What are seagrass meadows?
Seagrasses are flowering plants that live fully submerged in shallow, coastal waters where sunlight can reach the ocean floor. Unlike seaweeds (algae), seagrasses have true roots, leaves, and even flowers and seeds. These underwater meadows occur on every continent except Antarctica and thrive in sheltered bays, lagoons, and estuaries. You can find seagrass meadows in a few of our national marine sanctuaries, including Florida Keys, Greater Farallones, and Monterey Bay national marine sanctuaries, as well as shallow coastal waters near Gray's Reef National Marine Sanctuary. While true seagrasses are marine plants found in coastal sanctuaries, similar submerged aquatic vegetation thrives in the freshwater environments of sanctuaries like Thunder Bay and Mallows Bay-Potomac River. These underwater plant communities perform many of the same ecological roles—offering habitat, filtering water, and supporting diverse aquatic life.
Why are seagrass meadows important?
Seagrass meadows are among the most productive and valuable ecosystems on Earth. They provide nursery habitat for countless marine animals, including fish, sea turtles, conch, scallops, and manatees. Their dense blades slow water movement, trap sediment, and stabilize the seafloor, helping to keep coastal waters clear. These meadows also store large amounts of "blue carbon,"absorbing carbon dioxide and helping to mitigate climate change. In addition, seagrass meadows support coastal economies by sustaining fisheries, enhancing water quality, and providing beautiful areas for snorkeling and wildlife watching.
What threats do seagrass meadows face?
Seagrass meadows are vulnerable to a range of human impacts. Coastal development, boat propeller scarring, and pollution from stormwater runoff can damage or smother seagrasses. Poor water quality reduces the sunlight these plants need to grow, and climate change brings rising ocean temperatures and stronger storms that further stress these delicate ecosystems. In some regions, invasive species or diseases have caused widespread losses.
Fortunately, restoration projects across the National Marine Sanctuary System are helping to protect and rebuild seagrass meadows through improved water quality, boat-mooring programs, and planting initiatives. By reducing impacts from anchors and runoff, sanctuaries are giving these vital underwater meadows a chance to recover and flourish.
Seagrass Species Spotlights
Each species of seagrass plays a unique role in shaping coastal ecosystems and supporting marine life. From the temperate waters of California to the tropical shallows of Florida and the Caribbean, these underwater plants form vital habitats across the National Marine Sanctuary System.
Eelgrass (Zostera marina)
Where it grows: Found in temperate coastal waters across the Northern Hemisphere, eelgrass thrives in sandy or muddy bottoms from Alaska to Baja California. Within the National Marine Sanctuary System, it grows in Monterey Bay, Greater Farallones (Bolinas Lagoon), and Channel Islands national marine sanctuaries.
Why it matters: Eelgrass meadows provide nursery grounds for fish, crabs, and invertebrates. They help filter the water, stabilize sediment, and store carbon, making them essential for both biodiversity and climate resilience.
Turtle Grass (Thalassia testudinum)
Where it grows: The most common seagrass species in Florida Keys National Marine Sanctuary, turtle grass thrives in warm, shallow waters of Florida, the Caribbean and the Gulf of America.
Why it matters: With wide, ribbon-like blades, turtle grass forms lush underwater pastures that feed green sea turtles, manatees, and other grazers. These meadows support diverse marine life and act as important nursery areas for commercially valuable species such as snapper and lobster.
Manatee Grass (Syringodium filiforme)
Where it grows: Manatee grass is a common seagrass species in Florida Keys National Marine Sanctuary and other shallow Gulf and Caribbean coastal waters. It often grows alongside turtle grass in sandy, calm environments at moderate depths, typically less than 33 feet (10 meters).
Why it matters: Its slender, cylindrical blades form dense beds that provide cover for juvenile fish and invertebrates. As its name suggests, it is an important food source for manatees and green sea turtles, helping sustain populations of these gentle grazers.
Shoal Grass (Halodule wrightii)
Where it grows: Shoal grass thrives in warm, shallow coastal waters throughout the southeastern United States, the Caribbean, and Gulf. Within the National Marine Sanctuary System, it is common in Florida Keys National Marine Sanctuary and in nearshore coastal habitats adjacent to Gray's Reef National Marine Sanctuary, where it forms important nursery grounds that connect estuaries to offshore reef ecosystems.
Why it matters: Shoal grass is one of the first species to colonize disturbed or shifting sandy areas, helping stabilize sediments and prepare the seafloor for other seagrasses to take root and grow. This is why it is sometimes referred to as "pioneering shoal grass." Its narrow blades provide nursery habitat for small fish and invertebrates, while its root system reduces erosion and supports coastal water clarity.
Submerged Aquatic Vegetation (SAV) in Mallows Bay
Where it grows: In the low-salinity, tidal waters of Mallows Bay on the Potomac River, freshwater submerged aquatic vegetation such as wild celery (Vallisneria americana), Coontail (Ceratophyllum demersum), and water stargrass (Heteranthera dubia) form lush underwater meadows. These beds occur within the sanctuary's boundary and are accessible by kayak from nearby public camps and launch sites.
Why it matters: Although not true marine seagrasses, these SAV meadows perform many of the same vital functions—providing habitat and nursery grounds for fish and invertebrates, stabilizing sediment and improving water clarity. They also highlight the diversity within the National Marine Sanctuary System of underwater plant communities across the salt-to-freshwater continuum.
