1.10 The productivity of Mediterranean sea

In addition to salts, there are innumerable dissolved substances and particles suspended in water, including sediments, organic debris and microscopic organisms. Their concentration can also reduce the transparency of the water.

Among these substances, a fundamental role is played by some inorganic compounds essential for the plants life: the nutrients. The main nutrients are nitrogen and phosphorus. Nutrients are only partially formed in the sea following the decomposition of animals and plants; instead a great input comes from the river contribution.

The enrichment of nutrients, mainly nitrogen and phosphorus, in coastal and lagoon waters due to both natural and anthropogenic causes is called eutrophication. Eutrophication stimulates primary production leading to an increase in vegetal biomass.

The Mediterranean is generally a poor and poorly productive (oligotrophic) sea, with some exceptions (the high Adriatic) (Fig. 1_SES1.10).

The Adriatic Sea produces 50 % of the fish species fished for feeding of the whole Mediterranean because it is rich in nutrients brought by the rivers (Fig. 2_SES1.10).

The northern Adriatic Sea is rich in nutrient, but there is some difference between for example the Italian e the Croatian side. The seawater colour of the Italian side is usually green (Fig. 3_SES1.10), while it is blue in the Croatian side. This is due not only to a different kind of sediment but also by the nutrients carried by the rivers and the micro algal blooms.

Normally, all organic matter present in the sea, animals and plants, once dead, falls to the sea floor where it is transformed into inorganic substance thanks to the work of particular bacteria, called decomposers.

In the Mediterranean, due to deep water currents, most of this inorganic substance is lost, it travels from the Gibraltar Strait to the Atlantic Ocean. This loss of nutrients is one of the reason for the lower number of marine species in the Mediterranean than in other more productive seas.

1.10.1 Natural causes of the presence of nutrients in the sea

There are innumerable dissolved substances and particles suspended in water, including salt, sediments, organic debris and microscopic organisms. Their concentration can also reduce the transparency of the water.
Among these substances, a fundamental role is played by some inorganic compounds essential for the plants life: the nutrients. The main nutrients are nitrogen and phosphorus. Nutrients are only partially formed in the sea following the decomposition of animals and plants; instead a great input comes from the river contribution.
The enrichment of nutrients, mainly nitrogen and phosphorus, in coastal and lagoon waters due to both natural and anthropogenic causes is called eutrophication. Eutrophication stimulates primary production leading to an increase in vegetal biomass.
The Mediterranean is generally a poor and poorly productive (oligotrophic) sea.
The Adriatic Sea produces 50 % of the fish species fished for feeding of the whole Mediterranean because it is rich in nutrients brought by the rivers.
The northern Adriatic Sea is rich in nutrient, but there is some difference between, for example, the Italian and the Croatian side. The seawater colour of the Italian side is usually green while it is blue in the Croatian side. This is due not only to a different kind of sediment but also by the nutrients carried by the rivers and the micro algal blooms.
Normally, all organic matter present in the sea, animals and plants, once dead, falls to the sea floor where it is transformed into inorganic substance thanks to the work of particular bacteria, called decomposers.
In the Mediterranean, due to deep water currents, most of this inorganic substance is lost, it travels from the Gibraltar Strait to the Atlantic Ocean. This loss of nutrients is one of the reason for the lower number of marine species in the Mediterranean than in other more productive seas.

1.10.2 Anthropogenic causes of the presence of nutrients in the sea

The word eutrophication, derives from the Greek "eutropia" and indicates a condition of nutritive richness in a given environment, in particular an overabundance of nitrates and phosphates in an aquatic environment.
The term is also used to indicate nutrient pollution that is precisely that process in which an exaggerated number of nutrients, in particular nitrogen and phosphorus, are added to the aquatic ecosystem and can act as fertilizers, causing excessive proliferation of algae. As a result, global oxygen consumption increases, and the lack of oxygen causes fish death over time.
This phenomenon has been recognized as a problem of pollution both in Europe and in North America during the mid-twentieth century and from that moment onwards it developed actively.
An exaggerated release of wastewater into the aquatic ecosystem has determined, inexorably, in many parts of the planet and mainly in developing countries, the release of a very high amount of nutrients that stimulates the disproportionate growth of algae. In industrialized countries, however, wastewater can be flooded directly into water bodies in an outrageously abusive way.
To counteract eutrophication, interventions are needed to reduce nutrient inflows to water bodies (reduction of fertilizers in agriculture, purification of civil and industrial waste, treatment of crop wastewater through sequestering agents and phytodepuration plants). Global warming is expected to contribute to worsening the phenomenon of eutrophication. The heating of surface waters, in fact, decreases the solubility of gases (and therefore also of oxygen).

