Seagrass

Seagrasses are submerged flowering plants, found mostly along the coastline, covering an estimated global area of 300,000-600,000 km2. Healthy seagrasses protect the shore, promote biodiversity, store carbon, cycle nutrients, and help support numerous industries (e.g. fishing, tourism).

Seagrasses are threatened by reductions in water clarity caused by eutrophication, erosion, and increased concentrations of particulate matter.

There are 72 known species of seagrass, of which 10 are at risk of extinction and 3 are endangered (Short 2011).

Seagrasses have declined in area by about 29% since the beginning of the twentieth century, at an annual rate of about 1.5% and faster in recent years, replaced with unvegetated, mud and sand soils (Fourqurean et al. 2012).

Seagrass Ecosystem: Loss of Habitat
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Which Goals Does This Affect?


How Was It Measured?

No globally complete databases or maps of seagrass extent currently exist, so seagrass extent was calculated from vector-based data obtained from the Global Distribution of Seagrasses database maintained by United Nations Environmental Program’s World Conservation Monitoring Center (UNEP-WCMC).

Seagrass Status and Trend data were calculated on a per-site basis using data from Waycott et al. 2009 and Short et al. 2011, which provide annual seagrass habitat extent data for several sites around the world.

When possible, the reference condition was calculated as the mean of the three oldest years between 1975-1985; alternative methods are described in Halpern et al. 2012.


What Are the Impacts?

Seagrass Biodiversity
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ECOLOGICAL IMPACT
Seagrasses uptake carbon dioxide from the atmosphere in a process known as ‘sequestration’. Two-thirds of seagrass biomass is buried as rhizomes and roots. If undisturbed, these structures, as well as seagrass litter in surrounding soil, are capable of storing carbon for centuries. When seagrass beds are damaged or destroyed, carbon is released back into the atmosphere.

Changes or disturbances to seagrass beds and meadows are often warning signs of increasing pressure from human activities. Seagrass’ sensitivity to these changes makes it an 'indicator species', acting as a barometer for the health of the surrounding ecosystem.

Seagrass beds occupy less than 0.2% of the area of the world's oceans, but bury between 4.2 and 8.4 Gt (1 GT = 1 billion metric tonnes) of organic carbon per year.

Rapid loss of seagrass habitat not only decreases carbon sequestration by seagrass ecosystems, but also releases stored carbon from the disturbed soils, contributing as much as 10% of the 0.5-2.7 Gt C per year released from changes in land use.

Seagrasses store approximately twice the amount of organic carbon per hectare as terrestrial soils. Though seagrass biomass is small compared with forests, the amount of carbon they store in soils may be nearly as high as that stored by terrestrial systems and mangroves.

Most terrestrial forests eventually return stored carbon to the atmosphere during forest fires, but seagrass soils can accumulate to depths of meters over millennia if undisturbed.

Over the last century, drainage of 1,800 km2 of wetlands for agricultural use in the San Joaquin Delta has released 4,000 years worth of carbon dioxide into the atmosphere, an amount of nearly 1 billion tons. Each year, between 5 and 7.5 million tons of CO2 continue to seep from the Delta, an amount equivalent to 1-1.5% of California’s annual greenhouse gas emissions.

Seagrasses provide an abundance of food and nutrients for surrounding species and neighboring habitats. They also offer protection from predators, and serve as nursery grounds for many young vertebrate and invertebrate species.

Seagrasses provide an essential habitat for culturally important species such as manatees, dugongs, and green turtles.  Certain sea birds, such as ducks, geese and swans, rely upon seagrass as a food source.
HUMAN HEALTH IMPACT
Seagrasses stabilize coastal sediments and prevent them from eroding.

Seagrasses provide shoreline protection by absorbing the impact of waves, although protection potential can be limited in extreme weather events, such as hurricanes and tsunamis.
ECONOMIC IMPACT
Seagrasses use and recycle nutrients found in water, providing services for which people would normally pay US $19,002 per hectare per year.

Seagrasses are important habitats or nursery areas for many commercially important species of fish, crustaceans (e.g. shrimp, spiny lobster) and shellfish (e.g. queen conch).


What Has Been Done?


Get More Information

Seagrass Watch
A community-based seagrass assessment and monitoring program.

Seagrass Recovery
A Unites States based organization dedicated to preserving seagrass through patented inventions and services for replanting and restoring damaged seagrass beds.

SeagrassNet
A global ecological monitoring program that “investigates and documents the status of seagrass resources and the threats to this important and imperiled marine ecosystem”.



References




PHOTO(S): © Keith A. Ellenbogen
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