Seagrass Beds

One of these, Johnson’s seagrass (Halophila johnsonii) is found nowhere in the world except in the Indian River Lagoon and was the first threatened seagrass to be listed under the Endangered Species Act.

Though often mistaken for seaweeds, seagrasses are evolved from terrestrial plants, and are the only class of flowering plant that is completely aquatic. Though they have leaves, roots, flowers and seeds, seagrasses lack strong supportive stems and trunks, an adaptation of terrestrial plants to overcome the force of gravity. Their natural buoyancy and flexibility allow them bend easily along with waves and currents.

The Value of Seagrasses

A single acre of seagrass can produce over 10 tons of leaves per year. By comparison, an acre of most grass forage crops like alfalfa and switchgrasses produce from 3 to 8 tons of vegetation per year. This high level of productivity has inspired comparisons of seagrass communities as the marine equivalent of tropical rainforests. In Florida, turtle grass (Thalassia testudinum) can grow .07 to .19 inches (2 to 5 millimeters) per day; manatee grass (Syringodium filiforme) can put on .33 inches (8.5 mm) of growth in a day; and shoal grass (Halodule wrightii) can produce one new leaf every nine days during spring.

This vast biomass provides food, habitat, and nursery areas for myriad adult and juvenile vertebrates and invertebrates. A single acre of seagrass may support as many as 40,000 individual fish and 50 million small invertebrates. The dense foliage shelters numerous species of juvenile fish, smaller finfish, and benthic invertebrates such as crustaceans, bivalves, echinoderms. These leaves also provide abundant attachment sites for small macroalgae and epiphytic organisms such as sponges, bryozoans and foraminifera (forams), which account for up to 30 percent of the total above-ground biomass in some systems. Finally, seagrasses’ dense networks of underground rhizomes also serve as protection, hindering predators’ attempts to dig up the abundant invertebrate prey living in the sediments where seagrasses grow.

Outside of their importance to biodiversity, seagrasses also offer several other valuable ecological benefits. Their sensitivity to changes in water quality makes them important indicator species that reflect the overall health of coastal ecosystems. And seagrass meadows also help dampen the effects of strong currents, providing protection to fish and invertebrates while also preventing the scouring of bottom areas.

Economically, Florida's 2.2 million acres of seagrass support both commercial and recreational fisheries that provide a wealth of benefits to the state's economy. Florida's Department of Environmental Protection (DEP) reported that in 2000, Florida's seagrass communities supported commercial harvests of fish and shellfish valued at over $124 billion. Including the nutrient cycling worth of seagrasses as well as recreational fisheries, DEP has estimated that each acre of seagrass in Florida has an economic value of approximately $20,255 per year, which translates into a statewide economic benefit of $44.6 billion annually.

Seagrass coverage throughout the Indian River Lagoon has been on the decline since mapping surveys began. In 1943, the IRL held over 71,000 acres (29,000 hectares); as of 2019, that figure had declined to just over 32,000 acres (13,000 hectares.)

Threats to Seagrass Communities

Seagrasses are subject to a number of naturally occurring stresses including storms, excessive grazing by herbivores and disease. Human-made threats include point and non-point sources of pollution, reduced water clarity, excessive nutrients in runoff, sedimentation and propeller scarring.

The effect of these stresses on seagrasses is dependent on both the nature and severity of the impact. Generally, if only leaves and above-ground vegetation are affected, seagrasses can recover from damage within a few weeks. However, when roots and rhizomes are damaged, a plant’s ability to produce new growth is severely impacted; plants may never be able to recover.

Storms, Grazing and Disease

Several natural processes can negatively affect seagrasses. Wind-driven waves from heavy storms can break or uproot seagrasses. Additionally, a number of small and large marine animals disturb seagrasses while foraging, including sea urchins and the endangered West Indian Manatee (Trichechus manatus). Other species, such as crabs, fishes, skates, and rays disturb rhizomes and roots, and can tear apart seagrass leaves as they forage for concealed or buried prey.

