Biological fixation of atmospheric nitrogen in the Mediterranean Sea

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Abstract
Nutrient concentration in the Mediterranean Sea is controlled by water exchanges through the Strait of Gibraltar and by atmospheric and terrestrial inputs. Various peculiarities in the nitrogen and phosphorus geochemical cycles are pointed out, namely a low N: P atomic ratio (6.4) in terrestrial discharges, and a budget well balanced for phosphorus (where terrestrial discharges amount to about 80% of the outflow) but apparently very deficient in nitrogen, despite a high N: P atomic ratio (22), in Mediterranean deep waters. This suggests the possibility of a surprisingly high rate of direct atmospheric N uptake by the Mediterranean ecosystem (possibly seagrasses Posidonia oceanica and pelagic bacterioplankton species).