Posidonia (Posidonia oceanica) is a marine plant endemic to the Mediterranean. Unlike algae, it has true roots, rhizomes, leaves, flowers and fruits.
Its long, ribbon-like leaves form shoots. Beneath the sediment, rhizomes grow horizontally or vertically, allowing the plant to gradually occupy space.
When thousands of shoots develop side by side, they form a meadow, functionally comparable in some respects to a grassland or forest on land.
Posidonia can reproduce sexually by producing flowers and then fruits containing a seed. But much of meadow expansion also relies on vegetative growth through its rhizomes.
Horizontal rhizomes gradually colonise the seabed, while vertical rhizomes allow shoots to keep growing as sediment accumulates.
This progress is slow. A well-developed meadow therefore results from a long history, very different from that of annual vegetation.
This slow growth also explains its vulnerability: what is destroyed quickly may take a very long time to recover.
Yet much of the meadow lies beneath the leaves and within the sediment.
The leaves
They carry out photosynthesis and offer an immense surface area available to many organisms.
The rhizomes
They connect the shoots, enable the plant's growth and help structure the seabed.
The roots
They anchor the plant in the substrate and help stabilise sediment.
The matte
Over time, rhizomes, roots, sediment and organic matter accumulate into a compact structure called the matte.
The meadow is therefore not simply vegetation covering the seabed.
It gradually builds its own habitat.
This matte can develop over long periods.
It stabilises the seabed and also forms an important carbon reservoir. Some carbon fixed through photosynthesis can thus be buried and preserved in sediment and meadow structures.
Posidonia therefore acts simultaneously above and below the seabed.
What the diver sees is only the most visible part of a much larger biological structure.
This photosynthesis releases oxygen.
But the importance of this production extends beyond the plant itself. Matter produced by the meadow enters many food pathways, either directly or after processing and decomposition.
Dead leaves notably contribute to a complex chain involving microorganisms and detritivores.
The meadow is therefore both a producer of living matter and a driver of ecological exchanges.
Its surface may be colonised by many organisms called epiphytes: algae, bryozoans, hydrozoans and other small sessile organisms.
These organisms in turn attract a varied microfauna.
A simple leaf thus becomes a living substrate.
The closer we look, the more the landscape changes scale: the meadow contains a multitude of small, overlapping habitats.
This structure offers refuges to many animals.
Crustaceans, molluscs, echinoderms and fish use the meadow to shelter, feed or hunt.
Some species live there long term. Others merely pass through.
The meadow is therefore not a closed environment: it continually exchanges organisms and matter with rocks, sandy seabeds and open water.
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