Do you have questions about seagrasses? Read this seagrass FAQ for answers.
Seagrasses are flowering plants found submerged in coastal marine waters. They evolved from land plants millions of years ago and are an important part of the marine environment.
Florida is estimated to have more than 2 million acres of nearshore seagrass. Florida's seagrasses are important natural resources that have many valuable functions:
- Seagrasses provide habitat for many species of fish, crustaceans and shellfish.
- Seagrass beds are nursery areas for much of Florida's recreationally and commercially important marine life, including fish and shellfish.
- Seagrasses and the organisms that grow on them are food sources for many animals.
- Seagrass roots and rhizomes (underground stems) stabilize the sea bottom in much the same way that land grasses reduce soil erosion. This function is important when tropical storms approach Florida's coastlines and threaten beaches, businesses and homes.
- Seagrasses help maintain water clarity by trapping fine sediments and particles. Bottom areas without seagrass are more often stirred up by wind and waves, thereby decreasing water clarity and affecting marine animal behavior and recreational quality of coastal areas.
Seagrasses are found throughout the coastal areas of the state from the Florida-Alabama line to Volusia County on the east coast, just north of the Indian River Lagoon. They live completely submerged in the marine and estuarine waters of protected bays, lagoons and the Gulf of Mexico along the continental shelf. Seagrasses require sunlight to grow, so the depth at which they occur is limited by water clarity. Florida seagrasses are most abundant in Florida Bay and the Gulf from Tarpon Springs to Apalachee Bay. These two areas contain some of the largest seagrass beds found in continental North America.
Seagrasses are plants and, therefore, need sunlight and carbon dioxide to create tissues. Through photosynthesis, seagrasses also generate oxygen; however, they cannot produce enough to support the living cells in roots and rhizomes, so seagrasses absorb additional oxygen from the surrounding water. Seagrasses also need an adequate supply of nutrients, such as nitrogen and phosphorus, and stable sediments in which to grow.
There are seven seagrass species found in Florida. The most common are shoal grass (Halodule wrightii), manatee grass (Syringodium filiforme) and turtle grass (Thalassia testudinum). Widgeon grass (Ruppia maritima), star grass (Halophila engelmannii) and paddle grass (Halophila decipiens) are less common. Johnson’s seagrass (Halophila johnsonii) is found only along Florida’s southeast coast and is listed as a threatened species under the Endangered Species Act.
More than 2 million acres of seagrass are found along Florida’s coastline and within estuaries. The Seagrass Integrated Mapping and Monitoring (SIMM) program coordinated by FWC attempts to collate the most recent seagrass mapping and monitoring data. This statewide reporting network has found that seagrass beds are stable and healthy in many areas, but they are vulnerable to water pollution and other stresses. Seagrass beds require clean water to flourish, and the health and abundance of seagrasses declined in Florida and worldwide in response to water pollution during the middle and late 20th century. These declines were especially dramatic in areas subject to rapid human population increases and urban development, such as the Tampa Bay region. Since that time, increasing awareness of the value of seagrass beds as nurseries and food sources for fish, shellfish, manatees, and sea turtles has led to concerted efforts to clean up coastal waters to promote seagrass growth and expansion. For example, Tampa Bay and Sarasota Bay now have more acres of seagrass than were present in the 1950s. In many other areas of the state, seagrass beds are generally stable or slightly increasing in area. However, recent losses of seagrass in Florida’s Big Bend, Florida Bay, and the Indian River Lagoon have occurred as the result of extreme weather, heat, water pollution, and algal blooms. FWC scientists and partners continue to monitor seagrass abundance around the state and work with managers to protect and restore seagrasses.
Sea turtles, manatees, parrotfish, surgeonfish, sea urchins and pinfish feed on Florida seagrasses. Many other smaller animals feed on the epiphytes and invertebrates that live on and among seagrass blades.
Epiphytes are organisms that grow on seagrass blades. They include microscopic algae, and invertebrates such as sponges, crustaceans, barnacles and spirorbid (coiled tube) worms. As seagrass blades age, the amount of epiphytes covering them increases. In healthy seagrass beds unaffected by nutrient pollution, epiphyte loads are fairly low and generally consist of calcareous algae, diatoms and spirorbid worms. As nutrient pollution increases, so does the number of epiphytes present on seagrass blades; they may include larger filamentous algae and a variety of small invertebrates. Heavy epiphyte loading can reduce the amount of light available to photosynthetic tissues in the seagrass blades, which in turn will limit the plant’s growth.
Because seagrasses need sunlight to survive, any event that reduces water clarity – which could decrease the amount of light reaching the bottom – may damage beds or kill seagrass plants. High river flow or storm runoff may contribute particles that reduce water clarity. Also, dissolved nutrients contributed by rivers, storm runoff, sewage or industrial discharges may fuel the growth of algae. High concentrations, or blooms, of phytoplankton (microscopic single-cell algae), particularly, can reduce the amount of light reaching seagrass beds. Phytoplankton blooms driven by nutrient pollution have been a significant factor in the reduction of seagrass beds in Florida coastal waters in the last 50 years. Seagrass beds can also sustain physical damage from boat propeller scarring and dredging.
Tropical cyclones often cause minimal and short-term physical damage to seagrass beds because the plants are protected by overlying waters. Storms may cause seagrasses to lose blades, but the rhizomes and roots in the sediment will likely remain intact and new blades may emerge after the storm. Debris and wrack associated with waves and surge may accumulate over seagrasses and damage them. Also, storm runoff and prolonged high river flow associated with heavy rain on land can result in darkly colored waters in estuaries and coastal areas for weeks or months, reducing the amount of light available to seagrasses. Extended periods of reduced light availability may cause seagrass losses months after a storm.
In general, climate change likely will not directly affect seagrasses, but indirect effects may cause long-term changes in the location of seagrass beds and the type of seagrasses found in a particular location. For example, rising sea levels might cause a seagrass bed to expand shoreward, but retreat along its deeper edges where light will become less available at the bottom. Climate change could alter rainfall patterns and any resulting changes in the amount and characteristics of river runoff may affect seagrasses. Less runoff could improve conditions for seagrasses by reducing pollution and sediment inputs, but increased runoff could reduce light available to seagrasses by carrying more particles and nutrients into coastal waters. Increasing water temperatures may also cause shifts in the distribution of seagrasses, with tropical and subtropical species expanding north and south into previously temperate regions.