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The Development of Knowledge of the Ligurian Sea

1.1. The first naturalists on the shores of the Ligurian Sea

The mild winter climate on the shores of the Mediterranean attracted not only the European aristocracy but also wealthy naturalists who discovered a host of unknown living creatures under the blue surface of the sea. In the 17th Century, learned societie s became passionate about this world of animals without vertebrae. A few distant expeditions allowed some scientists to collect, draw or describe many marine and terrestrial species. However, many people explored the marine world only with the help of local fishermen.

Lazzaro Spallanzani, a professor in Bologna, but a frequent traveler, traveled the coasts of the Riviera of the Levant and Ponant to study marine animals and illustrate his lectures. In the 1780s, he even set up a small laboratory in Porto Venere, near La Spezia, which is considered the first marine station [TOD 97]. The Earl of Marsilli made an inventory of marine animals, and measured the temperature and density of the water. The report he made to the Paris Academy of Sciences, of which he was a corresponding member, opened the way for discoveries by naturalists who would frequent the shores of the Ligurian Sea in the 19th Century and even constitutes the outline of an "oceanography" [MAR 25].

Among the naturalists who explored the Ligurian Sea, it is worth noting Péron and Lesueur, who described and named some jellyfish, including Pelagia noctiluca [PER 10]. Antoine Risso collected, described and detailed many marine fish species [RIS 13], nudibranchs as well as crustaceans, and attracted European naturalists to Nice and Villefranche. Some, such as Owens, Edwards, Vogt, Haeckel, Leuckart, Gegenbaur and Fol stayed for a few days or months.

The establishment of marine stations on the seaside at the end of the 19th Century satisfied the interest of zoologists for marine fauna, acalephs or other unknown zoophytes. In 1882, a laboratory of the École des Hautes Études was set up in Villefranche-sur-Mer by Jules Barrois and Hermann Fol. Alexis Korotneff and many Europeans came to study planktonic and benthic organisms. The anatomy and embryology of marine species was developed under these conditions. Some species even become biological "models" such as sea urchins and their larvae. The laboratory became the Russian zoological station, on the model of the Naples zoological station, under the direction of A. Korotneff.

On their part, the zoologists of the University of Genoa, attracted by the sea, in particular Raphael Issel and Alexandre Briand, created a marine laboratory finally located in Quarto dei Mille [REL 00a].

The marine laboratory, an essential tool for biologists, even became a laboratory for other disciplines. Fol and his colleague E. Sarasin, a physicist in Geneva, came to Villefranche in 1884 to study the penetration of light into the sea, as Forel had done in Lake Geneva. In 1903, Idrac installed a device in the Bay of Villefranche that would give the first recordings of the water temperature variation under the effect of the Mistral wind. In the 1960s, other disciplines, physicists and chemists came to work on the Ligurian Sea.

Following the round-the-world voyage of the English ship, HMS Challenger, designed to search for animals capable of living in the deep sea, A. Milne-Edwards and other zoologists, including A.F. Marion, a teacher at the Faculty of Science in Marseille, organized a deep-dredging mission in the Mediterranean. In 1881, they dredged at different points on a line from Villefranche-sur-Mer to Ajaccio. They noted the poverty of the macrofauna at the bottom of the Ligurian Sea and diverted their exploration towards the Atlantic. It was not until bathyscaphe dives of the 1950s [PER 64] and then 1990 that the exploration of life on the bottom was again addressed, and cores in deep sediments were taken [GUI 02] that it was quantified. Macrobenthos are rare, but sediments are populated by meiobenthic species.

In fact, oceanography, a multidisciplinary study of the sea, became a reality in the Mediterranean owing to Prince Albert I of Monaco, who, from 1886 onwards, began missions on a specially equipped ship, had original devices manufactured for physics and chemistry, for the capture of pelagic or benthic animals, and invited chemists, physicists, zoologists or physiologists to participate in his expeditions. Finally, he created a museum in Monaco to house his collections, which would become a laboratory for the study of the Sea of Monaco. He organized conferences at the Sorbonne and set up an Institute in Paris for public education, oceanographic research and teaching. In addition, Prince Albert I favored international action to produce a world map of the ocean floor and encouraged the formation of an International Commission for the Scientific Exploration of the Mediterranean Sea, similar to the International Commission for the Exploitation of the Sea created in Copenhagen by the countries of Northern Europe (ICES). While he favored the Atlantic and Arctic for his summer expeditions, he commissioned the director, Jules Richard, of the Oceanographic Museum, to launch a research program on the sea off Monaco.

