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Underwater Bay Grass Abundance (Baywide)

In 2016, preliminary data indicates an estimated 97,433 acres of underwater grasses were mapped in the Chesapeake Bay: 7,433 acres greater than the Chesapeake Bay Program’s 2017 restoration target and 53 percent of the partnership’s 185,000-acre goal.

April 22, 2011

Bay grasses, also known as submerged aquatic vegetation, are an integral part of the Chesapeake Bay ecosystem. Lee Karrh from the Maryland Department of Natural Resources (DNR) explains what bay grasses need to grow and why their survival is important to Bay critters.

April 22, 2011

What is submerged aquatic vegetation, and why is it important to the Chesapeake Bay?  Commonly known as “bay grasses” they among the most critical inhabitants of the Bay’s ecosystem.  Find out more about submerged aquatic vegetation, and see what people around Baltimore’s Belvedere Square had to say about it.


Restoring underwater grasses to the rivers, streams and shallow waters of the Chesapeake Bay will dramatically improve the Bay ecosystem. Grass beds provide food and shelter to fish, crustaceans and other species, add oxygen to the water, absorb nutrient pollution, reduce shoreline erosion and help suspended particles of sediment settle to the bottom.

Underwater grasses—also known as submerged aquatic vegetation or SAV—are sensitive to pollution but quick to respond to improvements in water quality. This means their abundance is a good indicator of Bay health. As pollution declines and water clarity improves, scientists expect underwater grass beds to expand.


In 2014, the Chesapeake Bay Program renewed its goal to achieve and sustain 185,000 acres of underwater grasses in the Chesapeake Bay. Progress toward this goal will be measured against a target of 90,000 acres by 2017—which was surpassed in 2015—and 130,000 acres by 2025. 

Past goals

In 1993, the Bay Program set a goal to restore 114,000 acres of underwater grasses to the Bay. Based on recovery rates at the time, partners expected to achieve this goal by 2005.

In 2000, the Bay Program recommitted to this 114,000-acre goal but agreed to “revise… restoration goals and strategies to reflect historic [underwater grass] abundance, measured as acreage and density from the 1930s to the present.” In 2003, the Bay Program adopted the Strategy to Accelerate the Protection and Restoration of Submerged Aquatic Vegetation in the Chesapeake Bay, which set a new Bay-wide restoration goal of 185,000 acres by 2010.

Historic data

Reviews of photographs from a number of sites dating back to 1937 suggest that close to 200,000 acres of underwater grasses may have once grown along the shorelines of the Chesapeake Bay. By 1984, nutrient and sediment pollution had weakened or eliminated grass beds in many areas, contributing to a Bay-wide SAV decline.

Long-term trend (1984-2016)

Between 1984 and 2015, underwater grass abundance increased from 38,958 acres to 97,433 acres.

Short-term trend (2007-2016)

Between 2007 and 2016, underwater grass abundance increased from 64,918 acres to 97,433 acres.

Change from previous year (2015-2016)

Between 2015 and 2016, underwater grass abundance in the regions of the Chesapeake Bay that were mapped in both years increased eight percent from 90,334 acres to 97,427 acres. Underwater grass abundance across the entire Bay measured 92,315 acres in 2015, and conservative estimates for the areas of the Bay that were not mapped suggest this number may have been at least 99,409 acres in 2016. This total surpasses the Chesapeake Bay Program’s 2017 restoration target and marks a 54 percent achievement of the partnership’s 185,000-acre goal.

Researchers attribute the rise in underwater grasses to a strong increase in the tidal freshwater and moderately salty regions of the Bay. The iconic grass beds at the mouth of the Susquehanna River, for instance, continued their four-year recovery following damage from Hurricane Irene and Tropical Storm Lee. And at over 10,000 acres, the grasses that stretch from Smith Island to Tangier Island have become the biggest contiguous grass bed in the Bay. Widgeon grass, in particular, expanded in this bed and other moderately salty waters, but because it is a “boom and bust” species whose abundance can rise and fall from year to year, a widgeon-dominant spike is not guaranteed to persist in future seasons. Researchers observed a drop in the eelgrass that grows in the very salty waters of the lower Bay, where beds had increased in recent years following losses that occurred during the hot summers of 2005 and 2010.

Additional Information

Collecting data

This data is collected through an aerial survey flown from late spring to early fall by researchers with the Virginia Institute of Marine Science (VIMS). VIMS processes the photography in fall and winter and provides preliminary area totals the following spring. Visit the VIMS SAV website for more information about the survey or for segment-specific survey results.

Reporting data

For almost 30 years, underwater grass acreage was reported in three geographic zones. Zone 1 was above the Chesapeake Bay Bridge, Zone 2 covered the area between the Bay Bridge and the mouth of the Potomac River and Zone 3 covered the area between the mouth of the Potomac and the mouth of the Bay. But these zones span a wide range of salinities and do not account for salinity-related changes in underwater grass communities. Because the makeup of grass communities changes as salinity increases—and because these communities respond differently to heat, drought, storms and other events—it makes more ecological sense to report underwater grass acreage by salinity zone than by geographic zone. This can also help scientists find patterns in how underwater grasses are growing.

Source of Data

Virginia Institute of Marine Science

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