Seagrass

Taxobox
name = Seagrass
image_width = 240px
regnum = Plantae
divisio = Magnoliophyta
classis = Liliopsida
ordo = Alismatales

Seagrasses (or sea-grasses in British English) are flowering plants from one of four plant families (Posidoniaceae, Zosteraceae, Hydrocharitaceae, or Cymodoceaceae), which grow in marine, fully-saline environments.

Ecology

These 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 dugongs, manatees, fish, geese, swans, sea urchins and crabs.

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.

Uses

Seagrasses are 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 part of the 20th century, seagrass was used by the French and to a lesser extent the Channel Isles as a form of mattress (paillasse) filling, and was in high demand by the French forces during World War I. Lately seagrass has been used in furniture, and woven like rattan.

Disturbances and threats to seagrasses

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, however, in global decline, with some 30,000 square kilometers lost during the last decades. The main reason for this decline 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 results in less sunlight reaching the seagrass leaves, which reduces photosynthesis and primary production. 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 the growth of algae, 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 reducing nutrient levels and pollution, protection using marine protected areas, and restoration using seagrass transplantation.

Genera of seagrasses

*Family Posidoniaceae
**"Posidonia"

*Family Zosteraceae
**"Zostera"
**"Heterozostera"
**"Phyllospadix"

*Family Hydrocharitaceae (Frogbit family)
**"Enhalus"
**"Halophila"
**"Thalassia"

*Family Cymodoceaceae
**"Amphibolis"
**"Cymodocea"
**"Halodule"
**"Syringodium"
**"Thalassodendron"

ee also

*Alismatales
*Salt marsh

References

Bibliography

* den Hartog, C. 1970. "The Sea-grasses of the World". "Verhandl. der Koninklijke Nederlandse Akademie van Wetenschappen, Afd. Natuurkunde", No. 59(1).
* Hemminga, M.A. & Duarte, C. 2000. "Seagrass Ecology". Cambridge University Press, Cambridge. 298 pp.
* Short, F.T. & Coles, R.G.(eds). 2001. "Global Seagrass Research Methods". Elsevier Science, Amsterdam. 473 pp.
* Green, E.P. & Short, F.T.(eds). 2003. World Seagrass Atlas. UNEP World Conservation Monitoring Centre, UCP, Berkely. 286 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. [http://www.doc.govt.nz/upload/documents/science-and-technical/sfc269.pdf]

External links

* [http://www.seagrasswatch.org/ Seagrass-Watch - the largest scientific, non-destructive, seagrass assessment and monitoring program in the world]
* [http://www.seagrassnet.org/ SeagrassNet - global seagrass monitoring program]
* [http://www.fiu.edu/~seagrass/class/bot5647/maureen.htm Taxonomy of seagrasses]
* [http://wsa.seagrassonline.org/ World Seagrass Association]
* [http://www.seagrassli.org/ SeagrassLI]
* [http://www.unepscs.org/index.php?option=com_content&task=view&id=52&Itemid=84 Seagrass Science and Management in the South China Sea and Gulf of Thailand]
* [http://www.blackwell-synergy.com/toc/mae/27/4 "Marine Ecology" (December 2006)] - special issue on seagrasses