Phosphorus and nitrogen behaviour in the Mediterranean Sea

Mediterranean nutrient studies differ from one another by their rates of exchange with the Atlantic Ocean and by atmospheric and terrestrial sources, which sometimes vary by six times. During the Medatlante cruises (in 1988 and 1989), increases of phosphate and nitrate concentrations were confirmed in deep western waters and may be related to increasing agricultural, industrial and urban activities around the sea since the 1960s. In a non-steady-state model, this evolution of deep water concentrations constrains uncertainties in the nutrient budgets; we propose a reduced range for atmospheric and terrestrial sources of nutrients. In the Western Basin in the late 1980s the total atmospheric and terrestrial source amounted to 8–10 × 10⁹ mol y⁻¹ of phosphate, and 190–220 × 10⁹ mol y⁻¹ of nitrate; about two-fold greater than estimates based on measurements of atmospheric and terrestrial inputs (the latter is calculated from the Rhône river concentrations over the 1971–1988 period). Consequently, phosphate budgets suggest that some of the riverine particulate input of phosphorus dissolves when entering the sea and constitutes a main source of phosphate, a scenario proposed by Froelich (1988). Likewise, dissolved inorganic nitrogen budgets require biological fixation of molecular nitrogen by plankton species and seagrasses. This process may constitute the main nitrogen source and explain the peculiar molar ratio N/P in the Mediterranean Sea (about 21–23) instead of about 15 in the global ocean.