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. Highδ ¹⁵N values (8.04 ± 2.03 %o) of the particulate organic nitrogen were in accordance with the observed low water column nitrogen fixation rates across the Mediterranean Sea.