Submerged Aquatic Vegetation (SAV)

Though forests and pastures are the most obvious vegetation to "landubbers" in Virginia, the underwater grasses in the Chesapeake Bay are being monitored very closely by those trying to measure if we are "saving" or "losing" the Chesapeake Bay.


submerged aquatic vegetation at Cape Charles
Submerged Aquatic Vegetation (SAV) mapped at Cape Charles
Source: Virginia Institute of Marine Science, Bay Grasses (SAV) in Chesapeake Bay and Delmarva Peninsula Coastal Bays

The 17 million people who live in the Chesapeake Bay watershed have upset the natural balance through a number of different stressors. Life adapts over thousands of years of time, but in the last 400 years, the impacts of increased urbanization have dramatically lowered the productivity and biodiversity of the Chesapeake Bay, especially the Submerged Aquatic Vegetation (SAV).

Restoring the SAV is essential for bringing back the crabs, the oysters, the fish, and the cultural patterns that were based on harvesting the food from the Chesapeake Bay. Cattle eat hay, deer browse on branch tips, caterpillars chew on leaves - grazing animals on land depend upon vegetation to provide food. Similarly, many of the animals in the Chesapeake Bay depend upon the submerged vegetation. The food chain is based on vegetation at the bottom. In the water, various animals feed on the invertebrates that eat the SAV, comparable to the way land-based birds eat caterpillars that eat leaves.

Algae is a source of food, but after it dies it decays and sucks oxygen out of the water. Excessive growth of algae, such as phytoplankton blooms, is followed by masses of decaying matter on the bottom of the bay, creating "dead zones" with inadequate levels of dissolved oxygen. Even SAV, which generates oxygen in the daytime, needs sufficient levels of dissolved oxygen in the water at night to fuel its normal metabolic operations.

SAV is killed and submerged lands become underwater deserts when sediments block sunlight, or when oxygen levels drop to zero
SAV is killed and submerged lands become underwater deserts when sediments block sunlight, or when oxygen levels drop to zero
Source: US Geological Survey (USGS), Synthesis of USGS Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management

Without underwater pastures of SAV in the water, aquatic invertebrates have no food. Without the invertebrates in the food chain, estuaries no longer serve as nurseries for baby fish that feed on the invertebrates. According to the US Fish and Wildlife Service:1

Since the 1960s, well over half of the SAV has disappeared from the Bay waters.

In theory, once the SAV is restored the number of critters that graze on the SAV will increase - and then the larger animals that eat the grazing critters (such as crabs and rockfish) will have more food. The 2014 Chesapeake Bay Agreement included a specific "outcome" for SAV:2

Sustain and increase the habitat benefits of SAV (underwater grasses) in the Chesapeake Bay. Achieve and sustain the ultimate outcome of 185,000 acres of SAV Bay-wide necessary for a restored Bay. Progress toward this ultimate outcome will be measured against a target of 90,000 acres by 2017 and 130,000 acres by 2025.

References

1. US Fish and Wildlife Service, "Submerged Aquatic Vegetation: Where Have All the Grasses Gone?" http://www.fws.gov/chesapeakebay/cbsav.html (last checked February 20, 2010)
2. "2014 Chesapeake Bay Watershed Agreement," Chesapeake Bay Program, p.6, http://www.chesapeakebay.net/chesapeakebaywatershedagreement/page (last checked June 17, 2014)


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