Article by Luca Turchetto
photo by Massimo Boyer
A few nights ago, I was eating pizza with a friend who was telling me about her day at the beach. Equipped with fins and masks, they and a friend of hers began exploring the seabed, but a few fin strokes from shore, as soon as the seabed got deeper, they stopped. "Luca, the seabed was dark because it was full of that seaweed that sticks to your body and from below it feels like it's rubbing the soles of your feet." I quote.
I've been working by the sea for years now, and this is something I've always heard grandmothers say to their grandchildren... don't go too far, the sea turns black, there's that seaweed that makes the sea dark...
Indeed, if we were to take aerial photographs, we would clearly see patches of darker color a few meters from the shore... well... what many people call algae is NOT algae but rather a plant. And it is probably one of the most important marine plants found in the aquatic environment: Posidonia oceanica.
Obviously it's strange to think of a plant in the sea; since the beginning of time, plants have been on land and algae in the sea...and yet that's how it is.
But let's proceed step by step without creating too much confusion, otherwise we will continue to think that Posidonia is an algae...or even worse, calling it algae or plant depending on how we wake up in the morning.
There are some major differences between algae and plants. The main one is that algae lack a clearly differentiated anatomical structure, including tissues, roots, stems, leaves, flowers, and fruits; all characteristics that are common in aquatic plants.
Obviously, they are plants adapted to aquatic life through certain measures: the ability to grow in a submerged environment, to tolerate being in a saline environment, resistance to waves and currents, and the maintenance of pollination by seed dispersal in an aquatic environment.
Posidonia oceanica belongs to the group of marine phanerogams as do other aquatic plants: Cymodocea nodosa , Zostera marina , Nanozostera noltii and Halophila stipulacea.
"Our" Posidonia oceanica —I say "our" because it is ultimately a species endemic to the Mediterranean, strictly confined to this sea—is made up of stems called rhizomes, which can grow horizontally (plagiotropic growth) or vertically (orthotropic growth), forming particular structures called mats . These consist of new plants in the upper part, while the lower part contains roots and rhizomes compacted by the sediment. From each rhizome, roots branch out downwards and a bundle of leaves branches upwards.
Posidonia has retained a vascular system for transporting nutrients, water, and gases, and the ability to reproduce through the use of flowers, fruits, and seeds.
Flowering occurs in autumn (September to November), while the fruits ripen over a period of six to nine months. Between May and July, they detach and float for a while on the sea surface, sometimes drifting ashore depending on the direction of the currents. The fruits are also called sea olives due to their size and dark green color.
It's not uncommon to see small balls of dead leaves, tangled together in fibers, on beaches after violent storms. These balls are called "egagropili," very light, light brown sea balls.
Posidonia oceanica meadows play a role in coastal marine environments similar to that of forests in terrestrial environments; they represent the main resource of Mediterranean coastal waters, both in terms of the surface area they occupy (20 to 50% of the seabed between 0 and 50 m deep) and, above all, for the fundamental biological role they play in maintaining coastal balance and related economic activities.
Among the main functions performed by this true "lung" of the sea we recall:
- a high production of oxygen: it is estimated that 1 m2 of prairie produces between 4 and 20 liters of oxygen in 24 hours;
- seabed stability factor: the increase in stability is linked, on the one hand, to the increase in sedimentation and, on the other, to the presence of roots and rhizomes with accentuated horizontal and vertical growth;
- coastal protection: the meadow significantly dampens the action of currents and wave motion on the coast, consequently reducing erosion or accumulation on beaches and silting up in ports;
- factor of biological and economic wealth: it has the same function as a nursery, within the prairie many organisms find refuge and protection that also use the prairie as a source of nourishment and a safe place for reproduction;
- Biological indicators: These are excellent bioindicators for assessing environmental quality; they can record numerous parameters: average water turbidity, currents and hydrodynamics, sedimentation rate, stable pollutants, decline in salinity at the mouths of coastal rivers or groundwater, stress, organic matter, and nutrients.
We have seen how Posidonia oceanica meadows constitute a complex and vulnerable system, highly susceptible to anthropogenic pressures.
The main pressure factors can be identified as follows:
- a) water pollution: urban, industrial and agricultural waste;
- b) marine works: ports, landfills, revitalization works, breakwaters;
- c) fishing, especially trawling;
- d) recreational boating: as a source of pollution and dangers to the seabed linked to the use of anchors.
Given its ecological importance, Posidonia oceanica is included as a protected species in several international and national conventions.
This is the case of the Bern Convention , on the conservation of European wildlife and natural habitats, which lists Posidonia oceanica in Appendix I together with Cymodocea nodosa and Zostera marina . These three species are listed as worthy of protection. In addition to the protection of the species, the convention provides that (Chapter II – Article 4):
- Each Contracting Party shall adopt necessary and appropriate laws and regulations to protect the habitats of species of wild fauna and flora, in particular those listed in Appendices I and II, and to safeguard natural habitats at risk of extinction.
- The Contracting Parties, in their planning and development policies, shall take into account the needs related to the conservation of protected areas referred to in the preceding paragraph, in order to avoid or minimize the deterioration of such areas.
- The Contracting Parties undertake to coordinate, where necessary, their efforts to protect the natural habitats referred to in this Article when they are located in frontier zones.
In addition to international conventions, there are also Community Directives, such as the Habitats Directive of 21 May 1992 (92/43 EEC/Natural Habitats), which provides the legal basis for the policy of conserving the natural habitats of wild flora and fauna and the conservation of biodiversity within the European Union.
The Directive includes six annexes, the first of which identifies the natural habitat types of Community interest, the conservation of which requires the designation of Special Areas of Conservation (SACs). This annex includes, among coastal habitats and halophytic vegetation, Posidonia oceanica meadows , identified as a priority habitat (no. 1120).
Article by Luca Turchetto
photo by Massimo Boyer
