Underwater grasses—also known as submerged aquatic vegetation or SAV—are plants that grow in the shallow waters of the Chesapeake Bay and its streams, creeks and rivers. Underwater grasses are a critical part of the Bay ecosystem: they provide wildlife with food and habitat, add oxygen to the water, absorb nutrient pollution, trap sediment and reduce erosion. Like all plants, underwater grasses need sunlight to grow, which makes improving water clarity an important step in underwater grass restoration.
What are underwater grasses?
Also known as submerged aquatic vegetation, or SAV, underwater grasses are plants that grow in the shallow waters of the Chesapeake Bay and its streams, creeks and rivers, and are a critical part of the Bay ecosystem.
Why are underwater grasses important?
Underwater grass beds are critical to the Chesapeake Bay ecosystem. They offer food to small invertebrates and migratory waterfowl; shelter young fish and blue crabs; and keep our waters clear and healthy by absorbing excess nutrients, trapping suspended sediment and slowing shoreline erosion.
These grasses also act as an excellent measure of Chesapeake Bay health. Although underwater grasses are sensitive to pollution, they respond fairly quickly to improvements in water quality. This means their abundance is a good indicator of restoration progress. You can watch changes in underwater grass abundance over time using this interactive map.
Providing food, habitat and oxygen
Bay grass beds provide food and shelter to a number of wildlife species, many of which depend on each other for survival.
Microscopic zooplankton feed on decaying underwater grasses, ridding grass beds of waste. In turn, these zooplankton become food for larger critters.
Small invertebrates–including barnacles, sponges, sea slugs and sea squirts–feed on and attach themselves to the stems and leaves of underwater grasses. Small crustaceans consume harmful algae that might otherwise grow on underwater grasses and stunt their growth.
Young crabs and fish–including spot, croakers, weakfish, Atlantic menhaden, white perch and American shad–find protective nurseries in underwater grass beds. Scientists have found 30 times more juvenile blue crabs in underwater grass beds than in areas with no grasses.
Small fish dart between underwater grasses as they hunt for prey and hide from predators.
Molting blue crabs seek refuge in underwater grass beds while their soft shells make them vulnerable to predation.
Migratory waterfowl feed on underwater grasses and the animals that live in underwater grass beds.
Underwater grasses also add oxygen to the water during photosynthesis. Underwater critters need oxygen to survive.
Keeping the water clear and healthy
Underwater grass beds help keep the Chesapeake Bay clean and healthy by:
Absorbing excess nutrients like nitrogen and phosphorous;
Trapping particles of sand, silt and sediment, which might otherwise cloud the water and suffocate shellfish; and
Reducing erosion by slowing water currents, anchoring bottom sediment in place and softening waves that break along the shoreline.
How much underwater grass is in the Chesapeake Bay?
In 2021, an estimated 67,470 acres of underwater grasses were mapped in the Chesapeake Bay and its tidal tributaries. This is a 7% increase from the 2020 total of 63,132 acres, but it only 37% of the 185,000 acres needed for a restored Bay.
Experts attribute the losses largely to a decline in widgeon grass, a species whose abundance can rise and fall from year to year in response to changes in water quality. For example, in 2018, widgeon grass increased in various parts of the Bay before subsequently declining in 2019. This mirrors a similar rapid increase in 2001 and 2002 that was followed by about a 50% decline in 2003. Though the precise cause for the fall in widgeon grass is unknown, a contributing factor may have been higher than average freshwater entering the Bay from rivers through 2019, which reduces water clarity and blocks sunlight from reaching the plants.
How do underwater grasses grow?
Just like plants on land, underwater grasses go through photosynthesis to convert sunlight into food. Sunlight, therefore, is the most important factor determining grass survival. Water must be clear enough for sunlight to pass through it and reach the grasses that grow on the bottom of rivers, streams and the Chesapeake Bay.
How does pollution affect underwater grasses?
Underwater grass growth is hindered by pollutants that cloud the water. These pollutants include excess nutrients—which fuel the growth of dense algae blooms—and suspended particles of sand, silt and sediment.
Healthy underwater grass beds can trap and absorb some of this nutrient and sediment pollution, but too much of it can block sunlight from reaching the plants.
How does weather affect underwater grasses?
Extreme weather, including high temperatures or excess rainfall, can harm underwater grasses.
Some species, like eelgrass, cannot grow in water that is too warm. In 2005, high temperatures caused large beds of eelgrass in the lower Chesapeake Bay to die. It can take several years for underwater grass beds to recover from these kinds of large-scale losses.
Precipitation—and the water-clouding pollution it pushes into rivers and streams—has a big influence on underwater grass growth.
Higher than average rainfall can push nutrient and sediment pollution into the Bay and its rivers and streams. In 2012, scientists recorded a 21 percent decline in underwater grass abundance, attributed in part to the strong storms—like Hurricane Irene and Tropical Storm Lee—seen in the late summer and fall of 2011.
Lower than average rainfall can result in clearer water, which can boost underwater grass growth.
How are underwater grasses being restored?
Modern science allows researchers to target underwater grass restoration to those areas where grasses once grew. But poor water quality, irregular weather and a lack of funding have slowed restoration progress.
Chesapeake Bay Program partners use four initiatives to restore and maintain the health of underwater grasses: improving water clarity, planting underwater grasses, protecting existing grass beds and enhancing underwater grass-related education and outreach.
Improving water clarity
Improved water clarity is critical to underwater grass restoration because clouded water can block sunlight from reaching aquatic plants. When water is clear, more sunlight can reach the bottom of rivers, streams and the Chesapeake Bay, fueling the growth of new grasses and the expansion of existing grass beds.
Maryland, Virginia and the District of Columbia have outlined water clarity standards for the shallow waters where underwater grasses grow or once could be found. These standards list the amount of sunlight that must be able to pass through the water.
Pollution reduction efforts—including the Bay “pollution diet,” or Total Maximum Daily Load (TMDL)—aim to slow the flow of nutrients and sediment into the Bay and its waterways. Upgrades to wastewater treatment plants and on-farm conservation practices, for example, are expected to improve water clarity.
Planting underwater grasses
Although underwater grasses can naturally colonize an area faster than we can spread them through planting, planting underwater grasses can lead to restoration success, enhancing the natural expansion of healthy grass beds.
The Maryland Department of Natural Resources (DNR), for instance, led two large-scale eelgrass restoration projects on the Patuxent and Potomac rivers between 2003 and 2007. Scientists found that distributing seeds collected from healthy grass beds elsewhere could accelerate the natural expansion of weaker eelgrass beds. Scientists need sustained funding and adequate plant and seed supplies to continue this kind of work.
Protecting existing grass beds
Underwater grass beds can be damaged by a number of things, including human activities, invasive species and climate change.
Shellfish dredges and boat propellers can pull underwater grasses up from the bottom of rivers, streams and the Bay. In shallow waters, “scars” from these human activities are often visible across grass beds.
Invasive species can threaten existing underwater grass beds. Mute swans, for instance, can eat more than eight pounds of grasses in a single day, pulling plants up from their roots and depleting entire grass beds. And water chestnut floats on the water’s surface, blocking sunlight from reaching grasses growing underneath.
Rising temperatures that accompany climate change could make the Bay an unsuitable habitat for certain underwater grass species. And the flooding and shoreline erosion that accompany sea level rise could lead to a further decline in water clarity.
What can you do to help?
To support underwater grasses in the Bay watershed, boaters can follow posted speed limits and no-wake laws to avoid harming grass beds. Boaters can also steer clear of any grasses growing in shallow waters.