Chesapeake Bay Species Article Series: Submerged Aquatic Vegetation

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Monitoring and restoring the diverse communities of underwater grasses in the Chesapeake Bay

Beneath the shallow waters of the Chesapeake Bay, underwater grasses sway in the current. Although different species vary in appearance, underwater grasses, also known as submerged aquatic vegetation (SAV), are all critical to the Chesapeake Bay ecosystem. Indeed, SAV is closely monitored by scientists because its health is dependent on good water quality, and, therefore, its abundance is an excellent barometer of the overall health of the Bay. 

Each year, thanks to the funding support of its partners, the Virginia Institute of Marine Science (VIMS) maps the SAV beds of the entire Chesapeake Bay using aerial photography and satellite imagery. The surveys, which are typically conducted between May and October, estimate the annual acreage of SAV. VIMS began monitoring SAV annually in 1984.

According to data released on July 31, 2024, by the Chesapeake Bay Program and Maryland’s Department of Natural Resources (DNR), an estimated 82,937 acres of underwater grasses were distributed throughout the Chesapeake Bay and its tidal tributaries in 2023—a 7 percent increase over 2022 acreage. The bay-wide gains observed in 2023 can largely be attributed to increases in three species: widgeon grass, sago pondweed, and eelgrass. 

SAV abundance remained fairly steady in Maryland’s portion of the Bay in 2023, reaching 37,770 acres. Scientists mapped 37,297 acres of underwater grasses in Maryland during the annual survey in 2022. 

Signed in 2014, the Chesapeake Bay Watershed Agreement established interim goals for SAV restoration. The ultimate goal seeks to restore 185,000 acres of underwater grasses with a target of 130,000 acres by 2025. The 2023 data indicates a 45 percent attainment of the ultimate goal and a 61 percent achievement of the 2025 target.

“A 7 percent bay-wide increase is really good,” says Brooke Landry, program chief of living resource assessment for the DNR and chair of the Chesapeake Bay Program SAV Workgroup. “When things are trending up, we typically see about a 3 percent increase, so we’re very happy. We still haven’t recovered from the significant losses in 2019—about one-third of the acreage—when underwater grasses experienced a rapid decline following two years of above-average rainfall, heavy river flows, and stormwater runoff. But I’m confident that we will continue to make progress. I’m especially excited to see the expansion in recent years of species like Horned pondweed and sago pondweed in the mid-Bay.” 

According to the DNR, there are 15 commonly occurring species of underwater grasses in the Chesapeake Bay. Like terrestrial vascular plants, SAV has leaves, roots, conducting tissues, flowers, and seeds, and it makes its own food through photosynthesis. Since underwater grasses would dry out and overheat on land, they must be submerged by water at all times. And they can only grow in shallow water, so sufficient sunlight can reach them. Underwater grasses evolved to lose structural compounds that terrestrial plants have because they don’t need them, but they do have air pockets in their tissue that keep them vertical/upright, and they will float if they break loose. 

SAV plays a critical role in the Chesapeake Bay ecosystem. It serves as food for waterfowl, fish, shellfish, and invertebrates, provides habitat and nursery sites, and offers shelter from predators. In addition, SAV absorbs nutrients, produces oxygen during photosynthesis, improves water clarity, settles suspended sediments, stabilizes bottom sediments, absorbs wave energy, and reduces shoreline erosion. 

Threats to SAV

Although underwater grasses provide several benefits to the ecosystem, they also face threats that impact their survival and abundance. Monitoring the distribution, density, and diversity of underwater grasses helps scientists determine factors that adversely affect SAV—enabling them to forecast emerging issues and implement management strategies.  

“Although it’s difficult to identify a primary threat to SAV—since many of the challenges are interconnected—poor water quality is an immediate threat,” says Emmett Duffy, chief scientist and director of Tennenbaum Marine Observatories Network at the Smithsonian Environmental Research Center (SERC), located in Edgewater. “Pollutants, such as sediment from runoff, cloud the water, and nutrients, including nitrogen and phosphorus, fuel algae growth in the water column and on the plants. This prevents sunlight from reaching the plants, causing SAV beds to die.” 

