Abstract
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 degrees 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 delta N-15 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.