Underwater Grasses Hit Record In Lower Chesapeake Bay Last Year

By Bay Journal, Maryland Matters

August 11, 2025

Underwater grasses continued to mount a comeback in the lower Chesapeake Bay last year, while a steady recovery also continued in upper portions of the Bay.

Areas in between were a different story, as declines in the mid-section of the Chesapeake more than offset improvements elsewhere.

Overall, data from the 2024 aerial survey showed a mixed bag of results, with the Baywide acreage of underwater meadows — one of the Chesapeake’s most critical habitats — decreasing by about 1% but with diverse trends in different areas.

“This year really shows just how dynamic the system is, that we can have two very different trends emerge in different areas,” said Chris Patrick, a researcher with the Virginia Institute of Marine Science who oversees the annual aerial survey of grass beds.

Indeed, last year saw more underwater grasses in the high salinity waters of the lower Bay than had been observed since the survey began four decades ago. The Susquehanna Flats in the upper Bay also saw significant expansion. But the mid-Bay saw large losses, especially along the Eastern Shore.

Altogether, the 2024 survey found 82,778 acres of submerged aquatic vegetation, or SAV, down from 83,419 acres in 2023.

That is well below the Chesapeake Bay Program goal of 185,000 acres, though it is significantly more than the 38,227 acres observed in 1984 when the survey began.

Underwater grass beds provide an important refuge for juvenile blue crabs and fish, as well as food for water­fowl. Plus, they pump oxygen into the water, their roots help stabilize sedi­ment and their leaves buffer wave action.

They are also a closely watched indicator of Bay health because the plants require clear water to get the sunlight needed to survive. They die off when sediment and nutrient-fueled algae blooms cloud the water.

But grass beds are not the same throughout the Chesapeake. Beds in different areas consist of different species based on the salinities of the water, and they can be impacted by local factors as opposed to Baywide conditions.

That might have been at play last year in the mid-Bay, where higher-than-normal spring river flows shifted sharply to lower-than-normal summer flows. That might have contributed to Eastern Shore losses, said Brooke Landry, a biologist with the Maryland Department of Natural Resources and chair of the state-federal Bay Program’s SAV Workgroup.

“It shows that we’re still susceptible to climate change, with super hot water and highly variable river flows potentially impacting the grasses as well as overall water quality,” she said.

Here’s how the Bay fared in different regions last year.

High salinity zone

The best news was in the high salinity, or polyhaline, portion of the Bay, which stretches south from the Rappahannock River and Tangier Island to the mouth of the Bay. The survey found 24,800 acres of grass in that area, a 14% increase from 2023 and the most recorded since the survey began.

It’s especially good news because the area is dominated by eelgrass, a species that is especially sensitive to warming temperatures and poor water quality and has suffered repeated setbacks over the years. As Bay temperatures have increased, polyhaline SAV coverage declined to just 11,975 acres in 2019.

Patrick said the comeback was driven in part by better water clarity, which means the plants get more light and allows eelgrass to move into deeper waters — sometimes as deep as 9 feet.

Last year saw notable increases in Mobjack Bay, Poquoson Flats and nearby Western Shore areas.

“We’ve seen a lot of big expansions,” Patrick said, including areas where grasses had not previously been observed. “It certainly looks like this is tied to improving water clarity. I mean, anecdotally, the water looks clear to me out there.”

The Bay Program goal for the polyhaline is 33,647 acres.

Tidal fresh zone

The tidal fresh zone of the upper Bay and upper portions of its tidal tributaries had 20,218 acres, a 2% increase from last year.

The large grass bed in the Susquehanna Flats, located near the mouth of the Bay’s largest tributary, increased to 10,925 acres last year. That was a 2% increase from 2023 and accounted for slightly more than half of all SAV in tidal fresh waters.

The tidal fresh zone, which hosts more than a dozen SAV species, also saw notable increases in Maryland’s Northeast River on the upper Eastern Shore and in Piscataway Creek off the Potomac River.

Not all the news was good. In Virginia, the upper Mattaponi River and upper portions of the tidal James River saw losses.

The Bay Program goal for the tidal fresh zone is 20,602 acres.

Low salinity zone

The slightly salty oligohaline zone, a relatively small area which includes portions of the up­per Bay and tidal tributaries, saw a 46% increase, to 4,730 acres.

Much of that was a bounce back from losses in recent years, particularly in and near Maryland’s Gunpowder and Middle rivers, which had seen recent declines. Other areas with increases included the Elk, Sassafras, Bush, and Back rivers in Maryland; the middle Potomac; and the middle Rappahannock in Virginia.

But the Chickahominy River in Virginia suffered a significant decline.

The oligohaline zone has the least amount of potential underwater grass habitat, with a Bay Program goal of 10,334 acres.

Mid-salinity zone

The mesohaline zone, with mid-range salinity, suffered a 14% decline, dropping to 33,031 acres. It has the largest amount of potential SAV habitat — its restoration goal is 120,306 acres — and it stretches southward from near Baltimore to the Rappahannock River and Tangier Island in Virginia. It includes large sections of most tidal rivers.

It is dominated by widgeon grass, which is notorious for rapid changes linked to water quality.

Most of the larger losses were along the Eastern Shore. In the lower Choptank River grass coverage fell by 2,200 acres, which made scientists think local conditions were at play.

Patrick said it is likely that heavy spring rains washed more nutrients off the land, which led to reduced water clarity in the Choptank and some other rivers in the area.

Landry said rapid salinity changes, spurred by high flow events in the spring and near-drought conditions in the summer, may have contributed to losses. “While widgeon grass does have a really broad salinity tolerance, it doesn’t respond well to rapid changes in salinity particularly when stressed by water quality,” she said.

While the area saw overall losses, some locations had notable increases, including the Patapsco River in Maryland and the Piankatank River in Virginia.