MSRP Map

Seagrasses

Seagrasses are vascular plants that can form dense vegetative communities in shallow water estuaries (Day et al. 1989). These plants have evolved the ability to carry out their entire life cycle completely submerged in the marine environment.

A remarkable similarity of vegetative appearance, growth, and morphology exists among the seagrasses. They have a linear form exhibited by a root system (rhizomes and roots) below ground and leaf structure (short shoots and leaf blades) above ground. Turtle grass, manatee grass, shoal grass, and widgeon grass are similar in appearance in that their leaves are long and either cylindrical (i.e., manatee grass) or flat. The flat blades are either broad (i.e., turtle grass) or narrow. The Halophila species differ from the other seagrasses in that the leaf structure is shorter with the blades resembling tufts or whorls. Turtle grass is the largest and most robust of South Florida's seagrasses; Johnson's seagrass the most diminutive. Manatee grass is distinctive in having cylindrical leaves which are quite brittle and buoyant. As seagrass blades break off, they are exported from the immediate area by winds and currents. Shoal grass is recognized as the pioneer species in the successional development of seagrass beds.

For all the Florida seagrass species, the leaf structure emanates vertically from the horizontal rhizomes at regular intervals. From the rhizomes, which are just under the sediment surface, emerge the roots and root hairs into the surrounding substrate. Seagrass rhizomes range in diameter from 1 mm (0.04 in) for the Halophila group to 1 cm (0.4 in) for turtle grass. These plant components form a well-developed anchoring system and constitute the below-ground biomass of the plant. The leaf structure consists of short shoots from which leaf blades emerge into the water column. Leaf blades from these species range in width from 1 mm to 1 cm (0.04 to 0.4 in) and in length from 5 mm to 1 m (0.2 to 40 in). The leaf varies in shape for the Halophila group from oval to linear while the other species are essentially elongate. These components represent the aboveground biomass or standing crop of the plant. Seagrass biomass consists of the weight of all living plant material (e.g., roots, rhizomes, leaf structure) and is expressed in terms of mass per unit area. Seagrass biomass and the standing crop of seagrass beds are terms used to quantify the density of seagrasses. The majority of seagrass biomass is usually below the sediment surface. The robust root and rhizome system of turtle grass contains between 55 and 90 percent of the plant's total biomass (Zieman and Zieman 1989). Despite shallower, less well-developed roots and rhizomes, both manatee grass and shoal grass have a greater portion of their total biomass (53 to 89 percent) below the sediment surface, followed by widgeon grass with 50 percent (Lewis and Phillips 1980). Each seagrass species can occur as a monotypic seagrass bed or can be found intermixed with the other species. In the Indian River Lagoon, some seagrass beds consist of all seven species with the Halophila species scattered throughout sparser areas within the bed.

Taken from: U.S. Fish and Wildlife Service. 1999. South Florida Multi-Species Recovery Plan. Atlanta, Georgia. Pp. 3:600-601.

Listed Species Occurring in Seagrass Habitat

  • American Crocodile
  • Green Sea Turtle
  • Loggerhead Sea Turtle
  • Hawksbill Sea Turtle
  • Leatherback Sea Turtle
  • Kemp's Ridley Sea Turtle
  • Wood Stork
  • Roseate Tern
  • Bald Eagle
  • West Indian Manatee