Caratteristiche oceanografiche ed ecologiche del Mediterraneo

The connection of the entire Mediterranean with the Atlantic Ocean is provided only by the Strait of Gibraltar, which limits exchanges only to the surface layer with an inflow of 1.75 Sv (1 Sv = 1 million m3/s) and an outgoing 1.68 Sv, due to the its depth of 330 m. From a meteorological point of view, the climate on the Mediterranean is closely linked to the North Atlantic baric cell. The variability of this cell is defined by a dimensionless index called «index NAO» (North Atlantic Oscillation), based on the difference of pressure between the minimum of the South Island and the maximum of the Azores High. This value has been calculated on time series data from 1865 to present and assumes positive and negative values cyclically. The circulation of the waters in the Mediterranean has four key points for its dynamics at all depths providing a significant activity: the Gulf of Lions in the western basin, the lower Adriatic, the Aegean Sea north of Crete and the Levantine Sea near Rhodes in the eastern Mediterranean, which is dynamically more active than the western The plankton community of the Mediterranean depends on hydrology and the long evolutionary history of this sea. The lack of a true deep planktonic fauna is the main feature that differentiates the Mediterranean from Oceans. Moreover, the current shape of the Straits of Gibraltar, resulting in significant hydrological differences between the Atlantic and the Mediterranean, enables the adaptation of the Atlantic species to the Mediterranean, so that their spread (Centropages chierchiae, Calanoides carinatus, Pleuromamma borealis, Psudocalanus elongatus, etc..) has been related only to the waters of the Alboran Sea. Although the opening of the Suez Canal in 1869 - an event that has put in communication the Mediterranean with the Indian Ocean - the topographical and hydrological obstacles allowed only few equivalent species (Arietellus pavoninus, Calanopia elliptica, Labidocera madurae, Paraeuchaeta concinna, etc.) to spread into the eastern Mediterranean. A particular zooplankton community belongs to the northern Adriatic Sea, as the neritic community is characterized by the presence of two species of copepod, Pseudocalanus elongatus and Temora longicornis. These two species represent relict populations, confined to the area affected by the waters of the Po River, resulting in low salinity. In summer, these two species show the annual peak of abundance and, following the flow of the waters of the Po, spread both into the north and south of the Adriatic basin. In theory, for the Atlantic planktonic specimens of surface and subsurface, the Straits of Gibraltar should not be a barrier to their spread into the Mediterranean. Many deep-water species may not be hindered in this step because of their migratory behavior (nictemeral, seasonal and ontogenetic vertical migration). So, a large number of species «Atlantic water indicators» have been indicated, whose presence in the Mediterranean allows to identify the Atlantic current. Atlantic water indicator species have been reported throughout the Mediterranean basin, even in areas more distant from Gibraltar, as the Levantine Basin. Good examples of indicator species can be found in many zooplanktonic groups such as chaetognathes, copepods, jellyfish, pteropods, and euphausiids. Among the copepods Acartia tonsa, Temora longicornis and Centropages chierchiae are potential indicators of Atlantic waters. Among euphausiids Thysanoessa gregaria of Atlantic water and Euphausia brevis and Stylocheiron suhmi of Levantine waters. Compared to the neighboring Atlantic and Indian Oceans, the Mediterranean Sea is commonly known as a relatively oligotrophic sea and therefore has low amounts of plankton. The oligotrophy is mainly due to very low concentration of inorganic phosphate, which limits primary production by phytoplankton. Another feature is the west-east decreasing gradient of concentration of chlorophyll, recorded with the technique of remote monitoring and in situ data. Integrated Chl-a concentration has shown a decline along the east-west geographical gradient by a factor of 7 (from 0.48 to 0.07 mg C m-3). In the Mediterranean, low values of Chl-a (less than 0.2 g Chl a l-1) were recorded over large areas, with the exception of high values observed during late winter-early spring in the Ligurian-Provençal region. In addition, high values of phytoplankton, although limited spatially, were observed in the Alboran Sea, in front of the North Balearic Catalan, as well as along the coast near the mouths of large rivers (Northern Adriatic) or areas with wide continental shelves (North Aegean). In the Pelagic system of the Ligurian Sea, the highest values were recorded during late winter-early spring. In the eastern Mediterranean, the minimum and maximum values are in the range between 0.003 and 0.5 g Chl-a l-1. These characteristics are reflected in the structure and dynamics of planktonic communities. The species composition and the zooplankton epipelagic biomass did not differ significantly between day and night, due to the absence, in the Mediterranean, of species defined strong vertical migrant, if compared to the Atlantic Ocean. In the western part, the Alboran Sea is very productive for the presence of a cyclonic vortex, leading to «upwelling» of rich nutrients. The high primary productivity results in the large zooplankton biomass, and the values, with peaks up to 200 mg m-3, are among the highest observed in the Mediterranean. It should be remembered that the upwelling phenomenon of waters, rare in the Mediterranean, occurs not only in the Alboran Sea, but even in the Ligurian Sea, the Gulf of Lion and the Straits of Messina. These areas are highly productive, with a very strong fish production. Generally, the strong differences in plankton production in the different subregions of the Mediterranean are not related to the composition of plankton species. In the Mediterranean, the community structure is characterized by a substantial uniformity among the different basins, while significant differences are evident between