What is submerged aquatic vegetation?
Submerged aquatic vegetation (SAV), or underwater grasses, grow in the shallow waters of the Chesapeake Bay and its streams, creeks, and rivers. They provide food for marine life and waterfowl, shelter for juvenile fish and blue crabs, and improve water quality by absorbing excess nutrients. They help stabilize the sediment where they grow, preventing erosion and providing extra protection from waves and storms. These underwater grasses deliver some of the highest-value ecosystem services on Earth. SAV is sensitive to water quality changes, making its abundance an excellent measure of the overall health of aquatic ecosystems.
Monitoring SAV at Mallows Bay
The Virginia Institute of Marine Science and other partners monitor underwater grasses through an annual aerial survey program that began in 1978. This data helps researchers monitor changes in SAV distribution and abundance across the Chesapeake Bay, as well as in smaller segments like the Potomac River. Sanctuary staff use this data to study long-term changes in SAV within the sanctuary and better understand the ecosystem health of the sanctuary.
Since 2022, in partnership with the Maryland Department of Natural Resources and Chesapeake Bay SAV Watchers, sanctuary staff have conducted annual surveys of SAV to better understand the species diversity and density within the sanctuary. This data provides insight into the role of underwater grasses in the sanctuary, particularly their relationship to the shipwrecks of Mallows Bay and their role in stabilizing sediment, providing protection from waves and storms, and providing habitat around the wrecks.
SAV Species and Distribution
Dense beds of SAV in the sanctuary are made up of a dozen species of freshwater and slightly brackish species. These species include wild celery (Vallisneria americana), water stargrass (Heteranthera dubia), common waterweed (Elodea canadensis), hydrilla (Hydrilla verticillata), Eurasian watermilfoil (Myriophyllum spicatum), curly pondweed (Potamogeton crispus), slender pondweed (Potamogeton pusillus), redhead grass (Potamogeton perfoliatus), coontail (Ceratophyllum demersum), and four species of naiads - southern (Najas guadalupensis), northern (Najas flexilis), slender (Najas gracillima), and spiny (Najas minor).
While most of these species are native, non-native hydrilla has become the dominant grass in Mallows Bay. Hydrilla was introduced to the Potomac River in the 1980s and rapidly spread throughout the Chesapeake Bay. While hydrilla does offer some benefits, such as great habitat for fish and food for waterfowl, it can grow very thick and block out sunlight, potentially crowding out native grasses.
Between 1990 and 2023, SAV coverage within the 18-square mile sanctuary boundary varied from year to year with an overall decline in SAV coverage over time. SAV cover within the sanctuary peaked in 1993, reaching a total of 825 acres, but has averaged only 200 acres annually for the past ten years. Where SAV was present it was consistently dense, with 70-100% coverage in most areas. The yearly variation and recent decline of SAV in the sanctuary reflects a broader trend in the middle Potomac River over the same time period. Temperature, precipitation, salinity, water clarity, storm events, pollution, erosion, and other factors all influence the changing SAV abundance and distribution in the region.
How Can I Get Involved?
Volunteers have been a key part of carrying out surveys at Mallows Bay and sanctuary staff plan to coordinate a volunteer survey each year to continue this monitoring effort.
In 2022, the first SAV survey at Mallows Bay was conducted in partnership with Force Blue, a team of Special Operations veterans and military-trained combat divers who work alongside scientists on special conservation projects. Watch the video below to learn more about Force Blue’s mission at Mallows Bay and ongoing Sanctuaries Tour of Duty Campaign.
To learn more about how to participate in an SAV Survey, visit our Community Science page.