Springtime contribution of dinitrogen fixation to primary production across the Mediterranean Sea

Abstract. Dinitrogen (N₂) fixation rates were measured during early spring across the different provinces of Mediterranean Sea surface waters. N₂ fixation rates, measured using ¹⁵N₂ enriched seawater, were lowest in the eastern basin and increased westward with a maximum at the Strait of Gibraltar (0.10 to 2.35 nmol N L⁻¹ d⁻¹, respectively). These rates were 3–7 fold higher than N₂ fixation rates measured previously in the Mediterranean Sea during summertime and we estimated that methodological differences alone did not account for the seasonal changes we observed. Higher contribution of N₂ fixation to primary production (4–8%) was measured in the western basin compared to the eastern basin (∼2%). Our data indicates that these differences between basins may be attributed to changes in N₂-fixing planktonic communities and that heterotrophic diazotrophy may play a significant role in the eastern Mediterranean while autotrophic diazotrophy has a more dominant role in the western basin.