The Physical Environment

Geology/geomorphology

The western Mediterranean developed as a result of the convergence of tectonic plates. The Liguro-Provençal Basin opened up as a result of the rotation of the Corso-Sardinian Block. The morphology of the Sanctuary thus includes island areas.

The seabed of the Sanctuary was formed gradually as a result of large-scale folds and thrust faults in the Alpine orogeny, subsidence phenomena that occurred because of extensions in the Neogene period, the morphogenesis of sea-level oscillations in the Quaternary period, atmospheric, marine and fluvial erosion and recent and on-going sedimentation.

These different geomorphological phenomena explain the existence of mountainous areas, plains that extend into rocky coastlines of varying steepness and low coastal sections, often accompanied by lagoons and wetlands. Apart from the plains on the eastern side of Corsica and the Tuscan coast, the coastlines that run along the perimeter of the Sanctuary are rocky and often very steep. Most of the water regimes are torrential, consisting of steep and often interdependent catchment areas. The mouths of the rivers are generally small, with stony sediments.

Underwater topography

The Sanctuary area is characterized by markedly heterogeneous topography and hydrodynamic diversity.

The Liguro-Provençal Basin and the area adjacent to it alongside Corsica and across the Sea of Sardinia lie on a continental platform with a very small border that lies between 2.5 and 10 nautical miles from the coast.

Deep underwater canyons run along paleofluvial channels and tectonic fractures at a depth of over 2000 m in places.

The continental shelf broadens from west to east and is very steep in the Provence region and along Corsica’s western coast, becoming wider in the eastern section and extending as far as 25 km from the Tuscany coasts.

The deepest seabeds are located to the east of the meridian, 5°30' E, and can reach 2,700 m. The northern Tyrrhenian Sea is less deep (reaching a maximum depth of around 1,700 m). It is separated from the Ligurian Sea by the Strait of Bonifacio and the Gulf of Follonica - island of Elba. Its large continental shelf, from which isolated underwater elevations arise, links the two parts.

Climate

The climate of the Sanctuary area is Mediterranean, with hot, dry summers – a result of the Azores High – mild, relatively rainy winters. Local winds vary in direction and intensity, reaching a peak in winter: the Liguro-Provençal Basin is affected by the mistral and tramontane from the N-NW, the northern Tyrrhenian Sea by N-NE winds and the sirocco from the S-SE, and the east coast of Corsica is impacted, to a lesser degree, by the libeccio from the S-SO.

 

Hydrology and Currentology

The water balance of the Mediterranean is negative: high levels of evaporation are not offset by precipitation and water supplied by catchment areas. This negative balance is therefore partly offset by the inflow of Atlantic waters that is higher than the outflow of Mediterranean waters.

The Mediterranean’s northwestern water masses circulate along the continental slope in a cyclonic circuit induced by the density gradients of three different water zones:

  • a surface zone up to 300-400 m deep, composed of Atlantic waters and additional freshwater from rain and rivers,
  • an intermediate zone between 200 and 500 m deep, which is denser, richer in nutrients and relatively warm,
  • more homogenous deep water, with a near-constant annual temperature of 12.7°C. Wind-induced water exchanges are more intense in winter when subsurface waters rise to the surface in upwellings.

The intermediate waters also form part of this current. They flow into the Tyrrhenian Sea and divide in order to cross the Canal of Corsica, passing west beyond Sardinia and Corsica. By the Cap Corse, there is a second current from the eastern Mediterranean. The two currents meet to the north of Corsica, forming the Liguro-Provençal current that flows up to the north towards the Gulf of Genoa before turning to the west along the coast towards Spain. As a result of cyclonic flow, the current gives the Liguro-Provençal Basin its dome-shaped point of divergence and frontal zone structures, which are both significant in terms of their biological productivity.

The intermediate waters also form part of this current. They flow into the Tyrrhenian Sea and divide in order to cross the Canal of Corsica, passing west beyond Sardinia and Corsica. By the Cap Corse, there is a second current from the eastern Mediterranean. The two currents meet to the north of Corsica, forming the Liguro-Provençal current that flows up to the north towards the Gulf of Genoa before turning to the west along the coast towards Spain. As a result of cyclonic flow, the current gives the Liguro-Provençal Basin its dome-shaped point of divergence and frontal zone structures, which are both significant in terms of their biological productivity.

Oceanographic processes and structures

For management purposes, different categories of zones were identified:

  • static bathymetry (canyons),
  • stable hydrographic phenomena,
  • ephemeral and mobile hydrographic phenomena (upwellings, frontal systems).

Biological productivity is high, particularly in areas where there is upwelling and in frontal zones, where plankton-eating marine mammals congregate.

The Sanctuary is also home to a large number of the different marine and coastal habitats of the Mediterranean. Numerous studies have sought to distinguish cetaceans’ habitats by classifying the temperature of surface waters in which different species live, and the different water masses present, depths and topographic structure, hydrological phenomena including upwellings and fronts or the combination of multiple spatial-temporal factors. According to such findings, preferred habitats seem to be the continental shelf and pelagic zone; the presence of marine mammals is often determined by the distribution of the prey they feed on.

 

Ecology: A changing scientific discipline

L'Ecology is the scientific study of living things, their environment and how they interact.

The term ‘ecology’ comes from the Greek oikos (‘house’ or ‘habitat’) and logos (‘study of’ or ‘knowledge’): it is the study of habitat. It was invented in 1866 by Ernst Haeckel, a German Darwinian biologist. In his work A General Morphology of Organisms, he used the term to refer to: ‘the science of the relationship between organisms and the world around them, in a broader sense, then, the science of the conditions of life.’

