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Climate change

The increase in temperature of the Mediterranean seawater has an impact on the populations of marine plants and animals: the species of southern affinity are favoured at the expense of northern species.

THE TEMPERATURE OF THE SEAWATER IS INCREASING IN THE MEDITERRANEAN
Meteorological services throughout the world have long disposed of series of measurements of the air temperature, but this is not the case for the seawater temperature for which the measurements are more recent. Long series of measurements are necessary in order to provide evidence of general trends disturbed by seasonal variations.

The current warming of the waters of the Mediterranean was initially established on the basis of hydrological data for the deep waters of the north-western Mediterranean. From 1959 to 1989, the extremely stable temperature of these deep waters increased by 0.12°C in 30 years.

For the coastal waters, down to depths of about a hundred metres, measurements started later at various points along the north-western coasts of the Mediterranean (Estartit on the Spanish Catalan coast, Villefranche-sur-Mer, the islands of Hyères, Marseille). All confirmed the same trend: warming of the order of 1°C of the coastal waters over the past 30 years.

Although bathers may have difficulty in perceiving the warming of the water, they may note the increase in thermal anomalies: milder winters, summer heat waves.

These anomalies also concern the seawaters and have an impact on biodiversity. The two major abnormal periods occurred in 1999 and in 2003: the summers were particularly windless and long.

Habitually, on the coasts of Provence, the end of summer is marked by numerous violent gales, generally from a northerly direction, such as the Mistral; they cause a mixing of the warm surface waters with colder waters situated below the thermocline.

In 1999, as in 2003, the surface water temperature remained abnormally high for a long period and was the cause of massive mortality of benthic invertebrates.

SOME LIKE IT… AND SOME DON’T
Six million years ago, the play of movements of tectonic plates caused the collision between the African plate and the Eurasian plate and led to the closing of the Mediterranean at Gibraltar. The Mediterranean then dried up, until the threshold at Gibraltar broke open and the waters of the Atlantic again filled it up.

Thus, the living organisms that populated the Mediterranean are naturally of Atlantic origin. They represent today 50% of the marine fauna, with the addition of almost 18% of cosmopolitan species.

In the course of the succeeding millennia, certain species became differentiated and became specific to the Mediterranean: they are called endemic species, and they represent 18% of the population.

All these animal and plant species adapted or were distributed geographically according to their affinities with regard to the environmental conditions and in particular according to the water temperature :

those that like the warmth of the eastern basin or the south of the western basin;
those that prefer the cooler waters of the north of the western basin or the northern Adriatic;
those for which the temperature is not a determining factor are found throughout the Mediterranean.

With the current warming of the waters of the Mediterranean, these adaptations are occurring before our eyes. Now that the temperature there suits them, certain species of southern affinity are extending their geographical range towards the north: the ornate wrassse Thalassoma pavo, the trigger fish Balistes carolinensis, the barracuda Sphyraena viridensis and the dolphin fish Coryphaena hippurus among the fishes, for example.

Others, such as the grouper Epinephelus marginatus, which already occurred in northern waters, now find conditions suitable to reproduce there. In contrast, species of northern affinity such as the sprat Sprattus sprattus, blocked in the northern Mediterranean, have no escape and their population is declining.

This tropicalisation does not only affect the fishes. All the animal or plant phyla are concerned. These changes are often discreet and are only detectable by specialists; however, they may sometimes be spectacular, as in the case of massive mortality of invertebrates, and in particular of the gorgonians, in 1999 and 2003.

Consult
Links to websites:

- Laboratory of the littoral marine environment: www.unice.fr