
Research by the Virginia Institute of Marine Science last year aimed to get more information from farther down the water column, where it’s harder for heat to penetrate. Their study found that summer heat waves stay closer to the surface, while winter temperature increases go “all the way down to the bottom.”
Wenegrat said he’s excited by the potential to combine both types of data for a fuller picture of what’s happening in the bay. He hopes the growing sources of information help local officials take action to prepare.
“Once you have the conditions for a marine heat wave in place, there's not a lot we can do directly to stop it,” Wenegrat said. “But there is a lot that can be done from a management perspective.”
That includes helping protect marine species, setting up an early warning system and addressing potential economic impacts to industries that rely on the water.
