Posidonia oceanica

Taxonomy

Assessment Information

Geographic Range

Population

Habitat and Ecology

Threats

Conservation Actions

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Taxonomy [top]

Kingdom	Phylum	Class	Order	Family
PLANTAE	TRACHEOPHYTA	LILIOPSIDA	NAJADALES	POSIDONIACEAE

Scientific Name:

Posidonia oceanica

Species Authority:

(L.) Delile

Common Name/s:

English

–

Neptune Grass, Species code: Po

Assessment Information [top]

Red List Category & Criteria:	Least Concern ver 3.1
Year Published:	2010
Assessor/s:	Pergent, G., Semroud, R., Djellouli, A., Langar, H. & Duarte, C.
Reviewer/s:	Cuttelod, A., Pergent-Martini, C., Abdul Malak, D., Ballesteros, E., Short, F. & Livingstone, S.
Contributor/s:	Carruthers, T., Short, F.T., Waycott, M., Kendrick, G., Fourqurean, J.W., Calladinea, A., Kenworthy, J. & Dennison, W.
Justification: Posidonia oceanica is endemic to the Mediterranean, and is the most-widespread seagrass species in the region. It is an important habitat forming species and provides habitat for many species. There have been declines in the population of P. oceanica due to mechanical damage from trawling and boats, coastal development and eutrophication, with losses particularly observed in the western Mediterranean. The overall decline in the Mediterranean has been measured as approximately 10% over the last 100 years, which does not trigger any of the threatened categories, but further information is needed. P. oceanica is a very slow growing species and takes a long time to recolonize areas from where it has been removed, although there is some evidence that it has recolonized in some areas in the Mediterranean where it is protected. This species is listed as Least Concern.

Geographic Range [top]

Range Description:	Posidonia oceanica is endemic to the Mediterranean Sea. Posidonia oceanica is the dominant seagrass in the Mediterranean Sea covering about 50,000 km² of coastal to offshore sandy and rocky areas to depths of 45 m.
Countries:	Native: Algeria; Croatia; Cyprus; Egypt; France; Greece; Israel; Italy; Libya; Morocco; Slovenia; Spain; Syrian Arab Republic; Tunisia; Turkey
FAO Marine Fishing Areas:	Native: Mediterranean and Black Sea
Range Map:	Click here to open the map viewer and explore range.

Population [top]

Population:	Posidonia oceanica is abundant in the Mediterranean, however there is evidence that the population is declining in the western Mediterranean. Several local studies have shown serious declines in P. oceanica meadows. However, accurate data are generally very localised and are lacking for many parts of the Mediterranean. Therefore, cases of observed regression are not representative of the region as a whole. Relatively healthy P. oceanica meadows, whose limits have changed little since the 1950s, can thrive in highly developed areas. In some areas, there is evidence of recolonization by P. oceanica after the human impact ceased or was reduced, but the process of recolonization is extremely slow, i.e. a few centimetres per year (Pergent-Martini et al. 1995). Considering all Mediterranean seagrass species area combined, Jackson et al. (2006) estimated total losses at 446 km² over the last 100 years. A recent study looking at the population status of six Mediterranean P. oceanica areas showed positive population dynamics in some localities for all studied parameters (González-Correa et al. 2007). It is estimated that the overall decline in area for P. oceanica is less than 10% (Thomas et al. 2005) over three generation lengths (100 years), and declines have mainly occurred near urban areas (Boudouresque et al. 2006).
Population Trend:	Decreasing

Population:

Posidonia oceanica is abundant in the Mediterranean, however there is evidence that the population is declining in the western Mediterranean. Several local studies have shown serious declines in P. oceanica meadows. However, accurate data are generally very localised and are lacking for many parts of the Mediterranean. Therefore, cases of observed regression are not representative of the region as a whole. Relatively healthy P. oceanica meadows, whose limits have changed little since the 1950s, can thrive in highly developed areas. In some areas, there is evidence of recolonization by P. oceanica after the human impact ceased or was reduced, but the process of recolonization is extremely slow, i.e. a few centimetres per year (Pergent-Martini et al. 1995). Considering all Mediterranean seagrass species area combined, Jackson et al. (2006) estimated total losses at 446 km² over the last 100 years. A recent study looking at the population status of six Mediterranean P. oceanica areas showed positive population dynamics in some localities for all studied parameters (González-Correa et al. 2007). It is estimated that the overall decline in area for P. oceanica is less than 10% (Thomas et al. 2005) over three generation lengths (100 years), and declines have mainly occurred near urban areas (Boudouresque et al. 2006).

