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

摘要

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°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.