Underwater grass abundance declines for second year in a row

Only small decrease from previous year, Chesapeake Bay Program reports

Posted 7/28/21

ANNAPOLIS — In 2020, the preliminary figure of 62,169 acres of underwater grasses was mapped in the Chesapeake Bay and its tidal tributaries, achieving 48% of the Chesapeake Bay Program’s …

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Underwater grass abundance declines for second year in a row

Only small decrease from previous year, Chesapeake Bay Program reports

Posted

ANNAPOLIS — In 2020, the preliminary figure of 62,169 acres of underwater grasses was mapped in the Chesapeake Bay and its tidal tributaries, achieving 48% of the Chesapeake Bay Program’s 2025 restoration target of 130,000 acres and 34% of the ultimate restoration goal of 185,000 acres.

Although the acres of underwater grasses observed in 2020 is a 60% increase from the 38,958 acres noted during the first survey in 1984, it is a 20% decrease from the current 10-year average of 78,168 acres.

The preliminary acreage observed in 2020 is a 42% decline from 2018 when it was estimated that the Bay may have supported up to 108,078 acres of underwater grasses. The Chesapeake Bay Program calls this “a staggering decrease.” But the good news while the loss from 2019 to 2020 was only 7%, it suggests that overall, underwater grass beds are beginning to stabilize after a difficult past few years.

In 2020, the largest decline in terms of total area — an estimated 5,684 acres — was again observed in moderately salty waters, particularly in the Tangier Sound, but also in the Eastern Bay, the mouth of the Choptank River and in the Little Choptank River. These losses are largely attributed to the continued decline in widgeon grass, which fluctuates from year-to-year as the species responds rapidly to impacts from extreme weather and changes in water quality.

Underwater grass abundance can vary from species to species and river to river. In 2020, local highlights included:

  • Upper Eastern Shore: In 2020, the Sassafras River lost most of the new grasses that it had gained in 2019, decreasing from 406 acres to 169 acres. Similarly, the Chester River lost nearly 900 acres of grasses from 2019 to 2020, dropping from 1,616 acres to 734 acres.
  • Middle Chesapeake Bay: Although underwater grass acreage in Virginia’s portion of Tangier Sound decreased substantially from the previous year, the abundance of grass beds in Maryland’s portion of Tangier Sound actually increased from 4,315 acres in 2019 to 4,725 acres in 2020. In the nearby Manokin River, underwater grass acreage increased from 583 acres in 2019 to 696 acres in 2020. The Little Choptank River, also in the middle of the Bay, decreased from 1,503 acres in 2019 to 550 acres in 2020 due to the loss of most of its widgeon grass.
  • Northern Maryland: Underwater grass abundance in and around the Susquehanna Flats region increased from 8,497 acres in 2019 to 9,194 acres in 2020. Additionally, the Northeast River increased from 87 acres in 2019 to 109 acres in 2020.
  • Washington, D.C.: Underwater grass abundance in the Anacostia River continued its resurgence and increased from 48 acres in 2019 to 53 acres in 2020.
  • Mid-to-Lower Chesapeake Bay: In the Virginia portion of the mid-to-lower Bay, underwater grass abundance did particularly well in the Potomac and Rappahannock rivers, as well as in Mobjack Bay. The Potomac River increased from 6,631 acres in 2019 to 7,534 in 2020, while the Rappahannock, including the Corrotoman River, increased from 1,789 acres in 2019 to 2,204 acres in 2020. In Mobjack Bay, underwater grass acreage increased from 5,027 acres in 2019 to 7,080 in 2020.

Underwater grasses — also known as submerged aquatic vegetation or SAV — are critical to the Chesapeake Bay ecosystem. They keep waters clean by absorbing excess nutrients, trapping suspended sediment and slowing wave action that helps to stabilize shorelines, protect wetlands and reduce erosion. Bay grasses also provide food for small invertebrates and migratory waterfowl and habitat for fish and blue crabs.

“While the 7% decline is disappointing, the silver lining is that the 2020 survey shows that underwater grasses are stabilizing following the losses experienced in the middle Bay in 2019. We are hopeful that this is a sign that we’re poised to start regaining that lost ground in coming years,” said Chris Patrick, head of the Chesapeake Bay SAV Monitoring and Restoration Program. He is also an assistant professor of biology at the Virginia Institute of Marine Science, College of William and Mary.

Chesapeake Bay Program partners are working to improve water clarity, protect and restore grass beds, enhance bay grass monitoring and research and expand education and outreach to restore underwater grasses and boost their habitat benefits in the watershed.

Underwater grasses in the Chesapeake Bay are measured and calculated each year by an aerial survey conducted by the Virginia Institute of Marine Science. In some instances, satellite imagery may be used to complement the results from the aerial survey. More information is available at the ChesapeakeProgress website.