Plankton

A jellyfish followed by a group of fish pulses rhythmically through the water above rocks which are hidden by algae.

A common sight along the Mediterranean coastline, uniting representatives of the three categories of marine organisms: plankton, nekton and  benthos.

These categories of living creatures transcend the official classification systems, by considering only the most intimate, essential relationships between organisms and their aquatic environment.

Plankton, their structure, either do not or cannot move independently of the currents and the waves.

Nekton, on the other hand, include only those animals such as fish, cetaceans and cephalopods, for whom the viscosity of the water and the strength of the currents represent no obstacle and which can swim thousands of kilometers from one point in the sea to another.

The Benthos, includes immobile animais and plants, as well as ones which moue on the ocean bottom or feed there.

The benthos includes immobile animais and plants, as well as ones which moue on the ocean bottom or feed there. This is not a rigid sub-division, and some organisms can switch categories.

For example, plankton includes the larval stages of many organisms which, in their adult forms, belong to the nektonic or benthic categories.

Summer cruising in the Mediterranean is very likely to take place on a perfectly cairn surface, through which the sun's rays penetrate easily and disappear into the blue deeps without meeting any obstacles in their way.

This liquid mass appears to be lifeless, but filter it through a thingauge net and there is a kind of soup, of varying density, made up of beings whose minute details can only
be seen under the microscope.  

The lens enlarges a universe of marvels which might have corne off a designer's drawing-board, or a out of a

glassblower's workshop, not to mention straight from the imagination of Sergio Rambaldi, creator of E.T. Shapes of astonishing elegance, geometrically perfect or strangely composite: triangles, cylinders, spheres, feathers, spirals, bells, cones and flowers, fili every dropof water as they move - wriggling, sliding or pulsing, as if driven by invisible oars.

But it would be a mistake to imagine that plankton is always so small and invisible to the naked eye.

Jellyfish, siphonophores, salps, hydrozoans, Chaetognates, molluscs and crustaceans, which are the most typical types of plankton, include large species (from a few centimeters to several meters in the case of colonial organisms).

Nor are they invisible and they always inspire a certain degree of fear in humans, often justified by their stinging defensive weapons. However, in the majority of cases, plankton is indeed small, but its importance is inversely proportional to its dimensions, because the life of these waters depends on it.

An abundance of plankton is considered one of the fundamental parameters for judging a sea's wealth and from this point of view the Mediterranean pays a heavy tribute to its internal circulation and the transparency of its waters.

Compared with other seas and a good many oceans the Mediterranean is, viewed overall, a fundamentally poor sea.

The Atlantic waters it receives are low in nutrients (mainly made of nitrogen and phosphorus) while the ones which flow out at Gibraltar are much richer.

The fact that most of the currents in the different basins described in the first chapter are very deep prevents the recycling of nutrients.

This is especially so for phosphorus, much of which is segregated in the waters furthest from the surface and therefore from the sun, once more a protagonist in the life of the Mediterranean.