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Underwater Grasses

Do you know how healthy your bay or coastal ocean is? Take a look at its underwater grasses. Underwater grasses, also called submerged aquatic vegetation, or seagrasses if they grow in the ocean, are flowering plants that live in shallow waters. These underwater meadows provide many benefits, including unique habitat for aquatic life, playing a major role in the reproductive cycles of many recreationally and commercially important species. For example, juvenile fish and shellfish feed on organic matter produced by decomposing underwater grasses; they also hide in the beds from larger predators. Sponges, sea slugs, scallop larvae, sea squirts, and other tiny animals feed on and attach themselves to underwater grasses.

Underwater grasses also help improve water clarity and protect shorelines from erosion. They filter polluted runoff and absorb nutrients such as nitrogen and phosphorus. Grasses also absorb wave energy, help settle suspended sediment, stabilize bottom sediments with their roots, and produce oxygen for fish and wildlife. Underwater grass beds also account for 15 percent of the ocean’s total carbon storage.

Different species of underwater grasses—such as eelgrass, shoalgrass, wild celery, and sago pondweed—prefer different water temperatures and salinity. They all require sunlight to grow and reproduce. As long as the water is clear, they can grow as tall as five or six feet.In murky water they are limited to more shallow depths, or they might be absent because there is not enough light reaching the bottom for them to grow.

Threats to Underwater Grasses

At the same time underwater grasses help improve water clarity, the degradation of water quality can be detrimental to grasses—it’s a delicate balance. Excessive nutrients from urban runoff cause algal blooms that block the sunlight from reaching the grasses. Another light-limiting factor is when large amounts of sediment are carried by streams and rivers from the surrounding watershed, decreasing water clarity.

Other threats include invasive species, dredging activities, certain fish harvesting and aquaculture methods, boat propeller scarring, and shoreline development. Rising temperatures and sea level could also impact underwater grasses because their range is limited by temperature. For example, eelgrass in the Chesapeake Bay is near the southern limit of its range, and hot summers might lead to diebacks in the following year.

Conserving Underwater Grasses in the Chesapeake Bay

At its most pristine, scientists believe the Chesapeake Bay might have supported more than 600,000 acres of underwater grass, but as of 2010, there are less than 80,000 acres of grasses.

To support the health of underwater grasses in the Chesapeake Bay, NOAA has been active in restoring grasses in rivers throughout the region. To monitor progress of restoration efforts and to keep tabs on how grasses are doing in the Bay, surveys partially funded by NOAA map underwater grasses around the Chesapeake and guide protection and restoration efforts.

From the mouth of the Chesapeake to Wachapreague inlet, NOAA helped fund underwater grass restoration for Virginia’s Eastern Shore, where 250 acres of eelgrass have now spread to 5,000 acres. Investments in habitat restoration projects like these will help protect the long-term economy of Virginia by stabilizing the natural resources that many industries depend on, from fishing to aquaculture to tourism.