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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).
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.
Seagrass condition was calculated using the equation: Current % cover, or hectares, of seagrass ÷ reference % cover or hectares. When possible, the reference condition was calculated as the mean of the extents for the three oldest years between 1975-1985; alternative methods are described in Halpern et al. (2012).
Like all of the habitats used in the OHI, seagrass is used as a component in calculating scores for many of the different goals. However, it is used differently depending on the goal in question. Although habitats such as seagrass are used in calculating these goal scores, countries are not penalized for not having a certain habitat type. Calculations are based on the rank of existing habitats, as opposed to using all possible habitat types.
Seagrass is used as a component in calculating Carbon Storage (Seagrasses), Coastal Protection (Seagrasses), and Biodiversity (Habitats: Seagrasses). Seagrass condition and extent were used directly in the calculation of the Status and Trend of these Goals. In calculating Coastal Protection, each contributing habitat type is ranked based on its level of contribution. Seagrass is ranked a 1 out of 4 where a higher rank indicates more protective ability.
What Are the Impacts?
'As is the case for all plants, seagrasses grow by taking up carbon dioxide from the atmosphere and using sunlight to turn it into roots, stems and leaves. 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 ('sequestering') carbon for centuries, thereby slowing global warming. When seagrass beds are damaged
or destroyed, oxygen reaches the buried material oxydizing the carbon to form carbon dioxide that re-enters the atmosphere.
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.
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.
Rapid loss of seagrass habitat not only decreases carbon sequestration, 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.
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
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.
Seagrasses shelter important seafood species and since seafood is an important source of protein for people worldwide, damage to seagrass habitats could be detrimental to human health.
use and recycle nutrients found in water, providing services valued more than $19,000 per hectare per year.
Seagrasses are important habitats and nursery areas for many commercially important species of fish, crustaceans (e.g. shrimp, spiny lobster) and shellfish (e.g. queen conch).
The annual economic value of seagrass to fisheries in the Mediterranean Sea is at least €190 million, including about €78 million to commercial fishing (based on value of seafood caught) and €112 million to recreational fishing (based on the overall economic impact of spending by anglers). Cuttlefish and scorpion fish are key species supported by seagrass that are important to both the commercial and recreational sectors. Further declines in seagrass would also impact octopus, which are commercially important and sea bass seabass (Dicentrarchus spp.) which are recreationally important. Seagrass also provides other ecosystem services and benefits, so its full economic value is much greater than the €190 million calculated for fisheries. The European Union’s Marine Strategy Framework Directive requires determination of the full value of marine degradation.
What Has Been Done?
On Virginia’s Eastern Shore, an industry worth US $2.8 million (adjusted) in 1929 all but disappeared when disease and extreme weather hit the region in the 1930s, wiping out the population of seagrass that had been home to a diverse array of species, including the valuable bay scallop. In 1997, the Virginia Institute of Marine Science and the Nature Conservancy came together to form the Virginia Coastal Zone Management program, which aimed to restore seagrass to the areas by planting seeds. In the past decade, over 38 million seeds were distributed, generating the growth of over 4,200 acres of seagrass. The seagrass brought back bay scallops whose harvest generated US $1.9 million in 2009.
In the last century, seagrass meadows in Tampa Bay, Florida declined nearly 75% because of pollution from industrial and municipal waste. Thanks to a federal grant from the US Environmental Protection Agency to upgrade sewage treatment plants, state legislation requiring higher standards of sewage treatment, and the community-based efforts of the Tampa Bay National Estuary Program, the loss of seagrass was reversed. The flow of excess nitrogen from industrial and domestic sources was reduced by 90%, and between 1982 and 1994, the abundance of seagrass increased 23%.
Get More Information
A community-based seagrass assessment and monitoring program.
A global ecological monitoring program that “investigates and documents the status of seagrass resources and the threats to this important and imperiled marine ecosystem”.
Crooks, S., Herr, D., Tamelander, Jerker, Laffoley, Dan & Vandever, Justin Mitigating climate change through restoration and management of coatsal wetlands and near-shore marine ecosystems: challenges and opportunities. (The World Bank: Washington D.C., 2011).
Fourqurean, J. W. et al. Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience 5, 505–509 (2012).
Heck Jr, K. L. H., Hays, G. & Orth, R. J. Critical evaluation of the nursery role hypothesis for seagrass meadows. Mar Ecol Prog Ser 253, 123–136 (2003).
McArthur, L. C. & Boland, J. W. The economic contribution of seagrass to secondary production in South Australia. Ecological Modelling 196, 163–172 (2006).
Watson, R., Coles, R. & Lee Long, W. Simulation estimates of annual yield and landed value for commercial penaeid prawns from a tropical seagrass habitat, Northern Queensland, Australia. Mar. Freshwater Res. 44, 211–219 (1993).