A wasting disease, thought to be caused by a marine slime mold, caused extensive damage to Zostera eelgrass beds in temperate coastal areas during the 1930s, causing a die-off of up to 90 percent of all eelgrass in North America. Introduced seaweed species can also compete with and displace native seagrasses.

Human Impacts

By far, human activities have the greatest impact on Indian River Lagoon seagrasses.

Reduced water clarity. Like land plants, seagrasses rely on sunlight, and water clarity determines how much light reaches their foliage. However, stormwater runoff from urban and agricultural areas carries household chemicals, oils, automotive chemicals, pesticides, animal wastes, and other debris, which are suspended as tiny particles in the water column. This clouding, or turbidity, reduces sunlight that reaches the seagrasses. In extreme cases, excessive sediments in runoff can even physically smother seagrasses.

Nutrient loading. As on land, nitrogen and phosphorous are important nutrients in freshwater, brackish and marine environments, allowing for the growth of microalgae and phytoplankton that serve as food for a great many organisms. But in overabundance, as when heavy volumes of stormwater carry excess fertilizers and animal wastes from land, nutrient loading causes massive blooms of algae that block sunlight into the deeper reaches of the lagoon.

Seasonal freshwater discharges from nearby Lake Okeechobee to manage lake levels also negatively impact lagoon water quality. In addition to reducing salinity levels, the freshwater releases also contain high levels of nutrients.

Dredging to maintain boating channels churns up sediments in the water column. While most impacts are short-term, if dredging is of a large enough scale and affects hydrodynamic properties of the area, such as the depth profile, water current direction, or current velocity, seagrasses may be severely threatened by sustained reductions of water clarity.

Prop scarring is caused by accidental or intentional boat groundings in shallow areas. When boaters enter water that is shallower than the draft of their vessel, propeller blades dig into seagrass beds as they pass over. This cuts not only seagrass blades but also frequently slashes underground rhizomes and roots. The resulting catastrophic furrow fragments the larger habitat, especially in areas where seagrass coverage is already sparse. Fragmented patches are susceptible to effects of erosion, and are vulnerable to increased damage as boaters continue to scar the meadow.

Management and Restoration

As of 2015, the Indian River Lagoon had approximately 59,000 acres of seagrass coverage, down from 80,000 in 2007, a decline of approximately 26 percent. Since the 1940s, some areas of the lagoon have completely lost their seagrass meadows. Some studies estimate the lagoon has lost 95 percent of its seagrasses since 1981.

As a general rule, seagrass coverage remains steady or increases in areas with relatively pristine environmental conditions. In the area encompassing the NASA protected zones, Merritt Island Wildlife Refuge, and Canaveral National Seashore, seagrass coverage has remained unchanged over the last 50 years; in the central Indian River Lagoon, near Sebastian Inlet, seagrass coverage has increased markedly from historic levels, though much of this increase is due to the opening of the inlet at its present location.

By contrast, seagrass coverage in areas heavily impacted by overdevelopment of shoreline areas and wetlands tend to experience significant declines. In the 50-mile stretch of the IRL between the NASA Causeway and Grant, Florida, seagrass coverage has decreased by over 70 percent in the last 50 years.

Managing water quality for seagrass health has improved overall water quality within the lagoon, increased overall amount and quality of available habitat, and is expected to increase biodiversity within seagrass meadows. St. Johns River Water Management District (SJRWMD) and South Florida Water Management District (SFWMD) are two of the organizations charged with managing water quality within the Indian River Lagoon. These organizations have actively pursued the goal of managing the lagoon in order to preserve and restore seagrass coverage to historic levels.

Management efforts include large-scale projects to remove muck from the lagoon bottom, construction of stormwater parks to intercept thousands of pounds of nutrients before they can enter the lagoon, and a 30-year monitoring program to document seasonal fluctuations in seagrass populations.

Seagrass restoration efforts are also widespread throughout the lagoon. Often, these projects also include clam planting and oyster reef rebuilding. In addition to nursery-grown material, some replanting efforts utilize seagrass fragments, dislodged by storms or boating activities, as starting material.

Further Reading

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