Healthy grass beds absorb some nutrient pollutants, which improves water clarity. But it’s a delicate balance. Since too much pollution is detrimental, the ultimate goal is to reduce the amount of pollution that enters the Bay. 

According to the Chesapeake Bay Program, pollution reduction efforts, such as the Chesapeake Bay Total Maximum Daily Load (TMDL), aim to slow the flow of nutrients and sediment into the Bay. A “pollution diet,” which was developed by the U.S. Environmental Protection Agency, the TMDL sets limits on nitrogen, phosphorus, and sediment pollution to meet water quality standards in the Bay and its tidal rivers. 

Weather conditions can also affect underwater grasses. For example, experts attribute the losses in 2019 largely to a decline in widgeon grass. Known as a “boom-or-bust” species, the abundance of widgeon grass can fluctuate significantly from year to year, and it’s highly sensitive to changes in the light environment. Scientists at VIMS identified the extremely wet spring of 2019, which brought a high volume of sediment-laden water into the Chesapeake Bay, as the most likely cause of the major crash in widgeon grass that year.   

In addition, heavy rains can alter salinity levels in the water. Salinity is one of the primary factors that determines where underwater grass species live in the Bay, which is divided into four salinity zones. Landry says there is a freshwater SAV community that prefers the tidal fresh and oligohaline (slightly salty) zones, a mesohaline community that thrives in the moderately salty zone, and a polyhaline community that is found in the very salty zone of the lower Bay. Changes in salinity due to heavy rains can impact SAV’s ability to survive, depending on the salinity zone in which it lives. 

And, of course, climate change is a major threat to underwater grasses. Warmer temperatures, an increase in severe storms and hurricanes, and rising sea levels can have serious consequences for SAV.

“The Bay is a temperate system, and some species, such as eelgrass, don’t do well when it’s too warm,” says Christopher J. Patrick, director of the SAV Monitoring and Restoration Program at VIMS and a member of the SAV Workgroup. “After high summer temperatures in 2005 and 2010 caused large meadows of eelgrass in the lower Chesapeake Bay to die, we saw an explosion of widgeon grass, which can tolerate heat better, in the following decade. As temperatures continue to get warmer, especially the frequency of extreme heat wave events, the concern is that these eelgrass losses will happen more frequently, which could lead to catastrophic losses of eelgrass.” 

SAV Monitoring Program 

The Chesapeake Bay Program began its annual SAV monitoring efforts in 1984 to estimate SAV acreage and density across the Bay. Since then, its work has evolved to include collecting detailed habitat and water quality data and providing educational volunteer opportunities to communities across the watershed. Today, the program utilizes a three-tiered approach, including aerial surveys, volunteer monitoring, and sentinel sites.

The first tier involves working with VIMS to conduct the annual bay-wide aerial SAV surveys. The data collected from the surveys are used not only to report the acreage and density of SAV but also to track progress and identify trends over time. 

“There were bay-wide mapping efforts in the late 1970s and regional efforts at various points over the last century,” Patrick says. “But the annual survey marked the start of a continued, sustained monitoring effort that allows us to keep close track of SAV throughout the Chesapeake Bay on a year-to-year basis.”

To restore and maintain the health of underwater grasses, the Chesapeake Bay Program and its partners focus on improving water clarity, planting underwater grasses, protecting existing grass beds, and promoting SAV-related education and outreach. The Chesapeake Bay SAV Watchers program, which is the second tier of the SAV Monitoring Program, is an integral part of these efforts.

Created in 2019 by the Chesapeake Bay Program and coordinated by the SAV Workgroup, SAV Watchers offers volunteer scientists an opportunity to engage in an educational experience that also provides useful data for Bay scientists and resource managers. Participants collect data on underwater grass diversity and habitat characteristics at sites throughout the Bay and submit photos of their observations. SAV Watchers is the Chesapeake Bay Program’s first official SAV monitoring program for volunteers.

