Ecology: A changing scientific discipline

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: