Uncoupling between dinitrogen fixation and primary productivity in the eastern Mediterranean Sea

In the nitrogen (N)-impoverished photic zones of many oceanic regions, prokaryotic organisms fixing atmospheric dinitrogen (N₂; diazotrophs) supply an essential source of new nitrogen and fuel primary production. We measured dinitrogen fixation and primary productivity (PP) during the thermally stratified summer period in different water regimes of the oligotrophic eastern Mediterranean Sea, including the Cyprus Eddy and the Rhodes Gyre. Low N ₂ fixation rates were measured (0.8-3.2 μmol N m^-2 d^-1) excluding 10-fold higher rates in the Rhodes Gyre and Cyprus Eddy (∼20 μmol N m^-2 d^-1). The corresponding PP increased from east to west (200-2500 μmol C m^-2 d^-1), with relatively higher productivity recorded in the Rhodes Gyre and Cyprus Eddy (2150 and 2300 μmol C m^-2 d^-1, respectively). These measurements demonstrate that N₂ fixation in the photic zone of the eastern Mediterranean Sea contributes only negligibly by direct inputs to PP (i.e., cyanobacterial diazotrophs) and is in fact uncoupled from PP. By contrast, N₂ fixation is significantly coupled to bacterial productivity and to net heterotrophic areas, suggesting that heterotrophic N₂ fixation may in fact be significant in this ultraoligotrophic system. This is further substantiated by the high N₂ fixation rates we measured from aphotic depths and by the results of phylogenetic analysis in other studies showing an abundance of heterotrophic diazotrophs. Key Points N₂ fixation is uncoupled from primary productivity in the eastern Mediterranean SeaN₂ fixation has negligible contribution to primary productionHeterotrophic N₂ fixation is important in the eastern Mediterranean Sea