There is a scene that repeats every summer on these islands' beaches. Someone reaches the shore, finds a brown band of plant material piled on the sand and pulls the same disappointed face: the beach is dirty.
It is not. What lies on the shore is the same thing that makes the water on that beach transparent, and what stops the sand from leaving with the first storm. Understanding what it is completely changes how you look at a cove.
01 What almost everyone calls seaweed
The mistake starts with the name. Those leaves get called seaweed out of habit, and that word suggests something simple, structureless, that turns up and rots. With that idea in mind, clearing them off the beach seems like common sense.
But they are not algae. They are leaves of Posidonia oceanica, a seagrass: a flowering plant with roots, stems, leaves, flowers and fruit, exactly like a land plant that had gone back to the sea.
And it is endemic to the Mediterranean, which means it grows in no other sea in the world. It occupies around 3% of the Mediterranean basin, some 38,000 square kilometres of seabed.
02 What it actually is
Underwater it forms continuous meadows covering the seabed. It lives between one and thirty-five metres deep, and that lower limit is not arbitrary: it depends on water clarity, because it needs light to live.
Its pace is the other thing worth keeping in mind. The horizontal rhizomes grow between one and six centimetres a year, and the vertical ones between 0.1 and 4. It is among the slowest-growing marine plants known.
That slowness is the key to everything else. A meadow advancing five centimetres a year takes a century to gain five metres. What is destroyed in an instant does not come back within a human lifetime.
03 Why the water is so clear
Here is the connection almost nobody makes. The turquoise, transparent water of these islands' coves, the kind that appears in every photograph, is not clear in spite of the posidonia: it is clear largely because of it.
The mechanism is simple. The meadow fixes the bottom with its rhizomes and traps fine sediment, so that sand and silt do not stay in suspension with every wave. Fewer particles in the water means more light and more clarity.
And there is a second function, on another scale: it produces oxygen and absorbs carbon dioxide at a remarkable rate, as well as stabilising sandy coasts and supporting a biodiversity that depends on it for shelter and breeding.
04 The barriers on the shore
Back to the brown band on the beach. It has a name of its own: it is called a banquette, and it forms when the dead leaves the plant sheds pile up on the shore and the waves compact them into layers.
That accumulation is not rubbish: it is a protective structure. It acts as a physical barrier that cushions winter waves and slows the loss of sand, which is exactly the problem suffered by the beaches it is removed from.
Hence removing it with machinery is counterproductive, and in several Mediterranean countries regulations restrict the use of these marine plant remains. Clearing the banquette so the beach looks clean is the quickest way to lose the beach.
05 A hundred thousand years and eight kilometres
And now to what makes these islands a special case within the Mediterranean. In 2006, south of Ibiza, a clonal colony was identified extending as far as La Savina and Es Pujols, already on Formentera.
Clonal means that all that tissue belongs to a single organism, spread by rhizome rather than by seed. The colony measures eight kilometres and is estimated at around 100,000 years old, placing it among the largest and oldest on Earth.
Put in context: that plant had been growing in that channel for tens of thousands of years when there was still no farming in Europe. And the World Heritage property declared around the Balearics includes some 55,000 hectares of this species.
06 What to do with this
First, change how you read the beach. A cove with a banquette on the shore is not a neglected cove: it is a cove that still has its defence system. The impeccably clean one, quite often, is the one that has lost its own.
Second, if you sail: do not anchor on it. An anchor tears the rhizomes out by the root, and with growth of between one and six centimetres a year, the scar left by a single anchoring takes decades to close. Along with warming water, it is one of its two main threats.
And third, the way of looking that repays the trip. When you are in the water and can see the bottom sharply several metres down, keep in mind that the sharpness has a specific cause and that it is probably right below you, in the form of green leaves. Clarity is not a property of the water: it is the result of a plant that has been there longer than almost anything else alive.
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