According to preliminary data from the Virginia Institute of Marine Science (VIMS), 91,559 acres of underwater grasses were mapped in the Chesapeake Bay in 2018. However, 22 percent of the Bay was not fully mapped due to prolonged turbidity, weather conditions and security restrictions. Using 2017 levels for the unmapped areas, it is estimated that the Bay may have supported 108,960 acres of SAV in 2018. This is a 4 percent increase from 2017 figures and 59 percent of the partnership’s 185,000-acre goal.
Submerged Aquatic Vegetation (SAV) Abundance (1984-2018)
Submerged Aquatic Vegetation (SAV) Abundance (2010-2018)
In 2018, adverse weather conditions, poor water clarity and security restrictions prevented researchers from collecting aerial imagery for a portion of tidal fresh and mesohaline Potomac River; the Susquehanna, Mattaponi, Middle, Choptank, and Honga rivers; and Fishing Bay. Satellite imagery was acquired to address these gaps, but was not sufficient to cover the entire missing area, approximately, 22 percent of the 2017 Bay total. In 2018, high density beds accounted for 61 percent of the total acreage, which is 17 percent higher than 2017.
While the overall trend is upward, the magnitude of changes from 2017 and 2018 varied by salinity zone:
- Tidal Fresh Zone: Underwater grass beds decreased from an estimated 19,903 acres to 19,012* acres, a 4.5 percent decrease and 92 percent achievement of the region’s 20,602-acre goal.
- Oligohaline Zone (slightly salty): Underwater grass beds decreased from an estimated 8,389 acres to 7,687* acres, an 8.5 percent decrease and 74 percent achievement of the region’s 10,334-acre goal.
- Mesohaline Zone (moderately salty): Underwater grass beds increased from an estimated 61,350 acres to 64,281* acres, a 5 percent increase and 53 percent achievement of the region’s 120,306-acre goal.
- Polyhaline Zone (very salty): Underwater grass beds increased from an estimated 15,251 acres to 17,980 acres, an 18 percent increase and 53 percent achievement of the region’s 33,647-acre goal.
*These areas were not fully mapped in 2018
Experts attribute the 2018 increases in the mesohaline and polyhaline zones to a continuing rapid expansion of widgeon grass. Because widgeon grass 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.
Underwater grass beds are critical to the Chesapeake Bay ecosystem. They provide food and shelter to fish and wildlife, sequester carbon, add oxygen to the water, absorb nutrient pollution, reduce shoreline erosion and help suspended particles of sediment settle to the bottom. Because they are sensitive to pollution but quick to respond to improvements in water quality, underwater grass abundance is a good indicator of the Bay’s health. Before Europeans colonized the region, up to 600,000 acres of underwater grasses grew along the shorelines of the Bay and its tributaries. By the mid 1980s, nutrient and sediment pollution had weakened or eliminated many of these grass beds. While climate change, shoreline hardening and stressors that reduce water clarity will continue to impact our restoration success, many of these stressors can be managed with on-the-ground efforts to reduce pollution and research has shown that nutrient reductions made under the Chesapeake Bay Total Maximum Daily Load (Bay TMDL) have played a critical role in underwater grass recovery.
More information about underwater grass abundance in the Chesapeake Bay can be found on the Virginia Institute of Marine Science (VIMS) website.