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Seagrass FAQ

Do you have questions about seagrasses? Read this seagrass FAQ for answers.

Underwater turtlegrass with fruiting shoot.

Seagrasses are flowering plants that have adapted to live in marine and estuarine environments in coastal waters around the world. Their evolutionary story involves multiple species returning to the sea over many millions of years, most with the same common ancestor which now makes up 70 living species.  Although none are directly related to typical terrestrial grasses, many species look and grow very similarly. All seagrasses are fully adapted to submerged aquatic life. Seagrass leaves are often long and flexible, which reduces drag in the water and helps them to avoid being ripped from the substrate by wind, waves, and tidal currents. Belowground structures like roots and rhizomes keep the plants anchored to the sediment and are important in nutrient uptake. Together, the above and belowground tissues help to cycle nutrients and gasses like oxygen and carbon dioxide between the sediment and the water column. The plants themselves can form large meadows and are often the dominant physical structure in otherwise sandy shorelines. In this way seagrasses are important ecosystem engineers and critical habitat for a variety of vertebrate and invertebrate life.

Although seagrasses live in marine waters, they evolved about 100 million years ago from land plants and have many of the same morphological features such as leaves, roots, flowers, seeds, and conducting tissues. Like land plants, seagrasses photosynthesize to create their own food and produce oxygen via chloroplasts. Similarly, they absorb nutrients from the sediment using their roots. However, unlike land plants, they do not need to overcome the force of gravity by having rigid structures such as stems and trunks. Instead, they rely on the buoyancy of their leaves (with air filled sacs called lacunae) to remain upright in the water column. Some species appear to lack stomata (tiny pores that control gas and water exchange) and instead rely on a thin cuticle layer for nutrient and gas diffusion.

Drawing showing a thick leafy structure depicting algae compared to thin blades of grass depicting seagrass.

Illustration of the difference in physical characteristics of seaweeds (left) vs seagrass (right). Image credit.

Seagrasses are often mistaken for seaweeds washed up on the beach; however, there are many important distinctions between the two. Seaweeds are not true plants, they are macroalgae that lack specialized structures like roots and rhizomes and can often be unicellular. Instead, macroalgae have a relatively simple holdfast that attaches the organism to hard surfaces, along with a stipe and blade. You can, however, find some unattached seaweeds floating in the water such as Sargassum. Seaweeds lack conducting tissues and use diffusion to extract nutrients from the water. Additionally, seaweeds do not flower but can sexually (gametes) or asexually (spores or fragmentation) reproduce. Seaweeds are classified into three major groups based on their accessory pigments: green, brown and red.

  • Seagrasses are typically considered a nursery habitat that provides structure for various life stages of commercially and economically important fish (such as drums and trout), crustaceans (stone crabs, shrimp, etc.), and shellfish (scallops, etc.) species. 
  • Seagrasses are highly productive habitats. They serve as important carbon and nutrient sinks and produce substantial amounts of oxygen for both the sediments and overlying water column.  
  • They are a food source for many animals including sea urchins, sea turtles, manatees, and some fish and crustaceans. Their leaves provide structure for numerous organisms that live on the leaf surface such as invertebrates (epibionts), micro- and macroalgae (epiphytes). These fouling communities have high rates of turnover and contribute substantially to coastal food webs.   
  • Seagrass roots and rhizomes (belowground structures) stabilize the sediment around coastal areas much in the same way that land grasses reduce soil erosion. This function is important because it helps protect Florida’s coastlines from tropical storms that threaten beaches, businesses, and homes.   
  • Seagrasses are water purifiers. Their leaves trap fine sediments and particles, while their belowground structures (roots and rhizomes) secure loose sediments. Sediments without seagrasses are easily stirred up by wind, waves, and currents thereby decreasing water clarity. Additionally, seagrass leaves absorb nutrients from the water column. Seagrasses promote clean and clear water which provides aesthetic value, and enhances tourism and water-related activities such as scalloping, fishing, boating, manatee-watching, etc.
Drawing demonstrating the carbon uptake and photosynthesis process.

Chloroplasts within the seagrass blades convert carbon dioxide and water into carbohydrates (sugar) and oxygen via photosynthesis. Seagrass belowground structures (roots and rhizomes) absorb and store nutrients, in addition to anchoring the plant within the sediments. (Image credit: Cullen-Unsworth et al. 2018)

Seagrasses are plants, and as such need carbon dioxide, water, and sunlight to produce their own food (carbohydrates = sugar) and oxygen through the process called photosynthesis. To grow, they also need ample nutrients (nitrogen/phosphorus) and stable sediments. Most importantly, seagrasses need clear water to receive enough light to photosynthesize. Although oxygen is produced during photosynthesis, they also require oxygenated bottomwaters and can be affected by anoxic events (depletion of oxygen). Seagrasses tend to prefer sheltered environments such as shallow bays, lagoons, and estuaries (where rivers flow into the sea) to thrive, but each seagrass species needs the right recipe of conditions to survive and grow. 