the neritic and pelagic systems. The zooplankton communities of the Mediterranean neritic ecosystem consist of few species that alternate in dominance during the seasonal succession. The copepods are the most important part of the population for most of the year. The main feature is the presence of neritic few opportunistic species of copepods, as Acartia clausi and Paracalanus parvus and cladocerans as Penilia avirostris and Evadne spinifera, able, through their reproductive strategies, to use more efficiently the large primary production of spring/summer period. Other copepod species of neritic community have very different annual cycles in relation to their reproductive strategies, characterized by low biotic potential. The seasonal pattern of these species depends mainly on their biological cycle that is unrelated to the maximum primary productivity, since most of them (Temora stylifera, Centropages typicus, Ctenocalanus vanus, Clausocalanus furcatus, C. arcuicorni) have omnivore food habits. Another quantitatively important zooplanktonic group is formed by the appendicularians. Their species are distributed throughout the year alternating in dominance. The genus Fritillaria is more abundant in winter with F. pellucida and F. borealis, while Oikopleura genus, as O. longicauda, is dominant during the spring and summer. In spring big swarms of gelatinous species like Thaliaceae can be seen, consisting of Thalia democratica and Doliolum nationalis. Among the predators of zooplankton, the chetognates, Sagitta setosa, S. enflata and S. minima, represent the largest group, followed by siphonophores and scyphomedusae. The pelagic Mediterranean ecosystem is characterized by a zooplanktonic community of high biodiversity and lower seasonal variations in abundance and species composition. The zooplanktonic pelagic Mediterranean community presents a vertical distribution pattern in which three zones can be identified: epipelagic 0-100 m; mesopelagic 100-600 m; deep below 600 m deep. The surface community is composed of a large number of pelagic species, to which several dominant species of the neritic ecosystem are added: during the spring-summer period of maximum abundance, they spread in the pelagic zone, leading to the seasonal qualitative and quantitative variations of the epipelagic community structure. In the Mediterranean, the vertical distribution of zooplankton biomass is characterized by a sharp decrease from the surface to the bottom. In the Tyrrhenian Sea, for example, below 1000 m deep, biomass values of 0.3 mg m-3, expressed as wet weight, have been recorded. Low values of zooplankton biomass observed in the deep waters of the Mediterranean are related to low production of oligotrophic surface waters and to the consequent reduced organic matter, transferred to the upper layers. The distribution of planktonic species, both on the surface into the deep system, shows a clear uniformity in all the Mediterranean basin. The Strait of Messina (80 m deep) and the Channel of Sicily (300 m deep) are not a barrier to the passage of mid-water species between the western and the eastern parts, because the upper limit of the vertical distribution of these exceeds 300 m deep. The dominant species are the same in the western as in the eastern sector. Moreover, the species vertical distribution pattern shows significant similarity in both eastern and western basins. The Mediterranean basin extends in a latitudinal range characterized by subtemperate environments (Gulf of Lion and Northern Adriatic Sea) to subtropical (northern coast of Africa and the Middle-East). During the year, the wide fluctuations in temperature values lead to a clear seasonal succession in hydrological and biological processes of the entire basin. The marine meteorological phenomena are the main forcings that drive the dynamics of plankton communities. As part of the seasonal succession, the abundance of zooplankton in pelagic ecosystem fluctuates within a narrow range, unlike the neritic ecosystem which is characterized by larger fluctuations. The zooplankton biomass peak was recorded during February, March and May in the Balearic Sea and in April in the Tyrrhenian Sea. The annual cycle of zooplankton along the neritic-pelagic gradient has the same temporal pattern. In winter, mainly in the northernmost regions of the Mediterranean, the cooling air causes a sharp decrease in temperature of surface waters, resulting in an increase of their density, which determines their sinking and ascent of deep water, warmer and richer in nutrients. During this period, other limiting factors, such as the limited amount of sunlight, low temperature and strong hydrodynamism, do not allow a high primary productivity. Scarce Phytoplanktonic resources are available for zooplankton, so that none becomes dominant and the zooplankton community is characterized by high values of indices of species richness and diversity. In spring, with the daily increase of light and temperature, combined with the wealth of nutrients derived from winter water column homothermy, there is an explosion of phytoplankton abundance, characterized by the dominance of diatoms. After three or four weeks from phytoplankton explosion, mainly in neritic ecosystem an increase of herbivorous zooplankton abundance can be observed, dominated by opportunistic species of copepods such as Acartia clausi and Paracalanus parvus. In summer, the temperature increasing leads to the establishment of a thermocline, resulting in vertical stratification of water masses, which does not allow the supply of nutrients from the bottom. The high consumption of nitrate by diatoms and the summer stratification, determines the disappearance of nitrate, so in summer the nitrogen is as ammonia or urea of metabolic origin. This phenomenon results in a significant change in the phytoplankton community, with the establishment of the dominance of small flagellates, which are able to take advantage of the nitrogen in ammonia form. The change in phytoplankton community structure affects the zooplankton, with the dominance of cladoceran species and micro-filter feedings on copepods, meso-filter feedings.