The strategy of regular measurements and observations without a priori, as meteorologists did at the time, was introduced in 1896 at the Russian Zoological Station in Villefranche-sur-Mer (seawater temperature and presence of macroplanktonic species). This series of observations was interrupted in 1917 by the revolution in Russia.

In 1909, the prince of Monaco assigned the Eider, which was equipped with a winch, to a complete study of the sea off Monaco. The museum's scientists regularly carried out hydrological measurements, water and plankton sampling on a transect extending offshore to 1000 m depths. Unfortunately, this time series was interrupted by World War I. The variations in hydrology (temperature and salinity) from surface to bottom suggested coastal mixing in winter. The study of nanoplankton revealed the existence of different species of coccolithophores in Mediterranean waters [BER 39], and other curious species of phytoplankton, alongside diatoms and dinoflagellates [PAV 37], and temporal variation in zooplankton. Monitoring of the sea was resumed in Monaco and the ship Winnaretta Singer allowed more distant studies in the Ligurian Sea.

The Monaco site and the Bay of Villefranche became privileged coastal water monitoring sites. Halim [HAL 60] described the annual cycle of dinoflagellates in the bay and, to explain it, introduced a strategy of regular measurements of temperature and salinity at different depths during the year, following the example of the Monaco series. He once again showed the importance of vertical hydrological structure by sampling the deep water off Cap de Nice.

Weekly measurements at a permanent station in the Bay of Villefranche (Point B) were carried out from 1957 by P. Bougis. This long series, subsequently enriched by additional variables (zooplankton, nutrients, pH), became a basis for the study of long-term variations in coastal waters due to variations in hydrodynamic and climatic forcings in the Ligurian Sea.

The temperature in deep Mediterranean waters (12.4-13°C) was measured as early as the end of the 19th Century [NIE 12]. The values much higher than those encountered in the oceans at the same depths have intrigued oceanographers, but the mechanisms responsible were only demonstrated in the 1960s.

1.2. Vertical structure of the Mediterranean and hydrodynamics

During the dredging of the Travailleur in 1881, A. Milne-Edwards, considering the temperature measurements in the deep layers, noted that "sa constance parfaite fait supposer qu'il y a donc une nappe d'eau presque immobile en profondeur"1. However, the measurement method was not perfect with a conventional thermometer. Aimé [AIM 45] proposed using a "reversing thermometer" that he used in Algiers and Toulon. This type of thermometer then replaced the ordinary Saussure thermometer in all shipments until the appearance of thermal probes (thermistors).

The Thor mission in the Mediterranean (1908-1910) described the spatial characteristics of the Mediterranean in summer and winter in its two basins. The vertical, three-layer hydrological structure, superficial, intermediate and deep, has been recognized [NIE 12]. Net phytoplankton, zooplankton, mesopelagic fish and fish larvae were also collected during these campaigns. However, the crossing of the Ligurian Sea in a few transects with stations too far apart does not reveal the typical spatial structures that would be discovered later.

The contrast between the high seasonal variability of surface water and the stability of intermediate and deep waters, represented on a TS (Temperature-Salinity) diagram, leads Tchernia [TCH 60] to consider them as "typical waters" with reference value. To characterize deep water in the Mediterranean, Lacombe and Tchernia [LAC 58] compiled an inventory of existing measures. A transect South of Toulon, and stations in the Ligurian Sea, were completed by a few stations in the South of the Western basin, in the Tyrrhenian and in the Eastern basin. They highlighted three "deep waters types", kinds of references (Crete Sea, Eastern Mediterranean, Western Mediterranean) and suggested that the Mediterranean basins no longer communicate with each other at depths greater than 2000 m.

While there has been a...