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 N 2 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 δ 15 N 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.