Underwater Grasses Are Key to Chesapeake Bay Health

Underwater grasses in the Susquehanna Flats, which has seen considerable submerged aquatic vegetation growth in recent years. Maryland Department of Natural Resources photo.

Across the Chesapeake Bay and its tributaries, below the water’s surface, ribbons of green sway in the current.

They are stems of pondweed, eelgrass, widgeon grass, stargrass, and a number of other species. Together they make up the underwater grasses that form a crucial habitat for Bay life, help clean the water, and mitigate effects of climate change.

For such fragile blades, the grasses carry a lot of weight in the Chesapeake Bay ecosystem, and Brooke Landry, program chief of Living Resource Assessment at the Maryland Department of Natural Resources (DNR), said underwater grasses are sometimes overlooked for their critical role.

“It can help to compare underwater grasses to terrestrial plants. If you think about the land without any plants or trees, it’s a barren landscape that doesn’t support much life,” Landry said. “We need plants to provide oxygen and habitat and a number of other ecosystem services. It’s the exact same underwater.”

On Wednesday, the Chesapeake Bay Program, Maryland DNR, and the Virginia Institute of Marine Science released the 2023 count of underwater grass abundance, reporting an estimated 82,937 acres, 7% higher than last year’s acreage. Of that total, 37,770 acres were in Maryland waters.

Every year, scientists from the Virginia Institute of Marine Science use aerial imagery to map the distribution of underwater grasses, also called submerged aquatic vegetation or SAV, in the Bay and its tidal tributaries. This data is used to track progress toward the restoration goal established most recently in the 2014 Chesapeake Bay Agreement, and helps Maryland,  Virginia, and the District of Columbia manage underwater grasses and establish protection zones for the aquatic plants.

Scientists in the 1970s raised the alarm about declining underwater grasses in the Bay which attracted Congressional attention and helped lead to the creation of the Bay Program, said Landry, who is also the chair of Chesapeake Bay Program’s Submerged Aquatic Vegetation Workgroup. 

As the population of the watershed grew, nitrogen, phosphorus and sediment polluting the Bay wiped out swaths of grasses and prevented recovery and more losses followed Hurricane Agnes in 1972.

Since then, the recovery of underwater grasses has faced continued challenges, but Landry sees their recovery as critical to the overall health of the Bay.

For one thing, the grasses provide a foundation for a vast network of life in the Chesapeake. Tiny invertebrates live on the grass blades, and the vegetation provides shelter and an oxygen-rich environment for fish and invertebrates like blue crabs. Waterfowl rely heavily on underwater grasses as well, eating their seeds and tubers.

But underwater grasses also help to filter out excess nutrients that pollute the Bay. They use up nitrogen and phosphorus, and grass beds slow the water to allow suspended sediments to settle, Landry said. That improves water clarity, and water is noticeably clearer in areas with underwater grasses.

The water outside an SAV bed is noticeably murky. DNR photo

Water clarity is markedly clearer in the direct vicinity of SAV beds. DNR photo

This same quality makes grasses beneficial for dissipating waves, which are an increasing concern as climate change leads to more extreme weather events and increased flooding.

“They’re the first barrier for your shoreline when storms are rolling in across the water,” Landry said.

Like other plants, underwater grasses act as carbon sinks, and they also help to neutralize the acidity of the Bay.

Despite these benefits, underwater grasses have something of a public image problem. Some coastal residents see them as a nuisance for boating and swimming, and Landry said people might have negative associations with all underwater plants in Maryland because of a general association with harmful algal blooms, macroalgae, and invasive species that are highly publicized. 

“They see green in the water and think it’s a bad thing rather than a good thing,” Landry said 

She said the state is trying to change that perception and encourage good stewardship of underwater grasses.

A 2024 Maryland law that takes effect in October will limit the amount of underwater grasses that property owners can remove for navigation purposes from a width of 60 feet to a width of 20 feet out to the navigable channel. It also requires anyone trimming underwater grasses for that purpose to contact DNR for guidance and to report information about the removal.

Landry said the law should “reduce the impact to a valued resource” by shrinking the area where grasses can be removed, and that the reporting requirement will help DNR track the impact of cutting back grasses.

Landry encourages people to embrace the grasses around them, such as by slowing down your boat, raising your propeller, or swimming around tangles of vegetation.

“Societally, we try to eliminate every single inconvenience to the detriment of the environment,” she said. “We need to get back to a point where we can live in some harmony with the natural world around us. If you want to swim off the shoreline, your feet might touch underwater grasses—they won’t harm you, and for the benefits they provide, it’s worth the inconvenience.”

She said she encourages everyone to strap on a mask and snorkel through a bed of underwater grasses.

“Some of our SAV species are absolutely beautiful and the beds are teaming with life,” she said. “And once you look at the plants, it’s less likely you’ll get the willies if you accidentally touch one.”

Underwater grasses hit 107,000 acres in 2018 and appeared to be on track for the 2025 target of 130,000 acres, but two years of heavy precipitation in 2018 and 2019 combined with the associated runoff set grasses back considerably. Acreage totals in the entire Bay are now 64% of the way to that goal.

The distribution of underwater grasses over time. Via VIMS

Measures to reduce nutrient and sediment pollution in the Bay have facilitated the recovery of underwater grasses (the survey only mapped 38,000 acres of SAV when it officially began in 1984), particularly the implementation of the Chesapeake Bay Total Maximum Daily Load. But Landry said it’s important to take additional steps toward restoration of underwater grasses, including direct planting. 

“We need to increase our capacity for direct SAV restoration using seeds either collected from donor beds in the Bay or grown in a controlled environment, like a greenhouse or aquatic plant nursery,” she said. 

Ultimately, Landry said more capacity to plant areas with grasses will promote further recovery as well as provide opportunities for education and outreach. 

Multiple groups are working on underwater grass restoration, including a team at Maryland DNR, as well as partners at Anne Arundel Community College and a number of Maryland’s riverkeeper organizations, including ShoreRivers on Maryland’s Eastern Shore, and Arundel Rivers Federation in Edgewater.

Landry said residents interested in helping underwater grasses can join the Chesapeake Bay SAV Watchers Program, which trains volunteers to help monitor underwater grasses throughout the Bay. She also hopes making more people aware of the importance of underwater grasses will help build momentum for recovery efforts.

“We need more outreach about the overwhelming benefits of underwater grasses,” she said. “The more people appreciate how beneficial the grasses are to the Bay, the better off we’ll be.”

By Joe Zimmermann, science writer with the Maryland Department of Natural Resources.