Population Trend:

Decreasing

Habitat and Ecology [top]

Habitat and Ecology:	Posidonia oceanica is the most abundant seagrass species in the Mediterranean. This species forms single species meadows in the Mediterranean bioregion (Peres and Picard 1964, Boudouresque et al. 1990, Short et al. 2007), from the surface to the maximum depth of 45 m. It is common on different types of substrate, from rocks to sand (Bethoux and Copin-Motegut, 1986), except in estuaries where the input of fresh water and fine sediments is high. It is more commonly found on sand. Posidonia oceanica is a large, long-living but very slow-growing seagrass. Its shoots, which are able to live for at least 30 years, are produced at a slow rate from rhizomes which grow horizontally by only 1-6 cm each year. Over centuries the rhizomes form mats which rise up into reefs that help to trap sediment and mediate the motion of waves, thus clarifying the water and protecting beaches from erosion (Boudouresque et al. 2006). Posidonia oceanica is a monoecious species, with male and female flowers in the same inflorescence. The biological characteristics of P. oceanica are not conducive to a rapid recolonization of dead matte: sexual reproduction is rare, natural reestablishment is not common, and horizontal growth of rhizome edges from a contiguous bed is very slow (Meinesz et al. 1991). For more detailed information, a synthesis of the current knowledge is available in Boudouresque et al. (2006). The meadows composed of this species are considered the basis of the richness of Mediterranean coastal waters, due to the surface area they occupy and to the essential part they play at biological level in maintaining the coastal equilibrium and their concomitant economic activities (Boudouresque et al. 2006). The role of Posidonia oceanica meadows in marine coastal environments is often correctly compared to that of a forest. Posidonia oceanica is an important habitat forming species and provides habitat for many species. Nursery grounds for the juveniles of many commercially important fishes and vertebrates, such as several species of the family Sparidae (e.g., Diplodus annularis), Serranidae (e.g., Serranus cabrilla), Labridae (e.g., Coris julis and Crenilabrus maculatus) and Scorpaenidae (e.g., Scorpaena scrofa and Scorpaena porcus), and the sea urchin Paracentrotus lividus. Posidonia oceanica is also grazed on by the Green Sea Turtle (Chelonia mydas). A recent study by Thomas et al. (2005) found that urchins have a relatively minor impact on the seagrass, while grazing by the fish Salpa salpa can outstrip locally the plants' leaf production.
Systems:	Marine

Habitat and Ecology:

Posidonia oceanica is the most abundant seagrass species in the Mediterranean. This species forms single species meadows in the Mediterranean bioregion (Peres and Picard 1964, Boudouresque et al. 1990, Short et al. 2007), from the surface to the maximum depth of 45 m. It is common on different types of substrate, from rocks to sand (Bethoux and Copin-Motegut, 1986), except in estuaries where the input of fresh water and fine sediments is high. It is more commonly found on sand.

Posidonia oceanica is a large, long-living but very slow-growing seagrass. Its shoots, which are able to live for at least 30 years, are produced at a slow rate from rhizomes which grow horizontally by only 1-6 cm each year. Over centuries the rhizomes form mats which rise up into reefs that help to trap sediment and mediate the motion of waves, thus clarifying the water and protecting beaches from erosion (Boudouresque et al. 2006).

Posidonia oceanica is a monoecious species, with male and female flowers in the same inflorescence. The biological characteristics of P. oceanica are not conducive to a rapid recolonization of dead matte: sexual reproduction is rare, natural reestablishment is not common, and horizontal growth of rhizome edges from a contiguous bed is very slow (Meinesz et al. 1991). For more detailed information, a synthesis of the current knowledge is available in Boudouresque et al. (2006).

The meadows composed of this species are considered the basis of the richness of Mediterranean coastal waters, due to the surface area they occupy and to the essential part they play at biological level in maintaining the coastal equilibrium and their concomitant economic activities (Boudouresque et al. 2006). The role of Posidonia oceanica meadows in marine coastal environments is often correctly compared to that of a forest.

