Seagrass


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A seagrass bed in waters off the coast of FloridaSeagrasses are flowering plants from one of four plant families (Posidoniaceae, Zosteraceae, Hydrocharitaceae, or Cymodoceaceae), all in the order Alismatales (in the class of monocotyledons), which grow in marine, fully-saline environments.



Contents [hide]

1 Ecology

2 Environmental services

3 Uses

4 Disturbances and threats

5 Genera

6 See also

7 Bibliography

8 References

9 External links





[edit] Ecology



White-spotted puffers are often found in seagrass areas.

Seagrass bed with several echinoids, Grahams Harbour, San Salvador, BahamasThese unusual marine flowering plants are called seagrasses because the leaves are long and narrow and are very often green, and because the plants often grow in large "meadows" which look like grassland: in other words many of the species of seagrasses superficially resemble terrestrial grasses of the family Poaceae.



Because these plants must photosynthesize, they are limited to growing submerged in the photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. They undergo pollination while submerged and complete their entire life cycle underwater. There are about sixty species worldwide (although the taxonomy is still disputed).



Seagrasses form extensive beds or meadows, which can be either monospecific (made up of one species) or multispecific (where more than one species co-exist). 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 highly 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 a highly important link in the food chain, with hundreds of species feeding on seagrasses worldwide, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs.



[edit] Environmental services

Seagrasses are sometimes labeled ecosystem engineers, because they partly create their own habitat: the leaves slow down water-currents increasing sedimentation, and the seagrass roots and rhizomes stabilize the seabed. Their importance for associated species is mainly due to provision of shelter (through their three-dimensional structure in the water column), and for their extraordinarily high rate of primary production. As a result, seagrasses provide coastal zones with a number of ecosystem goods and ecosystem services, for instance fishing grounds, wave protection, oxygen production and protection against coastal erosion.



Sea-grass meadows account for 15% of the ocean’s total carbon storage. The ocean currently absorbs 25% of global carbon emissions.[1]



[edit] Uses

Seagrasses were collected as fertilizer for sandy soil. This was an important activity in the Ria de Aveiro, Portugal, where the plants collected were named moliço.



In the early 20th century, The French and to a lesser extent the Channel Islands used it as a mattress (paillasse) filling, and it was in high demand by French forces during World War I.



Lately seagrass has been used in furniture, and woven like rattan.



[edit] Disturbances and threats

Natural disturbances such as grazing, storms, ice-scouring, and desiccation are an inherent part of seagrass ecosystem dynamics. Seagrasses display an extraordinarily high degree of phenotypic plasticity, adapting rapidly to changing environmental conditions.



Seagrasses are in global decline, with some 30,000 square kilometres (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.



The most-used methods to protect and restore seagrass meadows include nutrient and pollution reductions, protection using marine protected areas, and restoration using seagrass transplantation.



[edit] Genera



Evolution of seagrassesFamily Posidoniaceae

Posidonia

Family Zosteraceae

Zostera

Heterozostera

Phyllospadix

Family Hydrocharitaceae (Frogbit family)

Enhalus

Halophila

Thalassia

Family Cymodoceaceae

Amphibolis

Cymodocea

Halodule

Syringodium

Thalassodendron

[edit] See also

Alismatales

Salt marsh

Posidonia oceanica

[edit] Bibliography

den Hartog, C. 1970. The Sea-grasses of the World. Verhandl. der Koninklijke Nederlandse Akademie van Wetenschappen, Afd. Natuurkunde, No. 59(1).

Duarte, Carlos M. and Carina L. Chiscano “Seagrass biomass and production: a reassessment” Aquatic Botany Volume 65, Issues 1-4, November 1999, Pages 159-174.

Green, E.P. & Short, F.T.(eds). 2003. World Atlas of Seagrasses. University of California Press, Berkeley, CA. 298 pp.

Hemminga, M.A. & Duarte, C. 2000. Seagrass Ecology. Cambridge University Press, Cambridge. 298 pp.

Hogarth, Peter The Biology of Mangroves and Seagrasses (Oxford University Press, 2007)

Larkum, Anthony W.D., Robert J. Orth, and Carlos M. Duarte (Editors) Seagrasses: Biology, Ecology and Conservation (Springer, 2006)

Orth, Robert J. et al. "A Global Crisis for Seagrass Ecosystems" BioScience December 2006 / Vol. 56 No. 12, Pages 987-996.

Short, F.T. & Coles, R.G.(eds). 2001. Global Seagrass Research Methods. Elsevier Science, Amsterdam. 473 pp.

A.W.D. Larkum, R.J. Orth, and C.M. Duarte (eds). Seagrass Biology: A Treatise. CRC Press, Boca Raton, FL, in press.

A. Schwartz; M. Morrison; I. Hawes; J. Halliday. 2006. Physical and biological characteristics of a rare marine habitat: sub-tidal seagrass beds of offshore islands. Science for Conservation 269. 39 pp. [1]

[edit] References

^ Laffoley first=Dan (December 26, 2009). "To Save the Planet, Save the Seas". http://www.nytimes.com/2009/12/27/opinion/27lafolley.html?ref=opinion. Retrieved December 2009.

[edit] External links

Seagrass-Watch - the largest scientific, non-destructive, seagrass assessment and monitoring program in the world

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

SeagrassLI

Seagrass Science and Management in the South China Sea and Gulf of Thailand

Marine Ecology (December 2006) - special issue on seagrasses

Cambodian Seagrasses

관련링크: http://www.seagrassrecovery.com/index.htm