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Underwater Bay Grass Abundance in Four Salinity Zones

In 2013, there were an estimated 59,927 acres of underwater grasses in the Chesapeake Bay.

  • In the Tidal Fresh Salinity Zone (the Bay’s fresh waters), there were 13,990 acres, or 68 percent of the zone goal.
  • In the Oligohaline Salinity Zone (the Bay’s slightly salty waters), there were 5,590 acres, or 54 percent of the zone goal.
  • In the Mesohaline Salinity Zone (the Bay’s moderately salty waters), there were 25,579 acres, or 21 percent of the zone goal.
  • In the Polyhaline Salinity Zone (the Bay’s very salty waters), there were 14,768 acres, or 44 percent of the zone goal.






SAV Salinity Zones

Map: SAV Salinity Zones

Date created: Apr 21 2014 / Download

Up until 2013, submerged aquatic vegetation (SAV, also known as bay grasses) acreage totals for the Chesapeake Bay were aggregated into three zones: Upper Bay, Middle Bay and Lower Bay. For 2013, it was decided to report the totals by the four salinity zones found in the Bay: tidal fresh, oligohaline, mesohaline and polyhaline. Since different species of SAV are generally found in waters of a specific range of salinity, tracking changes in SAV abundance by salinity could help track changes in species abundance more easily.




May 05, 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.

Produced by Matt Rath
Music: “A Moment of Jazz” by Ancelin



May 05, 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.

Produced by Matt Rath
Music: “Neapolitan song” by Astrid & Ruediger Kramer
“2 percenter” by Williamson
“All of Me” by Double F

Importance

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 grasses to expand.

Goal

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 and 130,000 acres by 2025. 

While the Bay Program previously reported underwater grass acreage in three geographic zones, these data were re-aggregated in 2013 into four salinity zones. Each salinity zone has its own restoration goal.

  • The goal for the Tidal Fresh Salinity Zone is 20,602 acres.
  • The goal for the Oligohaline Salinity Zone is 10,334 acres.
  • The goal for the Mesohaline Salinity Zone is 120,306 acres.
  • The goal for the Polyhaline Salinity Zone is 33,647 acres.

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 (1984-2013)

  • Between 1984 and 2013, underwater grass abundance in the Tidal Fresh Salinity Zone increased from 6,911 acres to 13,990 acres. Abundance averaged 12,399 acres and ranged from 6,900 acres in 1995 to 25,481 acres in 2008.
  • Abundance in the Oligohaline Salinity Zone increased from 653 acres to 5,590 acres. Abundance averaged 6,680 acres and ranged from its 1984 low to 13,919 acres in 2005.
  • Abundance in the Mesohaline Salinity Zone increased from 15,636 acres to 25,579 acres. Abundance averaged 27,851 acres and ranged from its 1984 low to 48,444 acres in 2002.
  • Abundance in the Polyhaline Salinity Zone decreased from 15,027 acres to 14,768 acres. Abundance averaged 17,887 acres and ranged from 9,959 acres in 2006 to 24,016 acres in 1993.

Short-term trend (2004-2013)

  • Between 2004 and 2013, underwater grass abundance in the Tidal Fresh Salinity Zone decreased from 14,464 acres to 13,990 acres. Abundance averaged 18,350 acres and ranged from 12,148 acres in 2012 to 25,481 acres in 2008.
  • Abundance in the Oligohaline Salinity Zone decreased from 13,616 acres to 5,590 acres. Abundance averaged 10,515 acres and ranged from 5,511 acres in 2012 to 13,919 acres in 2005.
  • Abundance in the Mesohaline Salinity Zone decreased from 28,987 acres to 25,579 acres. Abundance averaged 25,359 acres and ranged from 19,251 acres in 2007 to 31,268 acres in 2005.
  • Abundance in the Polyhaline Salinity Zone decreased from 15,879 acres to 14,768 acres. Abundance averaged 14,156 acres and ranged from 9,959 acres in 2006 to 17,570 acres in 2009.

Change from previous year (2012-2013)

  • Between 1984 and 2013, underwater grass abundance in the Tidal Fresh Salinity Zone increased from 12,148 acres to 13,990 acres.
  • Abundance in the Oligohaline Salinity Zone increased from 5,511 acres to 5,590 acres.
  • Abundance in the Mesohaline Salinity Zone increased from 19,619 acres to 25,579 acres.
  • Abundance in the Polyhaline Salinity Zone increased from 10,917 acres to 14,768 acres.

To see Bay-wide trends in underwater grass abundance, visit our Underwater Bay Grass Abundance (Baywide) indicator.

Additional Information

Collecting data

This data is collected through an aerial survey flown by researchers with the Virginia Institute of Marine Science (VIMS) from late spring to early fall. 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 aerial survey or for segment-specific survey results.

Reporting data

When scientists began to collect underwater grass abundance data in 1984, the Chesapeake Bay was divided into 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.
  • Zone 3 covered the area between the mouth of the Potomac River and the mouth of the Bay.

For almost 30 years, underwater grass acreage was reported in these geographic zones. 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.

Based on an analysis of the distribution and long-term trends of underwater grass species in the Bay (Moore et al., 2000; Orth et al., 2010), there are three or four distinct underwater grass communities that can be delineated by salinity range. These correspond to the four salinity zones used to report underwater grass abundance:

  • Tidal Fresh Salinity Zone (no salt)
  • Oligohaline Salinity Zone (slightly salty)
  • Mesohaline Salinity Zone (moderately salty)
  • Polyhaline Salinity Zone (very salty)

Source of Data

Virginia Institute of Marine Science

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Annapolis, Maryland 21403
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