The Chesapeake Bay has a new seagrass champion, but it’s not necessarily good news for the Bay.

The Bay supports roughly 24 underwater grass species, but for decades, eelgrass dominated. The tall, deeply rooted plants formed vast meadows that accounted for two-fifths of the Bay’s total underwater grass acreage as recently as the late 1990s.

Widgeon grass, left, has replaced eelgrass, right, as the dominant underwater grass in the Chesapeake Bay. Scientists worry that its thin, less dense blades may not provide the same habitat quality for crabs and fish. (Virginia Institute of Marine Science)

But the Chesapeake is near the southern edge of its range, and eelgrass has since suffered a series of heat-related die-offs. By 2019, it accounted for just 19% of the Bay’s grass coverage, according to a new study. And a different grass reigns instead.

The paper, published in the Proceedings of the National Academy of Sciences, confirms what scientists have long suspected: Widgeon grass is now the dominant species of submerged aquatic vegetation, or SAV, in the Bay, accounting for up to 43% of the total acreage in recent years.

“Widgeon grass has been hanging around forever,” said Marc Hensel, a post-doctoral research associate at the Virginia Institute of Marine Science and the study’s lead author. “But it’s always been in the fringes, literally, because it lives in shallow water.”

A mix of factors has moved the fringe actor to center stage.

Gradually warming water caused eelgrass to retreat from over half of its area in 1991. Meanwhile, efforts to reduce nutrient pollution in the Bay brought clearer water — at least in some years — that allowed widgeon grass, which requires especially clear water, to expand.

The ability of the more heat-tolerant widgeon grass to move into areas vacated by eelgrass is good news because scientists had long been worried that nothing would replace eelgrass in mid– and-high-salinity parts of the Chesapeake. Those areas contain — by far — the greatest amount of potential SAV habitat, but they are off-limits to most of the Bay’s other grasses, which need low-salinity or freshwater areas.

Widgeon grass is often an unreliable replacement because of its wild year-to-year fluctuations.

It produces large amounts of seeds that can persist in the sediment for years. When conditions are right, vast beds suddenly spring forth.

But it is equally vulnerable to poor water quality. The study found that in years with high spring flows — which flush large amounts of sediment and nutrients into the Bay — widgeon grass beds can “totally collapse.” The sediment clouds the water, and the nutrients spur algae blooms, blocking the sunlight the plants need to survive.

The tall blades of eelgrass can better withstand such conditions because they extend close to the water surface, avoiding the worst of the murky conditions. Widgeon grass does not reach its full height until mid-summer, and the short springtime plants are more vulnerable to cloudy conditions.

Widgeon grass has long been known to have boom and bust cycles, but the full magnitude of those changes was previously unknown.

An annual aerial survey conducted since 1984 measures the overall SAV acreage but cannot differentiate among species. Each year, though, scientists spot-check survey results, during which time they identify the present species. Using that and other information, scientists in the new study constructed a model showing how eelgrass and widgeon grass abundance has changed.

While the amount of underwater grasses has always varied from year to year, the model shows how the rise of widgeon grass has exaggerated those swings. Half of the years between 1999 and 2019 had increases or decreases in Baywide acreage of more than 20%, largely driven by widgeon grass. Such large variations were rare in earlier years.

In 2019 alone, half of the widgeon grass acreage was lost, the study reported.

“The booms and busts, the peaks and valleys, from a Baywide perspective, appear to be widgeon grass peaks and widgeon grass valleys,” Hensel said.

Even when widgeon grass fills some of the same areas previously dominated by eelgrass, biologists worry that it may not provide the same benefits.

It is not only shorter, but its blades are thinner than those of eelgrass, so they may not provide the same shelter for juvenile blue crabs, spot, or black sea bass seeking refuge from predators. Unlike eelgrass, which persists much of the year, it provides habitat for only a few months.

Further, the scientists reported widgeon grass lacks the extensive eelgrass root system, making it more vulnerable to wave action and other physical disturbances.

More work is underway to provide a fuller look at the ecological implications of a Bay dominated by widgeon grass, said Chris Patrick, director of the SAV monitoring and restoration program at VIMS and a co-author of the study.

“It becomes more of an issue for the Chesapeake when widgeon grass stops being a supporting player and starts being the main event in terms of seagrass,” Patrick said, “because then we have to contend with these swings and how that’s driving increases and decreases in service provisioning from year to year.”

Scientists are also looking at the implications of eelgrass’s demise and the rise of widgeon grass for reaching the region’s 185,000-acre SAV goal. That goal was partly based on the expectation of returning eelgrass to areas where beds existed during the last century. It’s unclear whether widgeon grass, which generally needs more light and doesn’t survive in deeper areas, can occupy all of the areas vacated by eelgrass.

“Widgeon grass really wasn’t an important player in those discussions,” Patrick said. “It’s largely ignored by seagrass biologists in the Bay community until recently. It’s a different species. It’s got different light needs, and it’s got different tolerances for stressors.”

Patrick said the fact that widgeon grass was a largely ignored species until recently provides a case study of how climate change can alter systems in unpredictable ways — and how the Chesapeake of the future will be different from the Bay of the past.

In fact, the study underestimates the extent of the change, Patrick said. It only examines periods since 1991 for which data was available, but eelgrass is known to have covered a much greater area in the past.

“We’ve actually lost more ground than this paper really fully alludes to because we didn’t have an aerial survey, and we didn’t have the ground observation data,” Patrick said.

The good news is that, given unusually low river flows into the Bay this spring, the conditions are right for widgeon grass to thrive this summer, biologists say.

But how long those beds persist is anyone’s guess.

This article was originally published on the and is republished with permission.

Karl Blankenship is editor-at-large of the Bay Journal.

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