Ecology

Seagrasses are flowering plants that grow submerged in shallow estuarine (brackish) and marine environments. Of the sixty species found worldwide, seven grow in Florida waters. An estimated 2.7 million acres of seagrass meadows grow along Florida's extensive coastline, protected bays and lagoons. They are especially abundant along the Gulf of Mexico, including the continental shelf.

Seagrass meadows serve several important ecological functions, including supporting a myriad of recreational and commercial fish and shellfish such as redfish, sea trout, snapper, pink shrimp and blue crabs. They serve as habitat and feeding grounds for endangered manatees and green sea turtles. Pelicans, terns, and wading birds also forage in the grassbeds, feeding on tiny crustaceans and young fish that find shelter and food there in the early stages of life.

Florida's grassbeds also promote water clarity. The plants' extensive system of roots and rhizomes, or runners, help stabilize bottom sediments, while their leaves trap fine particles and sediment from the water column. Seagrasses are limited to relatively clear, shallow waters where enough sunlight is available for the production of food and oxygen through the process of photosynthesis.

Seagrasses are not true grasses. However, the sixty or so species did evolve from more than one land-based ancestor that adapted to life in the aquatic environment. Thus, the term "seagrass" does not refer to a taxonomic group, but to a collection of species that fills an ecological niche and has five basic adaptations in common: an ability to grow submerged, survivability in high and varying salinities, an anchoring (root/rhizome) system, a submerged pollination mechanism, and the ability to compete with other plants in the marine environment. Like their land-based ancestors, seagrass plants have an internal vascular system for transporting materials to and from the leaves and roots/rhizomes. They also reproduce in the same two basic ways as flowering plants on land. Sexual reproduction involves the production of flowers that are pollinated underwater. Asexual or vegetative growth occurs when growing tips produce new rhizomes, roots, and shoots.

Florida's Species

Of the seven species found in Florida's waters, three are most common: turtle grass, Thalassia testudinum, shoal grass, Halodule wrightii; and manatee grass, Syringodium filiforme. Turtle grass is the largest and most abundant species found in Florida. Extensive meadows of this species are found throughout the Caribbean. These productive habitats support many fish and invertebrates associated with nearby coral reefs and mangroves. Turtle grass meadows are considered to be the mature or "climax" species, existing in an area indefinitely unless environmental conditions change. The plant itself has a deep root and rhizome system and its broad flat leaves can be up to a foot in length. Turtle grass plants are not tolerant of freshwater or being exposed at low tide for long periods.

Manatee grass often grows in association with other seagrasses, especially turtle grass, with its thick root and rhizome system forming dense mats on the muddy and sandy bottom. This species is easy to recognize because its leaves are cylindrical like the whiskers of a manatee. The shape of the leaves, which reach up to 20 inches in length, may help protect the plant in strong currents.

Shoal grass is the most tolerant of changing conditions and grows in water that is too shallow for most seagrasses. The plant has ribbon-like flat leaves and is tolerant of being exposed at low tide and of freshwater inputs from nearby land areas. Shoal grass is a relatively fast-growing species that colonizes barren sandy areas in quiet waters. This species has been useful in restoring areas damaged by boat groundings and propeller scars. Transplants of shoal grass colonize the damaged areas relatively quickly, preventing further erosion and allowing for the establishment of other species of seagrass, macroalgae and marine life.

Widgeon grass, Ruppia maritima, is also a very shallow water seagrass that grows in both fresh and saltwater conditions in the estuaries and bays across the state. The remaining 3 species belong to the genus Halophila: star grass, Halophila engelmannii; paddle grass, Halophila decipiens; and Johnson's seagrass, Halophila johnsonii. Members of the Halophila genus have leaves that are shaped like paddles. They tend to be smaller and more fragile than other better known species. They are tolerant of lower light levels and form sparse grassbeds in deeper water, especially in the Gulf of Mexico offshore from Florida's "Big Bend" region.