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Uncoupling between dinitrogen fixation and primary productivity in the eastern Mediterranean Sea
Author(s) -
Rahav Eyal,
Herut Barak,
Stambler Noga,
BarZeev Edo,
Mulholland Margaret R.,
BermanFrank Ilana
Publication year - 2013
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/jgrg.20023
Subject(s) - photic zone , ocean gyre , diazotroph , oceanography , productivity , nitrogen fixation , heterotroph , mediterranean climate , environmental science , new production , mediterranean sea , primary production , nitrogen , biology , subtropics , ecology , geology , chemistry , nutrient , phytoplankton , bacteria , paleontology , ecosystem , macroeconomics , economics , organic chemistry
In the nitrogen (N)‐impoverished photic zones of many oceanic regions, prokaryotic organisms fixing atmospheric dinitrogen (N 2 ; 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 µ mol N m −2 d −1 ) excluding 10‐fold higher rates in the Rhodes Gyre and Cyprus Eddy (~20 µ mol N m −2 d −1 ). The corresponding PP increased from east to west (200–2500 µ mol C m −2 d −1 ), with relatively higher productivity recorded in the Rhodes Gyre and Cyprus Eddy (2150 and 2300 µ mol C m −2 d −1 , respectively). These measurements demonstrate that N 2 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, N 2 fixation is significantly coupled to bacterial productivity and to net heterotrophic areas, suggesting that heterotrophic N 2 fixation may in fact be significant in this ultraoligotrophic system. This is further substantiated by the high N 2 fixation rates we measured from aphotic depths and by the results of phylogenetic analysis in other studies showing an abundance of heterotrophic diazotrophs.