Due to their morphology and growth habit, seagrasses are also sometimes confused with marine macroalgae; however closer examination reveals significant differences. Structurally, seagrasses are more closely related to terrestrial plants and, like terrestrial plants, possess specialized tissues that perform specific tasks within each plant. Conversely, algae are relatively simple and unspecialized in structure. While algae possess only a tough holdfast that assists in anchoring the plant to a hard substratum, seagrasses possess true roots that not only hold plants in place, but also are specialized for extracting minerals and other nutrients from the sediment. All algal cells possess photosynthetic structures capable of utilizing sunlight to produce chemical energy. In seagrasses, however, chloroplasts occur only in leaves, thus confining photosynthesis to leaves. Further, algae are able to take up minerals and other nutrients directly from the water column via diffusion. Seagrasses however, transport minerals and nutrients in xylem and phloem. Finally, while most algae lack specialized reproductive structures, most seagrasses have separate sexes and produce flowers and seeds, with embryos developing inside ovaries.

The value of seagrasses:
Within seagrass communities, a single acre of seagrass can produce over 10 tons of leaves per year. This vast biomass provides food, habitat, and nursery areas for a myriad of adult and juvenile vertebrates and invertebrates. Further, a single acre of seagrass may support as many as 40,00 fish, and 50 million small invertebrates. Because seagrasses support such high biodiversity, and because of their sensitivity to changes in water quality, they have become recognized as important indicator species that reflect the overall health of coastal ecosystems.

Seagrasses perform a variety of functions within ecosystems, and have both economic and ecological value. The high level of productivity, structural complexity, and biodiversity in seagrass beds has led some researchers to describe seagrass communities as the marine equivalent of tropical rainforests. While nutrient cycling and primary production in seagrasses tends to be seasonal, annual production in seagrass communities rivals or exceeds that of terrestrially cultivated areas. In Florida, Halodule beaudettei, has an estimated annual production (as measured in grams of carbon per square meter) of 182 – 730 g/C/m^-2; Syringodium filiforme has an estimated annual production of 292 - 1095 g/C/m^-2; and Thalassia testudinum has an estimated annual production 329 - 5840 g/C/m^-2. Blade elongation in seagrasses averages 2-5 mm per day in Thalassia testudinum, 8.5 mm in Syringodium filiforme, and as much as 3.1 mm in Halodule beaudettei. In the Indian River Lagoon, Halodule beaudettei has been shown to produce one new leaf every 9 days during spring – the season of highest productivity (Virnstein 1982).

As habitat, seagrasses offer food, shelter, and essential nursery areas to commercial and recreational fishery species, and to the countless invertebrates that are produced within, or migrate to seagrasses. The complexity of seagrass habitat is increased when several species of seagrasses grow together, their leaves concealing juvenile fish, smaller finfish, and benthic invertebrates such as crustaceans, bivalves, echinoderms, and other groups. Juvenile stages of many fish species spend their early days in the relative safety and protection of seagrasses. Additionally, seagrasses provide both habitat and protection to the infaunal organisms living within the substratum as seagrass rhizomes intermingle to form dense networks of underground runners that deter predators from digging infaunal prey from the substratum. Seagrass meadows also help dampen the effects of strong currents, providing protection to fish and invertebrates, while also preventing the scouring of bottom areas. Finally, seagrasses provide attachment sites to small macroalgae and epiphytic organisms such as sponges, bryozoans, forams, and other taxa that use seagrasses as habitat. A number of studies have found epiphytes to be highly productive components of seagrass habitats (Penhale 1977, Heijs 1984, Tomasko & Lapointe 1991), with epiphytes in some systems accounting for up to 30% of ecosystem productivity, and more than 30% of the total above ground biomass (Penhale 1977, Morgan & Kitting 1984, Heijs 1984). Seagrass epiphytes also contribute to food webs, either directly via organisms grazing on seagrasses, or indirectly following the deaths of epiphytes, which then enter the food web as a detrital carbon source (Fry & Parker 1979, Kitting et al. 1984).

Economically, Florida’s 2.7 million acres of seagrass supports both commercial and recreational fisheries that provide a wealth of benefits to the state’s economy. Florida’s Department of Environmental Protection (FDEP) reported that in 2000, Florida’s seagrass communities supported commercial harvests of fish and shellfish valued at over 124 billion dollars. Adding the economic value of the nutrient cycling function of seagrasses, and the value of recreational fisheries to this number, FDEP has estimated that each acre of seagrass in Florida has an economic value of approximately $20,500 per year, which translates into a statewide economic benefit of 55.4 billion dollars annually. In Fort Pierce, Florida alone, the 40 acres of seagrass in the vicinity of Fort Pierce Inlet are valued at over $800,000 annually. When projected across St. Lucie County’s estimated 80,000 acres of seagrass, this figure increases to 1.6 billion dollars per year.

Threats to seagrass communities:
Seagrasses are subject to a number of biotic and abiotic stresses such as storms, excessive grazing by herbivores, disease, and anthropogenic threats due to point and non-point sources of pollution, decreasing water clarity, excessive nutrients in runoff, sedimentation and prop scarring. What effect these stresses have on seagrasses is dependent on both the nature and severity of the particular environmental challenge. Generally, if only leaves and above-ground vegetation are impacted, seagrasses are generally able to recover from damage within a few weeks; however, when damage is done to roots and rhizomes, the ability of the plant to produce new growth is severely impacted, and plants may never be able to recover (Zieman et al. 1984, Fonseca et al. 1988). Some of the major environmental challenges to seagrass health are discussed below.