“SAV Watchers is a great opportunity for members of the community to become involved in SAV monitoring and restoration efforts and help make a difference,” says Landry, who is the coordinator of the program. “In addition to volunteer engagement, some of our larger Riverkeeper organizations have expanded their interest from SAV monitoring to restoration. They now work in partnership with the SAV restoration team at the DNR to locate donor SAV beds, harvest and process seeds, choose restoration sites, and conduct the planting. So, we’re getting more people involved in Chesapeake Bay stewardship.”

In accordance with procedures established by the Chesapeake Bay Program, environmental organizations, including ShoreRivers and Arundel Rivers Federation, have created and coordinated volunteer SAV monitoring and restoration programs within their communities. 

ShoreRivers has been involved with SAV restoration efforts for more than eight years and works closely with the DNR and Anne Arundel Community College (AACC). In 2020, thanks to funding from the Chesapeake Bay Trust, the organization built a turbulator to increase its capacity to restore underwater grass beds. A turbulator is a tank that agitates harvested SAV to separate the seeds from the plant stems. After separation, the seeds are mixed with sand and planted in shallow waters, where sunlight can reach the bottom. 

“The turbulator has allowed us to process the seeds ourselves and effectively doubled the state’s capacity for SAV restoration,” says Captain Zack Kelleher, Sassafras Riverkeeper, ShoreRivers. “Over the past four seasons, we have harvested, processed, and planted more than 15 million seeds in our local Eastern Shore waterways, covering more than 75 acres of river bottom. Through this work, we have also engaged hundreds of volunteers and community members. More than 50 volunteers are involved with our SAV Watchers program, and they go out each summer to scout for and collect data about the grass beds in our local rivers and creeks.” 

In the spring of 2024, Arundel Rivers Federation launched the Arundel Rivers SAV Restoration and Outreach Program, which was made possible through funding provided by the Chesapeake Bay Trust and a partnership with the DNR, AACC’s Environmental Center, and Anne Arundel County’s Department of Recreation and Parks.

“In its first year of operation, Arundel Rivers and its partners processed more than two million native grass seeds, which could restore up to 17 acres of grass beds,” says Elle Bassett, the South, West, and Rhode Riverkeeper, Arundel Rivers Federation. “The South, West, and Rhode waterways haven’t seen substantial grass growth for many years, so this project will contribute to meeting our SAV restoration goals for these rivers. The program is also a great hands-on volunteer opportunity that raises awareness about the importance of SAV. Volunteers help scout for grass beds, monitor restoration sites, and process the seeds.” 

To learn more about SAV Watchers, visit chesapeakebaysavwatchers.com. For more information about the programs at ShoreRivers and Arundel Rivers Federation, visit shorerivers.org and arundelrivers.org.

Created in 2022, the third tier of the SAV Monitoring Program is the Chesapeake Bay SAV Sentinel Site Program. Sentinel sites are located in each of the Bay’s four salinity zones and are monitored using a standardized, in-depth data collection protocol in which scientists track changes in SAV habitat. The program is coordinated by the SAV Workgroup. 

Future Outlook

Although progress has been made in SAV restoration, the gains in recent years do not offset the losses in 2019. So, unfortunately, the interim goal for 2025 will not be met. The goal is not out of reach, but the timeline may have been too ambitious. However, restoration partners are optimistic about the future. They know that there is still a lot of work to be done, but they are committed to SAV restoration and will do everything possible to advance this critical effort.

“Perhaps we need to re-evaluate our interim goals,” Patrick says. “The timeline of current goals may be unrealistic. But although we won’t meet our interim goal for 2025, we are going in the right direction. With time, we will achieve our bay-wide goal—and perhaps even surpass it.”

“Humans have done a lot of harm to the planet, and we owe it to Mother Nature to pay back,” Duffy adds. “Our dedicated partners and our numerous, enthusiastic volunteers are all working together to restore SAV and ensure a healthier Bay for future generations.”