Low water column nitrogen fixation in the Mediterranean Sea: basin-wide experimental evidence.

The abundance and nitrogen fixation rates of Trichodesmium sp. integrated down to the deep chlorophyll maximum as well as nitrogen fixation rates in size-fractionated discrete surface water samples were measured across the Mediterranean Sea. The abundance of Trichodesmium sp. was generally low (<50 trichomes m-3), except at one station in the Tyrrhenian Sea where the water temperature was >20°C and Trichodesmium sp. was present at a higher density (ca. 700 trichomes m-3). Across the basin, size-fractionated nitrogen fixation rates were low (<250 nmol N2 m-3 h-1). The highest rate was measured at a station characterized by relatively high concentrations of total ­dissolved P and dissolved Fe. High δ15N values (8.04 ± 2.03‰) of the particulate organic nitrogen were in accordance with the observed low water column nitrogen fixation rates across the Mediterranean Sea.