In 2018, the Virginia Institute of Marine Science released a landmark scientific paper declaring success with its longtime efforts to boost the Chesapeake Bay’s seagrass population.

It was “very much, ‘mission accomplished,’” said Chris Patrick, now director of VIMS’ monitoring and restoration program for underwater plants.

But the following year, many of the grasses plummeted, killing off enough to cover Washington D.C.

One of the major culprits: a newly dominant species in the bay called widgeongrass. 

In new research published in the same journal, local scientists investigate how the widgeongrass domination will affect the bay — and what it reveals about climatic shifts that are already underway. The Chesapeake Bay Program, National Park Service, Smithsonian and several Maryland universities collaborated on the project.

“Our past work showed a record-setting resurgence of underwater grasses in response to nutrient management,” the team wrote. "But now we are seeing that the story is vastly more complex and in fact, is still being written.”

The marine science institute, which is part of William & Mary, wanted to learn more about the reasons behind such a large die-off in 2019. 

“We really started to put on our Sherlock Holmes hat to figure out what’s causing these year to year changes,” said lead author and research associate Marc Hensel.

VIMS started surveying the bay’s seagrass population in the mid-1980s.

At that time, the eelgrass species occupied about 50% of it. It “looks kind of like really thick pasta,” Patrick said.

The much thinner widgeongrass, on the other hand, took up only about 15% of the bay. (The remaining percentage includes freshwater and tidal plants elsewhere in the estuary.)

But by the mid-2000s, widgeongrass started on an upward trajectory. Its population has nearly tripled in the past three decades, while eelgrass habitat was cut in half. 

Both are good for the bay. They provide habitat for marine creatures, help clean the water and lessen shoreline erosion.

But here’s where things get complicated.

“This is a climate change story,” Hensel said. “Climate change is responsible for starting to edge eelgrass out of some areas of the Chesapeake.”

Though both species are seagrasses that live in the same water body, they tolerate environmental factors differently.

Widgeongrass have longer-lasting seeds and can tolerate much more heat, which is why it’s exploded amid climate-induced warming and marine heat waves.

It’s much more vulnerable, however, to changes in water quality. 

Those changes are often tied to heavy rainfall during springtime that push more nutrient pollution into the bay – and make the widgeongrass population somewhat more volatile in any given year.

Therefore, it’s likely responsible for both large rises and falls in the overall seagrass population.

The variability can have far-reaching consequences in both the bay and on land, causing economic and ecological instability in things like the fishery market, Hensel said.

All of this is separate from what goes on in the bays on the other side of the Eastern Shore, which feed into the Atlantic Ocean and have a whole different ecosystem. Eelgrass there are thriving amid ongoing restoration work.

Patrick said one of the study’s biggest takeaways should be changing the way officials manage the bay.

Rather than thinking about how climate impacts like rising heat will affect the bay as a whole, he said, scientists need to pay attention to what’s happening on a species-by-species level, and how that all fits together. 

The case of the shifting seagrass in the Chesapeake is one that defies conventional wisdom that “climate change winners” will migrate north and invade new habitats, the researchers say.

“At least in the short term, local species that were always present may step up and redefine their role.”

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