Submerged Aquatic Vegetation (SAV)

eelgrass (Zostera marina) recovery has bgun along the Atlantic Ocean side of the Easten Shore
Source: Virginia Institute of Marine Science, Eelgrass

Though forests and pastures are the most obvious vegetation to "landlubbers" 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 (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 underwater grass is essential for restoring the crabs, bay scallops, oysters, and fish populations. The cultural patterns that were based on harvesting the food from the Chesapeake Bay cannot be sustained without recovery at the bottom of the food chain.

Grazing animals on land depend upon vegetation to provide food. Cattle eat hay, deer browse on branch tips, caterpillars chew on leaves. Similarly, many of the animals in the Chesapeake Bay depend upon the submerged vegetation; the food chain starts with vegetation at the bottom. In the water, various animals feed on the invertebrates that eat the Submerged Aquatic Vegetation, 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 Submerged Aquatic Vegetation, 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
Source: US Geological Survey (USGS), Synthesis of USGS Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management

Without underwater pastures in the water, aquatic invertebrates have little food or shelter. Without the invertebrates in the food chain, estuaries no longer serve as nurseries for 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.

The recovery of the crab and rockfish populations will be enhanced if they have more food. The 2014 Chesapeake Bay Agreement included a specific outcome for Submerged Aquatic Vegetation: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.

A key species of Submerged Aquatic Vegetation, eelgrass (Zostera marina), disappeared from the Atlantic Ocean side of the Chesapeake Bay after a 1933 hurricane. Prior to that event, a mold had already spread through the grass beds and disease had dramatically reduced the eelgrass. After the eelgrass disappeared and its sheltering habitat was no longer available, the bay scallop population plummeted.

In 1999, scientists noticed a patch of eelgrass had started to grow in a seaside bay. Seeds had naturally drifted to the site. The Nature Conservancy (TNC) and the Virginia Institute of Marine Science (VIMS) initiated a seed collection and replanting program to restore eelgrass in additional areas. Seeds collected in early summer by volunteers were kept in saltwater holding tanks until the Fall. Planting was easier than seed collection. The seeds were tossed off boats into the water, and sank naturally to the bottom.

Two decades of work led to significant success:³

Starting from the mere remnant discovered in a seaside bay, TNC and VIMS have since broadcast more than 72 million seeds into 600 acres to help accelerate the natural spread of eelgrass, which now covers 9,000 acres in South, Spider Crab, Hog Island and Cobb Island bays.

pastures of eelgrass are essential for scallops, crabs, and fish
Source: National Park Service, Northeast Coastal and Barrier Network Species Spotlight: Eelgrass

References