Map of Florida showing seagrass coverage through Florida's coastal areas.

Recent statewide seagrass coverage. Dark and light greens indicate continuous and patchy seagrass, respectively.

Seagrasses are distributed throughout Florida’s coastal waters from the Florida-Alabama line to Volusia County (near Canaveral National Seashore) on the east coast, just north of the Indian River Lagoon. Much of the state’s seagrasses are in Florida Bay near the Florida Keys, and the Big Bend, located between the mouths of the Suwannee and Apalachicola rivers along the Gulf Coast. The paddlegrass (Halophila decipiens) has even been documented in the “deep” (up to 98 ft) west Florida Shelf (WFS) ranging from Cape San Blas (Apalachicola) to Cape sable and the Florida Keys. This morphologically small seagrass is a big primary productive player in an area that lacks larger seagrass species (Hammerstrom et al, 2006).

There are seven seagrass species in Florida. However, there are only five species in Tampa Bay:

  • shoal grass (Halodule wrightii),
  • manatee grass (Syringodium filiforme),
  • turtlegrass (Thalassia testudinum),
  • widgeon grass (Ruppia maritima), and
  • star grass (Halophila engelmannii).

The other two less common species found in Florida are paddlegrass (Halophila decipiens) and Johnson’s seagrass (Halophila johnsonii).

Johnson’s seagrass was delisted from the Federal list of threatened and endangered species on May 16, 2022 (Federal Register) as a result of years of genetic research (Waycott et al. 2021) that has concluded that Johnson's seagrass is in fact a single female clone of a widely distributed Indo-Pacific species, Halophila ovalis. Data from 132 samples of Johnson's seagrass spanning from Sebastian Inlet to Biscayne Bay showed remarkable similarity to Halophila ovalis populations in Africa and Antigua. Although Johnson's seagrass is no longer protected by the Endangered Species Act, the South Atlantic Fishery Management Council designates all seagrasses as “Essential Fish Habitat" and "Habitat Areas of Particular Concern" under the Magnuson-Stevens Fishery Conservation and Management Act. Therefore, all seagrass species in Florida retain federal protections (NOAA).

Map of Florida showing runoff events in the Panhandle, Big Bend region, southwest Florida, and Indian River Lagoon. Other bloom and bloom runoff events are in the areas. One heat drought event in the Keys.

Types of disturbances driving changes in seagrass distribution in Florida waters 2012-2016. Image credit: Yarbro and Carlson 2016

More than 2 million acres of seagrasses are found along Florida’s coastline and within its many estuaries. A dedicated and diverse group of local and state entities, academic institutions and nongovernmental organizations monitor the coverage, species composition and health of this valuable natural resource.  At the state level, the Seagrass Integrated Mapping and Monitoring (SIMM) program coordinated by FWC attempts to gather and analyze the most recent seagrass mapping and monitoring data to provide a broad view of seagrass status and trends. In 2016, seagrass beds were stable and healthy in many areas, but remained vulnerable to water pollution and other stresses associated with rapid human population growth and urban development. More recently, declines along Florida’s Gulf Coast, in Florida’s Big Bend, a die-off event in Florida Bay, continued declines in Biscayne Bay and the catastrophic loss of seagrass in the northern Indian River Lagoon have all highlighted the importance of watershed management and the risks that environmental changes and poor water quality pose to Florida’s seagrasses and the species that depend on them.

Seagrasses are impacted by human disturbances such as dredging, coastal development, and boating activities including mooring, anchoring, running aground, and propeller scarring. Other disturbances like runoff from rivers can reduce water clarity because of increased nutrient pollution and suspended sediments entering our estuaries. Excess nutrient loads can fuel the growth of algal blooms that block light from reaching seagrasses. Climatic disturbances including regional droughts, hurricanes and sea level rise can also affect seagrasses by changing the water conditions. Abnormally low precipitation and the timing of droughts (i.e., summer) can increase salinity and water temperatures and reduce oxygen solubility; no oxygenation within the sediments allows the buildup of sulfides which is toxic to seagrasses. Hurricanes and tropical storms increase river discharge and runoff, reducing salinities and oxygen and dumping excess nutrients in receiving water bodies. Sea level rise threatens to outpace the shoreward expansion of seagrass while increasing light limitation on the deeper portions of existing seagrass meadows.

Aerial photo of a body of water shows many long white stripes across seagrass beds.

Photo of propeller scars in the Florida Keys National Marine Sanctuary. Image credit.

