Long-term numerical evolution of the nitrogen bulk content in the Mediterranean Sea

The behavior of the Mediterranean ecosystem in response to realistic riverine inputs and dissolved matter exchange is investigated. The strategy is to evaluate the stability of the ecosystem subjected to various atmospheric inputs. For this purpose, we employ a generic three-dimensional biogeochemical description, based on inorganic nitrogen, phytoplankton, zooplankton and detritus. The model is developed for the oligotrophic Mediterranean Sea at 1/8° × 1/8° horizontal resolution. The total nitrogen content in the Mediterranean Sea remains stable, with an overall annual loss of approximately 150 × 103 ton N year-1, attributable to further diffused terrestrial discharges of nitrogen along the central and the northwestern coasts. The total contents of nitrogen in Western and Eastern Mediterranean exhibit opposite tendency in the intermediate layers below 180 m, increasing in the former and decreasing in the latter. The upper euphotic layer of the Western Mediterranean over a long time frame has high total nitrogen content, more than 600 mmol N m-2, compared with the Eastern Mediterranean, which is less than 250 mmol N m-2, showing the development of different typical trophic regimes from the initial homogeneous status. The vertically integrated chlorophyll concentrations evidence at the end of the long-term simulation a clear west-east decreasing gradient; the average of the Western Mediterranean turns out twice the amount of the Eastern Mediterranean.