1.10.3 Eutrophication: the consequences of this phenomenon.

Eutrophication is a process in which an aquatic environment changes its ecological balance to enrich itself with the nutrients that are scarce; it's now used to indicate the excessive growth of plant organisms and for the doses of nutrient abundance.
The accumulation of elements such as nitrogen and phosphorus, causes the proliferation of microscopic algae which determine a greater bacterial activity and the death of fish in the long run. This phenomenon has been recognized as a problem of pollution.
These structural changes depend mainly on 3 factors: use of fertilizers, discharge of wastewater into water bodies, reducing self-purifying capacity.
Eutrophication occurs mainly in the Adriatic Sea, in the Gulf of Lyon and in the northern Aegean. The perturbation of the aquatic balance can be more or less evident according to the enrichment of the water in nutrients.
The main effects caused by eutrophication are:
1. Abundance of particulate substances, like phytoplankton, that determine the turbidity and color of the water;
2. Abundance of inorganic chemical compounds that in the potabilization plants they induce the formation of harmful substances, for example, the nitrosamines suspected of mutagenicity;
3. Abundance of organic substances that impart odors and tastes unpleasant to water, barely disguised by chlorination in the case of drinking;
4. Disappearance or sharp decline of valuable fish with highly negative effects on fishing;
5. Interdiction to the tourist use of the lake and to bathing. Bathing is dangerous because some algae cause skin irritation;
6. Reduction of oxygen concentration, particularly in the deeper layers of the lake at the end of summer and in the autumn period.
In light of these heavy repercussions and the consequent serious economic and naturalistic damage, the need to curb the progress of eutrophication, avoiding the collapse of the ecosystems affected by the phenomenon, appears evident.

1.10.4 The rivers that flow in the Adriatic Sea.

The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan one. The countries with coasts on the Adriatic are Albania, Bosnia and Herzegovina, Croatia, Italy, Montenegro and Slovenia. The Adriatic Sea acts as a diluition basin, because the salinity of Adriatic Sea is lower than the Mediterranean salinity , that collects third of fresh water of the latter. The salinity is 38%. The Po is one of the most important italian rivers. The Po river flows in Adriatic Sea between the provinces of Rovigo and Ferrara, after it travels about 652 kilometers starting on Monviso Mount that is situated in Cozie Alps. The Po has a maximum width of about a kilometer and a medium depht between 2 and 4 meters when it goes from Saluzzo to its confluence with the Ticino. From there to mouth reaches also the 9-10 meters. The Po separes itself in 6 branches. The first one, the Po of Volano at 35 kilometrs from Adriatic Sea, The last one, the Po of Goro underlines the border between Veneto and Emilia Romagna.Other Italian rivers that flow in Adriatic Sea are: the Adige, the Isonzo, the Tagliamento, the Brenta, the Piave, the Reno, the Narenta, the Metauro, the Chienti, the Tenna, the Tronto, the Aterno-Pescara, the Sangro and the Ofanto. Cetina is a croatian river that also flows in Adriatic sea. When a river encounters a sea, it could happen that it come to be created an instable ecosystem, because the fresh water of the river mixes with salty water of sea, and these are coastal areas near estuaries of rivers.

1.10.5 Sediments: a definition and a couple of different examples.

Inside the marine and oceanic masses there is a continuous sedimentation of materials, mainly consisting of remains of organisms (generally with shell or calcareous or siliceous skeleton, when they are not totally dissolved by sea water between 3500 and 5500 m of depth ), which accumulate on the bottom mixing with debris coming from the continental slopes (where they come from the watercourses). Near the continental slope, coarser sediments (sands) prevail, while the finer ones (clays) are deposited in the deepest areas (together with materials of cosmic and volcanic origin), where they are also found with a certain frequency concentrations of so-called nodules of manganese, pebbles with a flat shape and an average diameter of about 5 cm, which are found in abundance on vast extensions of the ocean floor: they are particularly rich in manganese as well as iron, but can also contain copper, nickel and other metals. Although currently their recovery is very expensive, the nodules will become in the next decades the main source of supply of many metallic minerals.

Accumulations and organic remains (largely derived from plankton-rich areas, together with organisms which are passively transported by currents and waves) give rise to extensive formations of globigerine calcareous mud (unicellular protozoa) and radioactive siliceous sludges (unicellular protozoa) and diatoms (unicellular algae). The continual sedimentation activity in the oceans is at the base of the formation of sedimentary rocks.

1.10.6 Micro-algal bloom: a description.

The freshwater microalgae live where there are the conditions necessary for their development, that is in the continental waters of the entire terrestrial globe. They are plant organisms able to carry out photosynthesis as the higher plants. They, unlike the last ones, do not produce seeds, leaves or flowers. They are indispensable for the food chain. As these living beings are able to transform inorganic substances into organic substances such as macroscopic or superior plants. Microalgae are commonly understood as the set of all those microscopic cells visible through the microscope or in any case with a magnifying glass. The classification of species living in fresh water is very complex, as in the case of microalgae living in the sea. Certainly, the number of species that live in fresh water is much lower than those living in the sea, but just as fascinating and in recent decades it has been given great importance to their cultivation for food. Many species are rich in substances important for the human body. Freshwater microalgae can have various shapes, colors and sizes; it ranges from bright green to dark green to cyanotic green, to yellow, to brown, to bright red. The forms are infinitely varied and strange. It goes from round ones to those with the shape of a torpedo to rectangular, square, ovoid ones and can be unicellular and multicellular that tends to form filaments or spheroidal colonies movable or immobile. Some chemical elements can favor their reproduction and their proliferation. Among these we find phosphorus, nitrogen, iron and silicon which, in some cases, lead them to the so-called water flowering.