Oceanic Posidonia (L.) Delila, 1813 is an aquatic plant, endemic to the Mediterranean Sea, belonging to the family Posidoniacee (Angiosperms Monocotyledons). It has features similar to terrestrial plants, has roots, a rhizomatous stem and leaves up to a meter long ribbon and united in tufts of 6-7. It blooms in autumn and in spring produces floating fruits commonly called “sea olives”.
Posidonia lives between 1 and 30 meters deep, exceptionally and only in very clear water up to 40 meters, and withstands temperatures between 10 and 28 ° C. It is a plant that requires relatively constant salinity values for which there is hardly near the mouths of rivers or lagoons. It needs a bright light, so the light is one of the main limiting factors. Colonizes the sandy or debris which adheres by means of the rhizomes and on which form vast prairies, or posidonieti, high density (over 700 plants per square meter). A small part of this production (3-10%) is used by herbivores, a large part goes to the decomposers and another percentage is stored inside the mattes in leaves and rhizomes.
The grasslands have an upper and a lower limit. The first, the point at which begins the prairie starting from the coast, is rather sharp, while the second, the point where it ends, can be of three types:
progressive or climatic limit: with increasing depth, decreases the density of the foliar dossiers because the light becomes a limiting factor. It is characterized by the presence of rhizomes plagiotropi ending suddenly.
Net or edaphic limit: the type of substrate does not allow the progression of the rhizomes, passing for example from a sandy substrate to a rocky. It is characterized by the absence of matte and by the presence of a high density of leaf files. limit erosion: it is linked to the strong hydrodynamic that does not allow the prairie to progress. It is characterized by high density and the presence of matte.
Limit regressive: it is caused by pollution that makes the waters of a given area turbid, thus preventing light penetration deeper. There are only matte death. In sheltered areas and low hydrodynamic, which causes increased sedimentation, the matte can stand up to the leaves do not reach the water surface. It creates in this way a said recif barriers barrier. Between the barrier and the coast you can form a lagoon, and this prevents the progression of the grassland towards the coast. The recif barriers has a very important role in protecting the coastline from erosion.
In areas with high hydrodynamic, however, the rhizomes can be undermined, creating formations such intermatte, formed by erosion channels.
Nelle aree riparate e a basso idrodinamismo, che provoca una maggiore sedimentazione, le matte possono alzarsi fino a che le foglie non raggiungono la superficie dell’acqua. Si crea in questo modo una barriera detta recif barriere. Tra la barriera e il litorale si può formare una laguna, e ciò impedisce il progredire della prateria verso la costa. La recif barriere ha un ruolo importantissimo nella protezione della linea costiera dall’erosione.
Nelle zone a forte idrodinamismo, invece, i rizomi possono essere scalzati, creando delle formazioni dette intermatte, costituite da canali di erosione.
Oceanic Posidonia as bioindicator
For nearly twenty years, it is expanding its use as a biological indicator of Oceanic Posidonia. Beams blocks of oceanic Posidonia. The analysis of the beam density is one of the prairies study methods. In fact, the plant has all the characteristics of a good bioindicator:
It is a benthic species; It has a long life cycle; It has spread widely throughout the Mediterranean; It has a great capacity for concentration of pollutants in its tissues; It is very sensitive to environmental changes. Through the study of grasslands it is therefore possible to have a fairly reliable picture of the environmental quality of coastal marine waters.
Posidonia oceanica has roots that mainly serve to anchor the plant to the substrate, rhizome and ribbon-like leaves.
Detail of the rhizome
The rhizomes, up to 1 cm thick, grow both horizontally (plagiotropic rhizomes) and vertically (orthotropic rhizomes). The former, thanks to the presence on the underside of lignified roots up to 15 cm long, anchor the plant to the substrate while the latter, by increasing the height, have the function of counteracting silting due to continuous sedimentation. The two types of growth give rise to the so-called matte, a terraced formation consisting of the intertwining of layers of rhizomes, roots and trapped sediment. In this way the posidonia colonize an environment that is difficult for algae to use due to the lack of roots.
Sexual reproduction occurs through the production of flowers and fruits. The flowers are hermaphroditic and grouped in a spike-shaped inflorescence, green in color and enclosed between floral bracts. The floral axis attaches to the rhizome in the center of the bundle. The gynoecium is formed by a unilocular ovary which continues with a stylus and ends with the stigma; the androecium consists of three stamens with short anthers.
Flowering is regulated by environmental factors (light and temperature) and by endogenous factors (age and size of the plant) and occurs in September – October in the grasslands closest to the sea surface, while it is moved by two months in the deeper grasslands. The pollen inside the anther is spherical in shape but becomes stringy as it is released into the water. There are no recognition mechanisms between pollen and stigma that prevent self-fertilization.
Pollination is hydrophilic and can lead to fruit formation, although some abort before ripening which occurs after six months. Once ripe, the fruits detach and float to the surface.
The fruit, slightly fleshy and commonly called “sea olive”, is similar to a drupe and has a porous pericarp rich in an oily substance that allows it to float. When it rots, a seed is released, which falls to the bottom and if it finds the right depth conditions and the right type of sediment it germinates and gives rise to a new plant.