The Atlantic surface water entering through Gibraltar straits is more or less depleted of nutrient and any biological activity in the surface waters of the Mediterranean Sea tends to reduce the initial stocks. Therefore, the supply of new nutrients to the surface layer relies on mixing with intermediate water or upwelling, on continental weathering, on river discharges, and on wet or dry deposition. All these factors contribute to the west-east increase in the degree of oligotrophy in the Mediterranean Sea and most of these factors are very sensitive to the increasing pressure of the human activities.
Increasing atmospheric concentration of CO2 results in increased seawater CO2 concentrations and consequently modified the carbonate equilibrium decreasing the pH. In a general context calcifying taxa are largely exposed to the acidification although the real effect of high level CO2 seawater is still debated. In addition to the higher carbon input from the riverine waters, the Mediterranean Sea takes up a large part of the anthropogenic CO2 injected into the atmosphere through air-sea interactions.
The Mediterranean Sea provides a unique environment to observe changes in nutrient cycling in the water column. Unfortunately, today, data of the carbonate system properties in the Mediterranean Sea are very scarce compared with other ocean areas.