Seagrasses are flowering plants (angiosperms) belonging to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the order Alismatales (in the class of monocotyledons), which grow in marine, fully saline environments. Twelve genera comprising some 60 species are known.
The name seagrass stems from the many species whose leaves are long and narrow, who grow by rhizome extension and often spread across large " meadows", which resemble grassland: many species superficially resemble terrestrial grasses of the family Poaceae.
Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine pollination and complete their life cycle underwater.
Seagrasses beds/meadows can be either monospecific (made up of a single species) or in mixed beds. In temperate areas, usually one or a few species dominate (like the eelgrass Zostera marina in the North Atlantic), whereas tropical beds usually are more diverse, with up to thirteen species recorded in the Philippines.
Seagrass beds are diverse and productive ecosystems, and can harbor hundreds of associated species from all phyla, for example juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is an important link in the food chain, feeding hundreds of species, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs. Some fish species that visit/feed on seagrasses raise their young in adjacent mangroves or coral reefs.
Seagrasses trap sediment and slow down water movement, causing suspended sediment to settle out. Trapping sediment benefits coral by reducing sediment loads, improving photosynthesis for both coral and seagrass. 
Zosteraceae, also known as the seagrass family, includes two genera containing 22 marine species. It is found in
coastal waters, with the highest diversity around Korea and Japan.|
Hydrocharitaceae, also known as tape-grasses, include
Canadian waterweed and frogbit. The family includes both fresh and marine aquatics, although of the seventeen species currently recognised only three are marine.
 They are found throughout the world in a wide variety of habitats, but are primarily tropical.|
Posidoniaceae contains a single genus with two to nine marine species found in the seas of the
Mediterranean and around the south coast of
Species subtotal: 2 to 9
|Posidonia||2 to 9 species|
Cymodoceaceae, also known as the manatee-grass family, includes only marine species.
 Some taxonomists do not recognize this family.|
Their importance for associated species is mainly provision of shelter (through their three-dimensional structure in the water column) and to their high rate of primary production. As a result, seagrasses provide coastal zones with a number of ecosystem goods and services, for instance nursery habitat for commercially and recreationally valued fishery species,  fishing grounds,  wave protection, oxygen production and protection against coastal erosion. Seagrass meadows account for more than 10% of the ocean’s total carbon storage.  Per hectare, it holds twice as much carbon dioxide as rain forests. Yearly, seagrasses sequester about 27.4 million tons of CO2[ citation needed]. Global warming models suggest that some seagrasses will go extinct – Posidonia oceanica is expected to go extinct, or nearly so, by 2050. This would result in CO2 release.  
In the early 20th century, in France and, to a lesser extent, the Channel Islands, dried seagrasses were used as a mattress (paillasse) filling - such mattresses were in high demand by French forces during World War I. It was also used for bandages and other purposes.
In February 2017, researchers found that seagrass meadows may be able to remove various pathogens from seawater. On small islands without wastewater treatment facilities in central Indonesia, levels of pathogenic marine bacteria – such as Enterococcus – that affect humans, fishes and invertebrates were reduced by 50 percent when seagrass meadows were present, compared to paired sites without seagrass,  although this could be a detriment to their own survival. 
Natural disturbances, such as grazing, storms, ice-scouring and desiccation, are an inherent part of seagrass ecosystem dynamics. Seagrasses display a high degree of phenotypic plasticity, adapting rapidly to changing environmental conditions.
Seagrasses are in global decline, with some 30,000 km2 (12,000 sq mi) lost during recent decades. The main cause is human disturbance, most notably eutrophication, mechanical destruction of habitat, and overfishing. Excessive input of nutrients ( nitrogen, phosphorus) is directly toxic to seagrasses, but most importantly, it stimulates the growth of epiphytic and free-floating macro- and micro- algae. This weakens the sunlight, reducing the photosynthesis that nourishes the seagrass and the primary production results.
Decaying seagrass leaves and algae fuels increasing algal blooms, resulting in a positive feedback. This can cause a complete regime shift from seagrass to algal dominance. Accumulating evidence also suggests that overfishing of top predators (large predatory fish) could indirectly increase algal growth by reducing grazing control performed by mesograzers, such as crustaceans and gastropods, through a trophic cascade.
Macro algal blooms cause the decline and eradication of seagrasses. Known as nuisance species, macroalgae grow in filamentous and sheet-like forms and form thick unattached mats over seagrass, occurring as epiphytes on seagrass leaves. Eutrophication leads to the forming of a bloom, causing the attenuation of light in the water column, which eventually leads to anoxic conditions for the seagrass and organisms living in/around the plant(s). In addition to the direct blockage of light to the plant, benthic macroalgae have low carbon/nitrogen content, causing their decomposition to stimulate bacterial activity, leading to sediment resuspension, an increase in water turbidity and further light attenuation.  
When humans drive motor boats over shallow seagrass areas, sometimes the propeller blade can damage the seagrass.
The most-used methods to protect and restore seagrass meadows include nutrient and pollution reduction, marine protected areas and restoration using seagrass transplantation. Seagrass is not seen as resilient to the impacts of future environmental change. 
In various locations, communities are attempting to restore seagress beds that were lost to human action, including in the US states of Virginia,  Florida  and Hawaii.  Such reintroductions have been shown to improve ecosystem services. 
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- Project Seagrass - Charity advancing the conservation of seagrass through education, influence, research and action
- SeagrassSpotter - Citizen Science project raising awaress for seagrass meadows and mapping their locations
- Seagrass and Seagrass Beds overview from the Smithsonian Ocean Portal
- Nature Geoscience article describing the locations of the seagrass meadows around the world
- Seagrass-Watch - the largest scientific, non-destructive, seagrass assessment and monitoring program in the world
- Seagrass Ecosystem Research Group at Swansea University - Inter-disciplinary marine research for conservation
- Restore-A-Scar - a non-profit campaign to restore seagrass meadows damaged by boat props
- SeagrassNet - global seagrass monitoring program
- The Seagrass Fund at The Ocean Foundation
- Taxonomy of seagrasses
- World Seagrass Association
- Seagrass Science and Management in the South China Sea and Gulf of Thailand
- Marine Ecology (December 2006) - special issue on seagrasses
- Cambodian Seagrasses
- Seagrass Productivity - COST Action ES0906
- Fisheries Western Australia - Seagrass Fact Sheet