Posidonia oceanica is an important habitat forming species and provides habitat for many species. Nursery grounds for the juveniles of many commercially important fishes and vertebrates, such as several species of the family Sparidae (e.g., Diplodus annularis), Serranidae (e.g., Serranus cabrilla), Labridae (e.g., Coris julis and Crenilabrus maculatus) and Scorpaenidae (e.g., Scorpaena scrofa and Scorpaena porcus), and the sea urchin Paracentrotus lividus. Posidonia oceanica is also grazed on by the Green Sea Turtle (Chelonia mydas). A recent study by Thomas et al. (2005) found that urchins have a relatively minor impact on the seagrass, while grazing by the fish Salpa salpa can outstrip locally the plants' leaf production.

Systems:

Marine

Threats [top]

Major Threat(s):	Posidonia oceanica is threatened at depth by mechanical damage from trawling, boat anchoring, and turbidity. Coastal development including shoreline hardening, urban and harbour infrastructure, and sand mining affect the upper limit of Posidonia meadows. Eutrophication (fertilizer from agriculture and urban waste) and pollution, especially in coastal regions that are heavily populated, is a problem. Fish farm activities and aquaculture affect surrounding Posidonia meadows. Only meadows greater than 800 m away from the fish farms showed no impact from the fish farming activity and meadows up to one km from large fish farms may be affected (Marba et al. 2006). Invasive species also compete for habitat (e.g., seaweeds species such as Caulerpa taxifolia and Caulerpa racemosa). Climate change will be an additional threat through warming of waters (in excess of 28°C) and erosion from sea level rise. The lack of genetic variability and slow growth makes Posidonia oceanica less resilient to disturbance.

Major Threat(s):

Posidonia oceanica is threatened at depth by mechanical damage from trawling, boat anchoring, and turbidity. Coastal development including shoreline hardening, urban and harbour infrastructure, and sand mining affect the upper limit of Posidonia meadows.

Eutrophication (fertilizer from agriculture and urban waste) and pollution, especially in coastal regions that are heavily populated, is a problem. Fish farm activities and aquaculture affect surrounding Posidonia meadows. Only meadows greater than 800 m away from the fish farms showed no impact from the fish farming activity and meadows up to one km from large fish farms may be affected (Marba et al. 2006). Invasive species also compete for habitat (e.g., seaweeds species such as Caulerpa taxifolia and Caulerpa racemosa). Climate change will be an additional threat through warming of waters (in excess of 28°C) and erosion from sea level rise.

The lack of genetic variability and slow growth makes Posidonia oceanica less resilient to disturbance.

Conservation Actions [top]

Conservation Actions:	Posidonia oceanica is protected by EU legislation (Habitat directive), the Bern and Barcelona Conventions and national legislation (Spain, Algeria, etc.). Fishing regulations limit trawling activities near the shore (either above 50 m or a certain distance from the coast), which constitute an indirect protection measure for the species (EC Council Regulation N° 1967/2006 and national regulations). Posidonia oceanica is present in various marine parks in the countries along the Mediterranean Sea (Green and Short 2003). Efforts have been made to prevent physical damage caused by trawler-fishing by placing artificial reefs, consisting of spiked concrete blocks, along certain stretches of the coast, and also by mounting a coastal watch to prevent illegal trawling. Research into potential species conservation plans is needed, as is site protection and management, habitat restoration, increased awareness, and legislation at local, national and international levels.

Conservation Actions:

Posidonia oceanica is protected by EU legislation (Habitat directive), the Bern and Barcelona Conventions and national legislation (Spain, Algeria, etc.). Fishing regulations limit trawling activities near the shore (either above 50 m or a certain distance from the coast), which constitute an indirect protection measure for the species (EC Council Regulation N° 1967/2006 and national regulations).

Posidonia oceanica is present in various marine parks in the countries along the Mediterranean Sea (Green and Short 2003). Efforts have been made to prevent physical damage caused by trawler-fishing by placing artificial reefs, consisting of spiked concrete blocks, along certain stretches of the coast, and also by mounting a coastal watch to prevent illegal trawling.

Research into potential species conservation plans is needed, as is site protection and management, habitat restoration, increased awareness, and legislation at local, national and international levels.

Citation:	Pergent, G., Semroud, R., Djellouli, A., Langar, H. & Duarte, C. 2010. Posidonia oceanica. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 18 June 2013.
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