Nutrients in the Mediterranean Sea, mass balance and statistical analysis of concentrations with respect to environmental change

Experimental data obtained from the Algero–Provençal basin (Western Mediterranean) in 1994 illustrates the homogeneity of phosphate, nitrate and silicate concentrations in deep-water throughout this basin, comparable to temperature and salinity homogeneity. Nutrient mass balances across the straits of Gibraltar and Sicily enable us to estimate new production (NP) and f ratio (new vs. total production) in the inner basins. Rather low values of NP and f have been obtained. The f ratio ranges between 0.05 and 0.15. These values demonstrate the existence of spatial variability for Mediterranean oligotrophy. A statistical analysis of historical data together with new data acquired in 1994 demonstrate an increasing trend for phosphate and nitrate concentrations in deep water. In contrast, no change with time was detected for silicate concentrations. The trends for nutrient concentrations in deep water constitute new constraints for the study of atmospheric and terrestrial inputs of phosphate and nitrate. Nutrient concentration increases are much more dependent on these inputs than on the Atlantic influx across the strait of Gibraltar. The trends also provide information about the functioning of the Mediterranean ecosystem and could help to monitor the environmental change. The working out of marine budgets highlights an inadequate knowledge of external sources of nutrients and call into question the problems linked to external inputs: i) riverine particulate phosphorus transformation into phosphate at the land–sea interface and ii) the biological fixation of atmospheric nitrogen.