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Underwater Heat Waves: Chesapeake Bay’s Waters Could Suffer Extreme Heat for Half the Year by 2100

A heron is perched on a piling at sunrise on the Chesapeake Bay in Chesapeake Beach. Photo from yvonnenavalaney/

A study by two researchers from the Virginia Institute of Marine Science has found that the frequency of heat waves in the waters of the Chesapeake Bay is increasing, a trend connected to rising atmospheric temperatures that could have “devastating impacts to the bay ecosystem,” the scientists warn.

“The results are really robust. It’s really clear when you look at the trends; they are pretty alarming,” said Piero Mazzini, a physical oceanographer who with Cassia Pianca co-authored the study published earlier this month in Frontiers in Marine Science.

Using three decades of temperature records from eight buoys and stations spread throughout and near the Chesapeake Bay, Mazzini and Pianca found that between 1986 and 2020, the bay experienced an average of two marine heat waves per year, with each lasting about 11 days.

Since roughly 2010, however, these events — defined as five or more consecutive days when water temperatures are warmer than 90 percent of records for that location on that date — have been increasing. Prior to 2010, the maximum number of marine heat waves that occurred in the Chesapeake Bay annually hovered around four to five. Between 2010 and 2020, maximum frequency reached six to eight annually.

Furthermore, the study noted, while years without marine heat waves “were not uncommon prior to 2010,” since then they’ve been absent from the bay in only a single year: 2014.

If trends persist, within 50 years marine heat waves could occur on a monthly basis and by the end of the century could extend to cover over half the year, leaving the Chesapeake Bay in a “semi-permanent”  marine heat wave state.

Mazzini said the pair’s “analysis suggests very strongly that the heat is coming from the atmosphere.”

Most tellingly, the data showed that marine heat waves tended to occur around the same time all around the Chesapeake Bay.

“You could never explain a heat wave occurring in the ocean and in one or two days going all the way to Baltimore. You’d need unrealistic currents that would never happen in nature,” Mazzini said. They determined “there had to be large-scale forcing happening that would force the ocean and the estuary at the same time. And that’s the atmosphere, that’s the ocean-atmosphere exchange of heat that would be the only possible cause of that.”

‘Devastating impacts’

Exactly what regular extreme temperatures do to aquatic life in the Bay isn’t clear, although Mazzini and Pianca warned they could have “devastating impacts.”

“Probably some of those species that are in the bay currently that are more adapted to cooler water may be completely gone,” said Chris Moore, a scientist with the Chesapeake Bay Foundation. Summer flounder, which fisheries managers have already detected moving northward, would likely decamp for colder currents, along with striped bass. Other species traditionally found only farther south could move in to take their place. Shrimp, for instance, have always been present in the bay but in recent years have multiplied to the point that Virginia launched an experimental fishery.

Mass mortalities aren’t out of the question. Marine heat waves have been linked to large die-offs of kelp, coral reefs and some types of underwater vegetation and fish in various parts of the ocean. That could be due to the intensity of these short events compared to the longer-term warming scientists have charted in oceans worldwide, Mazzini said.

“Perhaps species, marine organisms, may have a better chance to adapt to these gradual changes and become resilient, while these short-term impacts, they can be quite devastating,” he said. “Sometimes they can knock out a population.”

Equally as worrying are the impacts of marine heat waves on the bay’s oxygen content. Just as oxygen in the atmosphere is critical for terrestrial life, so too does its presence or absence shape marine ecosystems. The so-called dead zones that have long plagued the Chesapeake Bay are due to hypoxia, or too little dissolved oxygen. Generally, these dead zones are linked to aggressive algal blooms, fueled by excess nitrogen pouring into the bay, but temperature also plays a role: the warmer the water, the less oxygen it holds.

More frequent marine heat waves, Mazzini and Pianca found, “could aggravate hypoxia in the bay,” potentially pushing its ecosystem “past a dangerous tipping point.”

Moore said the VIMS study may explain some of the events and trends seen in the bay over the years. While researchers have been aware of the long-term warming of the Chesapeake Bay, the effect of marine heat waves on estuaries like the bay has received little attention until now.

“This gives us a lot more information on what’s happening in the ecosystem on a day-to-day basis,” he said. “We know we have an increased algal load, but we also know in certain time periods we can see those blooms. … This is beginning to tell us that much like humans, our natural resources are being more stressed during certain periods of time.”


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Underwater Heat Waves: Chesapeake Bay’s Waters Could Suffer Extreme Heat for Half the Year by 2100