Underwater grass: The unsung hero of the Chesapeake Bay

Inland silversides swim past redhead grass in the Severn River in Anne Arundel County, Md., on July 9, 2024. Redhead grass, a species of bay grass or submerged aquatic vegetation (SAV), had largely disappeared from the river in previous years, but has since rebounded. (Photo by Will Parson/Chesapeake Bay Program)

Have you ever been boating, kayaking, or swimming on the Chesapeake Bay and seen ribbons of green grass at the bottom of the water? If you have, chances are you saw underwater grass beds—also known as Submerged Aquatic Vegetation (SAV.)

Though it’s easy to overlook, underwater grass is a foundational plant of the Chesapeake Bay ecosystem. Healthy SAV beds are bursting with small organisms like shrimp and clams, serving as food for larger fish and migratory waterfowl. Blue crabs use underwater grass as shelter to mate and raise their young safely away from predators. And important forage fish also use the grass as habitat.

Underwater grass is also extremely important for water quality. Often considered the “kidneys of the Bay,” the plants trap sediment and absorb nutrients that flow into the estuary from the larger watershed. This keeps the water clearer, allowing for more plant growth, and helps prevent the low-oxygen “dead zones” caused by excess nutrients. As they grow, underwater grasses even pump dissolved oxygen into the water itself.

Unfortunately, the Chesapeake Bay has historically had trouble holding onto its underwater grass. In the 1970s, when Bay grasses were at their lowest, researchers discovered that excess nutrient pollution from a variety of sources was making the water cloudier and causing small organisms called epiphytes to grow on the leaves. Both of these issues kept sunlight from reaching the plants and resulted in massive die-offs across the estuary.

Today, environmental organizations throughout the region are working to keep excess nutrients and sediment out of the Bay to protect underwater grasses and make the estuary cleaner as a whole.

Stan Kollar of Kollar Nursery in Pylesville, Md., is transformed by underwater grasses on the Susquehanna Flats near Havre de Grace, Md., on July 25, 2016. Kollar was participating in a workshop to learn how to identify and monitor various species of submerged aquatic vegetation (SAV) that live in the Chesapeake Bay—a useful skill to have considering the rise in grass abundance in recent years. A healthy patch of underwater grasses provides food and habitat for animals like fish and crabs, reduces shoreline erosion, and improves water quality by slowing the current and helping sediment filter out of the water column. (Photo by Will Parson/Chesapeake Bay Program)

At the Alliance, we’re focused on two major sources of sediment and nutrient pollution–agriculture and urban runoff. We help farmers reduce their runoff by installing forest buffers, livestock fencing and manure storage pits, and help urban and suburban areas limit runoff by installing green infrastructure. The Alliance’s robust reforestation work also plays a part, since trees absorb sediment and trap or hold onto excess sediment.

Each year, researchers with the Virginia Institute of Marine Science, in partnership with the Chesapeake Bay Program, estimate the amount of SAV in the Bay. According to the 2025 survey, an estimated 89,385 acres of underwater grass was growing in the Bay. This is above the long-term average and about 50,427 acres higher than the survey began in 1984. Researchers believe that this is in part due to efforts to keep nutrient and sediment pollution out of the Bay, coupled with favorable temperature and weather patterns.

So, next time you find yourself out on the water, keep an eye out for underwater grass and try to admire how important this simple and unassuming plant really is. If you want to help protect grasses, consider volunteering with the Alliance on one of the many projects that ultimately make the Bay healthier for underwater grasses. If you like to boat or kayak, practice “Clean, drain, and dry” so that you don’t spread any diseases to underwater grass or Bay wildlife.

Support the Alliance’s Efforts