Study shows a decline in Chesapeake Bay underwater grasses

July 11, 2020
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Annapolis, MD – In 2019, an estimated 66,387 acres of underwater grasses were mapped in the Chesapeake Bay and its tidal tributaries, achieving 36% of the Chesapeake Bay Program’s ultimate restoration goal of 185,000 acres.

Although the 66,387 acres mapped in 2019 are 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 underwater grass acres in 2018 which resulted in an estimate calculated by combining mapped acreage with 2017 data for the region that was not mapped to estimate the acreage in the Bay.

In 2019, the largest decline in terms of total area—an estimated 34,986 acres—was observed in moderately salty waters, particularly in the Tangier Sound area, which experienced a 18,452-acre decline. Experts attribute the losses largely to a decline in widgeon grass. Widgeon grass fluctuates from year-to-year as the species responds rapidly to impacts from extreme weather or changes in water quality. The decline in widgeon grass that we saw in 2019 mirrors a similar situation that occurred in 2001-2003, where a rapid increase in widgeon grass in 2001 and 2002 was followed by a 50% decline in 2003. Though the precise cause for the decline in 2019 is unknown, higher than average river flows may have contributed by reducing water clarity and blocking sunlight from reaching the grasses.

Underwater grass abundance can vary from species to species and river to river. In 2019, local highlights included:

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  • Southeast Virginia: Underwater grass abundance in the middle and upper Rappahannock River increased from 1,007 acres in 2018 to 1,453 acres in 2019. In the upper tidal fresh portion of the James River, underwater grass abundance increased from 402 acres in 2018 to 536 acres in 2019, while in the Chickahominy River it increased from 711 acres in 2018 to 1,059 acres in 2019.
  • Lower central Chesapeake Bay: In the Maryland portion of the lower central Chesapeake Bay, underwater grass abundance declined from 2,669 acres in 2018 to 901 acres in 2019. Underwater grass abundance in the Virginia portion of the lower central Chesapeake Bay declined from 2,719 acres in 2018 to 583 acres in 2019.

The Chesapeake Bay Program recently brought together experts specializing in underwater grasses, water quality, and land-use research and management to develop a series of fact sheets that examines the role that a growing human population—and its associated impacts—plays on the Bay’s grass beds. Each fact sheet gives a close-up view of what’s happening in each of the rivers across the Bay in order to provide a summary of information to guide local planning decisions and implementation of best management practices that encourage underwater grass recovery. These facts sheets can be found at https://gis.chesapeakebay.net/sav/.

 

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