Anthropogenic Threats:
[A more detailed look at some emerging human-induced threats facing the seagrasses of the IRL is available here.]

The health of seagrass communities obviously relies heavily upon the amount of sunlight that penetrates the water column to reach submerged blades. Water clarity, heavily affected by the amount and composition of stormwater runoff and other non-point sources of pollution, is the primary influence that determines how much light ultimately reaches seagrass blades. Stormwater runoff drains both urban and agricultural areas, and carries with it household chemicals, oils, automotive chemicals, pesticides, animal wastes, and other debris. Under normal conditions, seagrasses maintain water clarity by trapping silt, dirt, and other sediments suspended in the water column. These materials are then incorporated into the benthic substratum, where they are stabilized by seagrass roots. However, when sediment loading becomes excessive, turbidity in the water column increases and the penetration of sunlight is inhibited. In extreme cases, excessive sediment loading can actually smother seagrasses.

When heavy volumes of stormwater runoff carrying excessive amounts of nitrogen and phosphorous from fertilizers and animal wastes drains into canals, and eventually empties into estuaries, it accelerates the growth rate of phytoplankton. Under normal nutrient conditions, microalgae grow at manageable levels, and are an important food source for many filter feeding and suspension feeding organisms. However, excess nutrient loading in water bodies causes massive blooms of algae that reduce water clarity by blocking the amount of sunlight available. Reduction in light levels, as well as depletion of the nutrient supply, leads to the death and decomposition of these microalgal blooms. The process of decomposition further degrades water quality by depleting much of the dissolved oxygen available in the water column, sometimes leading to hypoxic conditions and fish kills.

A number of other anthropogenic factors often affect the health of seagrass meadows. Dredging churns up seagrass beds, increasing turbidity and suspended sediments in the water column. This period of poor water quality may be temporary, and have few long-term impacts on seagrasses. However, if dredging affects hydrodynamic properties of the area, such as the depth profile, current direction, or current velocity, seagrasses may be severely threatened. Prop scarring is another factor that threatens seagrasses. Accidental or intentional groundings of boats in shallow areas may lead to significant, localized impacts on seagrasses. Scarring occurs in water that is shallower than the draft of the boat. Boaters entering these shallows often dig up the seagrass beds as they motor, cutting not only the blades, but more catastrophically, slashing underground rhizomes and roots as well. Prop scarring often results in a continuous line of seagrass damage, which acts to fragment the habitat, especially in areas where seagrass coverage is sparse. Seagrasses that remain in fragmented areas are then susceptible to erosion effects and are vulnerable to increased damage as boaters continue to scar the meadow.

NaturalThreats:
Threats to seagrasses are not limited to anthropogenic factors. There are also a number of natural factors that damage or threaten seagrasses. A wasting disease, thought to be caused by a marine slime mold, caused extensive damage to eelgrass beds (Zostera spp.) in temperate coastal areas during the 1930s, diminishing seagrass coverage by over 90%. Storms can also cause widespread damage to established seagrass meadows, sometimes on a regular basis. Wind-driven waves may break or uproot seagrasses, having minimal effects when leaves and vegetative structures are damaged; and more lasting effects when rhizomes and roots are damaged. In addition, 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.

Management of seagrasses:
The Indian River Lagoon has approximately 80,000 acres of seagrass coverage at the present time, a decline of approximately 18% overall from seagrass coverage estimated from aerial photos taken during the 1950s. Some areas of the lagoon have experienced alarming declines in seagrass coverage. For example, in the 50 mile stretch of the IRL between the NASA Causeway and Grant, Florida, seagrass coverage has decreased by over 70% in the last 50 years. However, in other areas, seagrasses have maintained their historic coverage levels, or have actually increased. In the area encompassing the protected zones of NASA, 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. As a general rule, seagrass coverage has been observed to remain steady or increase in areas retaining relatively pristine environmental conditions, and has declined in areas heavily impacted by overdevelopment of shoreline areas and wetlands.

St. Johns River Water Management District (SJRWMD) and South Florida Water Management District (SFWMD) are 2 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. Two main focus areas for improving water quality in the lagoon have been addressed: 1) to assist local governments in controlling and managing stormwater runoff; and 2) to purchase, and to the extent possible, restore, fringing wetland areas. Managing water quality for seagrass health has improved overall water quality within the lagoon; has increased habitat quality and quantity; and over the long-term, is expected to increase biodiversity within seagrass meadows. Enriching biodiversity within the Indian River Lagoon will make large contributions to the economy of the area by enhancing commercial and recreational fisheries stocks, increasing tourism and recreational opportunities, increasing property values, and potentially creating additional jobs. Outreach and education efforts undertaken by SJRWMD and SFWMD have improved public awareness and support of seagrass restoration as an effective management strategy.

Click a highlighted link to read more about individual species:

FURTHER READING:

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Aspden, William Clarkson. 1980. Aspects of photosynthetic carbon metabolism in
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Report by:  K. Hill, Smithsonian Marine Station
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