What are Seagrasses?

Seagrasses are flowering plants that live underwater. Like land plants, seagrasses produce oxygen. The depth at which seagrasses are found is limited by water clarity because they require light. Although seagrasses occur throughout the coastal areas of the state, they are most abundant from Tarpon Springs northward to Apalachee Bay. Seagrasses occur in protected bays and lagoons and also in places along the continental shelf in the Gulf of Mexico.

Florida's estimated 502,000 acres of seagrasses are important natural resources that perform many significant functions: 1) they help maintain water clarity by trapping fine sediments and particles with their leaves; 2) they can stabilize the bottom with their roots and rhizomes in much the same way that land grasses retard soil erosion; 3) they provide habitat for many fishes, crustaceans, and shellfish; 4) seagrasses and the organisms that grow on them are food for many marine animals, and most importantly; 5) they are nursery areas for much of Florida's recreationally and commercially important marine life.

Seagrass leaves provide excellent protection for young marine animals from larger open-water predators. Some animals, such as manatees, eat seagrass blades. Other animals derive nutrition from eating algae and small animals that colonize seagrass leaves. These colonizing organisms provide an additional link in the marine food chain.

Florida's Seagrasses

Although approximately 52 species of marine seagrasses exist worldwide, only seven species are found in Florida waters. Four of these are widespread in Florida and extend beyond its borders.

Shoal-grass (Halodule wrightii) is an early colonizer of disturbed areas and usually grows in water too shallow for other species.

Ruppia maritima, commonly called widgeon-grass, grows in both fresh and saltwater and is widely distributed throughout Florida's estuaries.

Turtle-grass (Thalassia testudinum) the most common of the Florida seagrasses, characteristically has deeper root structures than any of the other seagrasses.

Manatee-grass (Syringodium filiforme) is easily recognizable because its leaves are cylindrical.

Johnson's seagrass
(Halophila johnsonii)

star-grass (Halophila engelmannii)

paddle-grass (Halophila decipiens)

The other three are species of Halophila: star grass (Halophila engelamannii), paddle grass (Halophila decipiens), and Johnson's seagrass (Halophila johnsonii). These small, fragile seagrasses are sparsely distributed in Florida and only limited information about them exists.

Seagrass Losses in Florida

Seagrasses are a valuable part of Florida's marine environment, but they are disappearing at an alarming rate. Dredge and fill projects and degraded water quality, as well as other activities, are responsible for their precipitous decline.

Along the Southwest Florida coast there are two major bay systems with similar physical features but dramatically different histories. Tampa Bay has experienced the stresses of a developed, urbanized bay system. Charlotte Harbor, on the other hand, is one of the most natural estuaries remaining in Florida. During the past 100 years, Tampa Bay has experienced an 81 percent decline in seagrass acreage. A 29 percent decrease in area of Charlotte Harbor seagrasses was documented through comparison of aerial photographs from 1944 to 1982.

Department of Environmental Protection, Florida Marine Research Institute scientists are studying changes in Florida's coastal fisheries habitats. By analyzing aerial photographs from 1940's and 1950's and satellite imagery and aerial photographs from the 1980's, the scientists are able to evaluate habitat change.

Several sites on the east Florida coast have been analyzed. Among them are Ponce Inlet, just south of Daytona Beach, and the Indian River from Sebastian Inlet south to St. Lucie Inlet. At the Ponce Inlet site, a 100 percent loss of seagrasses was noted. This destruction was due primarily to dredge and fill activities for development and the Intracoastal Waterway. A seven-mile stretch of estuary surrounding the Sebastian Inlet has experienced a 38 percent decline in seagrass habitat since 1951. Another study site just north of Fort Pierce Inlet was assessed for change in habitat over time. A 25 percent loss of seagrasses was documented in this area since 1958.

The studies documenting fisheries habitat alterations in Florida, such as the seagrass losses described earlier, are proving helpful to local and state officials. They are increasing public awareness about the problem of fisheries habitat losses and are providing incentive to address this serious problem in Florida's coastal zone.

This page was derived from a brochure developed by the Department of Environmental Protection, Florida Marine Research institute with funds provided by a grant from the U.S. Office of Ocean and Coastal Resource Management, NOAA.