As seagrass scientists, it’s our job to provide outreach and education to the public about the importance of this valuable marine ecosystem. But ultimately, it starts with you! The easiest way to protect seagrasses is by preventing damage in the first place. Responsible boating practices not only can save the life of your passengers, but also seagrasses. Propeller scarring is an important cause of damage to seagrass beds. When a boat’s propeller cuts through the seagrass, it fragments the bed and is detrimental to its growth and to the numerous organisms that rely on it for food, shelter, and protection. It can take months or even years to fully recover. Regulations can now hold boaters accountable on a state and federal level for costs related to assessing damage, restoring habitat, and long-term monitoring of the restored area. In addition, if you are a coastal property owner, work in a coastal town or just happen to love manatees there are other important things you can do to protect seagrasses, like reducing the amount of fertilizer used on your lawn and even more importantly, supporting responsible watershed management. Florida’s recreational and commercial economy depend on full, healthy seagrass meadows.

Climate change is affecting ecosystems around the world, including seagrasses. The combustion of fossil fuels since the Industrial Revolution has increased the atmospheric carbon dioxide (CO2) concentration, increasing the greenhouse effect by trapping more solar heat near the earth’s surface. Seagrasses and other marine creatures residing at the edge of their distributional range are vulnerable to the effects of rising ocean temperatures. Warmer water temperatures can cause changes in physiology (i.e., metabolic rates), behavior (i.e., increased grazing effort), resulting in altered population dynamics, susceptibility to disease, food and oxygen requirements and range shifts, all of which can alter biodiversity and ecosystem function. For example, “tropicalization” occurs as tropical species extend their range into temperate regions, altering foodwebs, introducing new diseases and parasites and potentially destabilizing temperate species populations. With the increase in atmospheric carbon dioxide, oceans continue to uptake more CO2 which lowers pH (i.e., more acidic) in a process known as ocean acidification (OA). OA hinders the calcification processes of important micro- and macroalgae communities as well as corals, mollusks and some crustaceans – particularly during early life history stages. Although negatively influenced by warming coastal waters, rising sea levels, and changing precipitation patterns, seagrasses can also be carbon limited and have been shown not only to benefit from increased CO2 but in some cases capable of offsetting local OA, raising estuarine pH and helping to oxygenate warming estuaries as a result of increased photosynthetic activity. It is difficult to predict how the myriad impacts of climate change will affect seagrasses in the future, but it is clear that the many ecosystem services they provide will be more important than ever in the function and resilience of Florida’s marine and estuarine ecosystems.

Close up of a seagrass blade underwater with small white clusters and white string-like pieces on the blade.

Epiphytes and epibionts are a diverse and dynamic community of organisms that grow on and around seagrass leaves. They primarily consist of micro- and macroalgae, but also include invertebrates such as sponges, crustaceans, barnacles and spirorbid (coiled tube) worms. In healthy seagrass beds unaffected by nutrient pollution, epiphyte loads are low and generally consist of calcareous algae, diatoms, and spirorbid worms. Epiphytic algae require nutrients and photosynthesize just like seagrasses. As nutrient pollution increases, the amount of macroalgal epiphytes present on seagrass leaves increases as well. Older seagrass leaves tend to be more heavily epiphytized since there has been more time for the epiphytes to establish and grow. Heavy epiphyte loading is detrimental to seagrasses by reducing the quantity and quality of light available to photosynthetic tissues in the seagrass leaves, which can limit growth. Grazing by invertebrate herbivores is a primary mechanism at limiting epiphyte growth on seagrasses; a reduction in epiphyte grazers results in heavier epiphyte loads than excess nutrient input would. Algal epiphytes are a main food source for small invertebrates and the base of the food web for this important ecosystem.

Image collage of four pictures showing status before and after Hurricane Irma. Top two, a and b, show seagrass blades being buried in sand. Bottom two, c and d, show seagrass with blades missing.

Photos of seagrass burial (a & b) and erosion/blade removal (c & d) in the Florida Keys following the landfall of Hurricane Irma, Category 4, in 2017 (Wilson et al. 2020)

Tropical cyclones often cause minimal and short-term damage to seagrass beds because the plants are protected by overlying waters. Each storm is unique and can directly and/or indirectly affect seagrasses. Direct effects often include immediate responses to wind, waves, and currents such as the shearing of leaves (much like cutting your grass), the ripping out of whole plants as sediments are eroded away, or their burial as sediments are moved and deposited. Fortunately, if the rhizomes and roots in the sediment remain intact, new leaves may emerge after the storm.

Indirect effects may reduce seagrass abundance over time and are often associated with elevated rainfall, reduced salinities, and impaired water quality and clarity. Storm runoff and prolonged high river flow linked to heavy rains 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. Additionally, stormwater runoff can deliver fresher and/or low oxygenated water which can inhibit seagrass growth and/or result in plant death.