According to preliminary data from the Virginia Institute of Marine Science (VIMS), 66,387 acres of underwater grasses were mapped in the Chesapeake Bay in 2019. This is 51% of the Chesapeake Bay Program’s 2025 restoration target of 130,000 acres and 36% of the partnership’s 185,000-acre goal.
Although the 66,387 acres mapped in 2019 is a 70% increase from the 38,958 acres observed during the first survey in 1984, it is a 17% decrease from the preceding 10-year average of 79,738 acres and a 38% decrease from 2018 when it was estimated that the Bay may have supported up to 108,078 acres of underwater grasses. Many factors prevented complete mapping of SAV acres in 2018 which resulted in an estimate calculated by combining mapped acreage (99,511 acres) with 2017 data (8,567 acres) for the region that was not mapped to estimate the acreage in the Bay.
Submerged Aquatic Vegetation (SAV) Abundance (1984-2019)
Submerged Aquatic Vegetation (SAV) Abundance (2010-2019)
Experts attribute the decline in underwater grasses to decreases in the moderately salty and very salty regions of the Bay, and reported the following totals in the Bay’s four salinity zones:
- Tidal Fresh Zone: Underwater grass beds decreased from an estimated 19,051* acres to 17,577 acres, an 85% achievement of the region’s 20,602-acre goal.
- Oligohaline Zone (slightly salty): Underwater grass beds increased from an estimated 7,901* acres to 9,014 acres, 87% achievement of the region’s 10,334-acre goal.
- Mesohaline Zone (moderately salty): Underwater grass beds decreased from an estimated 62,933* acres to 27,947 acres, 23% achievement of the region’s 120,306-acre goal.
- Polyhaline Zone (very salty): Underwater grass beds decreased from an estimated 18,192 acres to 11,849 acres, 35% achievement of the region’s 33,647-acre goal.
*These areas were not fully mapped in 2018 due to weather conditions and highly turbid water.
In 2019, the largest declines in terms of total area (an estimated -34,986 acres) were observed in the mesohaline zone, particularly in the Tangier South area (-18,452 acres). Experts attribute the losses largely to a decline in widgeon grass. Widgeon grass is a “boom and bust” species whose abundance can rise and fall from year to year. In 2018, widgeon grass increased in the mesohaline and northern polyhaline salinity zones and eelgrass was observed by the survey for the first time in an extensive region east of Tangier Island. The subsequent decline observed in 2019 mirrors a 50% decline in widgeon grass in 2003 which had been preceded by a rapid increase in 2001 and 2002. Though the precise cause for the decline is unknown, the high average river flows through 2019 may have contributed by reducing water clarity.
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 may have grown 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 the overall underwater grass recovery documented since the Bay-wide aerial survey began in 1984.
More information about underwater grass abundance in the Chesapeake Bay can be found on the Virginia Institute of Marine Science (VIMS) website.