BIOLOGY
Biology of Posidonia
The species Posidonia oceanica is endemic to the Mediterranean Sea. It forms extensive underwater meadows that grow on rocks and sandy bottoms in clean water from the surface to over 40 meter depth. The rest of the Posidonia species (of which there are 7-8) are found in Australian waters, which illustrates the antiquity of the genus.
Posidonia oceanica is made of creeping or erect stems usually buried in the sediment, called rhizomes. Creeping rhizomes are called plagiotropic, and erect rhizomes orthotropic. The rhizomes end in groups of 4-8 leaves (shoots) that can reach more than 1 m long. Rhizomes also have roots that can grow to 70 cm beneath the surface of sediment. Net leaves form all year around, and live between 5 and 8 months and more rarely up to 13 months.
Posidonia oceanica propagates mainly via vegetative reproduction through rhizome elongation and cuttings.
Posidonia oceanica can also flower under the sea. This event occurs infrequently during autumn every five to ten years. Its green flowers, hidden under the leaves, offer a rare spectacle for divers.
The flowers are hermaphrodite, i.e. both male and females at the same time. The fruits require 6 – 9 months to ripen. Between May and July, they drop off and float for a while. They can wash up on beaches in great quantities. The fruits have the shape and the size of an olive.
The species Posidonia oceanica is endemic to the Mediterranean Sea. It forms extensive underwater meadows that grow on rocks and sandy bottoms in clean water from the surface to over 40 meter depth. The rest of the Posidonia species (of which there are 7-8) are found in Australian waters, which illustrates the antiquity of the genus.
Posidonia oceanica is made of creeping or erect stems usually buried in the sediment, called rhizomes. Creeping rhizomes are called plagiotropic, and erect rhizomes orthotropic. The rhizomes end in groups of 4-8 leaves (shoots) that can reach more than 1 m long. Rhizomes also have roots that can grow to 70 cm beneath the surface of sediment. Net leaves form all year around, and live between 5 and 8 months and more rarely up to 13 months.
Posidonia oceanica propagates mainly via vegetative reproduction through rhizome elongation and cuttings.
Posidonia oceanica can also flower under the sea. This event occurs infrequently during autumn every five to ten years. Its green flowers, hidden under the leaves, offer a rare spectacle for divers.
The flowers are hermaphrodite, i.e. both male and females at the same time. The fruits require 6 – 9 months to ripen. Between May and July, they drop off and float for a while. They can wash up on beaches in great quantities. The fruits have the shape and the size of an olive.
Posidonia life cycle
The different stages throughout the year are:
Summer
During the warmer months, the Posidonia leaves are covered by a large number of epiphytic organisms that seek substrate, protection and food from the plant. This leads the plant to acquire a brownish hue, causing difficulties in photosynthesis and breathing. During these months, its growth in height is minimal.
Autumn
When autumn arrives, Posidonia sheds off its dry leaves and the epiphytes that lived in them. These reach the coast, contributing much of the sediment and creating a protective barrier to prepare for winter. At this time, new deep green leaves begin to sprout and flowering begins.
Winter
The flowering process continues. The meadows turn into deep green forests.
Spring
By spring, the water temperature increases therefore the growth of the leaves accelerates. The first fruits can emerge and the seeds can germinate.
The leaves and rhizomes support many flora and fauna, some of which are calcified. When they die, their remains fall off and form an autochthonous sediment (debris of sea urchin spines, mollusc shells, etc.). The leaves can also reduce the speed of the current and the kinetic energy of the sedimentary particles carried by the water, causing them to deposit on the seabed.
Rhizomes grow in height, and can relatively speed up their growth to resist being buried by the sedimentation.
The “matte” (or dead meadow) is the whole mass composed of rhizomes, roots and the sediments that fill the interstices. Its density can be high (up to 5000 / m2). They are not very putrescible and thus are conserved within the “matte” for several centuries or more. With time, the “matte” rises to the surface (elevation of 1 m per century maximum) where it is stopped by hydrodynamic conditions. There are different types of meadows.
Bibliography:
- Boudouresque C. F., Bernard G., Bonhomme P., Charbonnel E., Diviacco G., Meinesz A., Pergent G., Pergent-Martini C., Ruitton S., Tunesi L., 2012. Protection and conservation of Posidonia oceanica meadows. RAMOGE and RAC/SPA publisher, Tunis: 1-202. https://www.rac-spa.org/sites/default/files/doc_vegetation/ramoge_en.pdf
- Díaz-Almela E. & Duarte C.M. 2008. Management of Natura 2000 habitats. 1120 *Posidonia beds (Posidonion oceanicae). European Commission. https://uicnmed.org/bibliotecavirtualposidonia/wp-content/uploads/2014/04/MANAGEMENT-of-Natura-2000-habitats-Posidonia-beds.pdf
- Telesca, L. et al. Seagrass meadows (Posidonia oceanica) distribution and trajectories of change. Sci. Rep. 5, 12505; doi: 10.1038/srep12505 (2015).