Food Cycles

In the Sanctuary area, optimal usage of productive potential is apparent from the existence of multiple links between the ‘classical’ food web (of the ‘diatoms - copepods – carnivores’ type) and the ‘microbial loop’ that exploits the full range of particle sizes. Of particular interest is the fact that the microbial network rapidly recycles organic matter in the euphotic zone and that gelatinous macro-zooplankton form an important part of the food web within the water column. This pronounced branching in the food web optimizes the times in the annual cycle in which primary production is minimal.

Primary production

The Sanctuary area is characterized by relatively high mesotrophic productivity, with peaks in primary production capable of surpassing 500 gC.m-2.yr-1 in frontal zones in summer. This productivity is caused by a range of fertilization mechanisms that raise the level of primary production, namely enrichment from coastal waters and below-surface water intake, the delayed effect of winter mixing bringing nutrients to the surface, the frontal zone separating coastal waters involved in cyclonic flow and open water, local ‘upwelling’ phenomena and associated mesoscale whirlwinds which can be 100 km in diameter. This productivity also favors the existence of complex structures that combine divergent and convergent features.

These fertilization mechanisms are all the more significant that the thermal flux of the water is relatively high in the Sanctuary area.

The dome-shaped structure that spreads out from the Liguro-Provençal Basin – favoring rises in intermediate water zones, rich in minerals (the majority of which are brought in by rivers) and the displacement of lighter and less salty water to peripheral areas – is the reason for the higher productivity compared with coastal locations. The winter cooling of surface zones leads to vertical mixing, which enables phytoplankton, particularly diatoms, to flourish, especially in summer when the chlorophyll content of surface waters approaches 2mg.m-3.

There is a high-density frontal zone where the oligotrophic coastal waters and the richer central part come together. The Liguro-Provençal front, which extends to around 20 nautical miles from Corsica’s western coast and the Italian Riviera and around 15 to 25 nautical miles from the Côte d'Azur, is relatively productive, with a peripheral increase in phytoplankton in spring and autumn, as a result of chlorophyll content levels reaching as much as 10mg.m-3.

These phenomena are key to the structure of the higher levels of the food chain, particularly for tertiary consumers – the migrating fish and cetaceans that are especially abundant in summer.

Secondary production

Secondary production covers a wide range of species from the following categories: zooplankton, cephalopods and fish.

Zooplankton is composed of herbivores (crustaceans, tunicates, mollusks), carnivores (cnidaria, ctenophora, polychaetes, mollusks, crustaceans, coelenterates and chaetognaths) and the eggs of crustaceans, fish and pelagic mollusks. As zooplankton can move only horizontally, its distribution and abundance is conditioned by spatial-temporal variations in primary production.

 

Zooplankton feeds on phytoplankton. Nycthemeral vertical migration of zooplankton often conditions nocturnal feeding activities among predators – Surface zones can reach values some eleven times higher than those of the day at times, as is the case for micronekton in the summer period. Macro-zooplankton, particularly tunicates, appendicularians and jellyfish, play a key role in their capacity as filters. Some cetaceans, particularly Mysticeti, feed primarily on krill. Other families of crustaceans are part of the diet, including glass shrimp, sergestidae and oplophorous shrimp. The main krill species, Meganyctiphanes norvegica, is of northern origin. It is exceptionally abundant in the center of the Liguro-Provençal Basin (up to 900 ind./1000m3 in the surface zone on summer nights). Similarly, an abundant glass shrimp is the Pasiphaea sivado species. The other species recorded are from temperate or northern areas: Stylocheiron longicorn, Euphausia krohnii, Nemastoscelis megalops and Nyctiphanes couchii.

Gelatinous macro-zooplankton (salps, appendicularians, jellyfish etc.) represent an important feature of the food chain within the water column. Fauna of this kind – ‘gelatinous’ carnivores’ – help to make the food chain more efficient, ensuring that organic matter is brought to the seabed by producing mucus, which is a vital feature of the food chain, linking nano-plankton and large predators.

Cephalopods play a major role in the food chain – they are the prey of a number of fish, marine mammals and seabirds. In addition to seasonal migration as a result of reproduction, the majority undertakes nycthemeral vertical migration, living close to the seabed, at the slope/deep ocean level, returning to the surface at night to hunt their prey (small fish, other cephalopods, crustaceans, krill, siphonophora, polychaetes and mollusks). Other species remain in the benthic zone, like the majority of sepiolidae and octopodidae that are usually found only on the continental shelf. These cephalopods play an important role, transferring energy to lower levels, not only benefiting cetaceans but also benthic scavenger species. .

 

The cephalopods consumed by cetaceans are distributed as follows:

  • benthic and semi-benthic species that live on the continental shelf and upper shelf serve as prey to striped and Risso’s dolphins,  
  • pelagic species that serve as prey to the cetaceans found mainly on the lower slope, such as long-finned pilot whales and Cuvier’s beaked whales,  
  • species from all pelagic areas as well as benthic and semi-benthic species found mainly on the slope and in the deep ocean and generally consumed only by common, blue, white and Risso’s dolphins, long-finned pilot whales, Cuvier’s beaked whales and species of pelagic squid consumed by small oceanic dolphins, sperm whales and Cuvier’s beaked whales.

The fish species most frequently consumed by cetaceans are Engraulis encrasicolus anchovies, Spratus spratus sprats and Sardinus pilchardus sardines, as well as, to a lesser degree, Sardinella aurita sardinelle, Trisopterus minutus capelins, Merluccius merluccius hake, Trachurus mediterraneaus Mediterranean horse mackerel, Scomber scombrus mackerel, Boops boops bogues and Belone belone garfish. These species are also consumed by large pelagic fish (tuna and swordfish, for example), seabirds and humans (fishing activities).

 

 

To summarise, the interactions can be illustrated as follows: