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In Cala Montgó, where I have been diving, I have witnessed an estimated 40% loss of Posidonia beds, mainly as a result of unregulated anchoring by pleasure boats each summer. Where there were once lush meadows of Posidonia teaming with marine life, we now see barren sediment, often littered with trash from tourist boats.

It is a submerged marine plant (not an alga), which forms vast underwater thick meadows from 1 to 40 m depth. This plant has a subterranean rhizome from which green leaves, reaching 1m in lengh, grow slowly. The Poseidonia has green flowers and fruits who look like green olives


Posidonia flowers and produces fruit under the water surface. The fruit is free floating and is known in Italy as 'the olive of the sea' (l'oliva di mare).

Ecological importance

The Poseidonia meadows are the largest marine producers of oxygen and biomass and one of the most diverse marine communities, with more than 1000 associated species of algae, invertebrates and fishes. Besides, they control the flow of the sediments by their roots and decrease the strength of the waves by their leaves. In this way they protect non only of marine biodiversity, but also the shores against erosion. Poseidonia is very demanding in light. It can live only where the luminosity is strong, and therefore where the transparency of waters is good. Where the euthrophication is high, the water turbidity increases, the light penetration at depth reduces and the Poseidonia beds are endangered.
















About 30 years ago I was overwhelmed by the beauty and diversity found while diving Cala Montgo. One dive we even spotted Scyllarus arctus (Linnaeus, 1758). Scyllarus arctus is a species of slipper lobster which lives in the Mediterranean Sea and eastern Atlantic Ocean. It is uncommon in British and Irish waters, but a number of English-language vernacular names have been applied, including small European locust lobster, lesser slipper lobster and broad lobster.


Every dive you found a lot of Haliotis tuberculata. If found dead they where often on the back of a Sea urchin. Last time we found less Haliotis tuberculata and among the ones we found there were freak shells with deformations. And some sea urchins with a beer bottle cap attached to it.

Sardinella feeding in Posidonia beds. Picture from Gaye Rosier

Much rarer is the Pinna nobilis. Endemic to the Mediterranean Sea, this bivalve mollusc is noted for its size : over three feet in length.  It lives, planted upright on sand-banks at a depth of 3 to 30 meters, were posidoniae weeds prosper. The Posidonia only grows in clean unpolluted water and is considered by scientist an infallible testimony to a healthy marine environment. The sea grass is called the lung of the Mediterranean as it absorbs up to 16 litres of carbon dioxide per square metre/day. But the plant is already considered a threatened species requiring protection.


Syngnathus typhle, the elusive  Posidonia pipfish. Picture from Gaye Rosier



Campaign to Protect Posidonia oceanica in Cala Montgó