Evidence of active dinitrogen fixation in surface waters of the eastern tropical South Pacific during El Niño and La Niña events and evaluation of its potential nutrient controls. Issue 3 (18th August 2013)

• Record Type: Journal Article
• Authors: Dekaezemacker, J.; Bonnet, S.; Grosso, O.; Moutin, T.; Bressac, M.; Capone, D.G.
• Is Part Of: Global biogeochemical cycles. Volume 27:Issue 3(2013)
• Page Start: 768
• Page End: 779
• Publication Date: 2013-08-18
• Subjects: Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/gbc.20063
• Languages: English
• ISSNs: 0886-6236

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>[1] Biological N<sub>2</sub> fixation rates were quantified in the Eastern Tropical South Pacific (ETSP) during both El Niño (February 2010) and La Niña (March–April 2011) conditions, and from Low‐Nutrient, Low‐Chlorophyll (20°S) to High‐Nutrient, Low‐Chlorophyll (HNLC) (10°S) conditions. N<sub>2</sub> fixation was detected at all stations with rates ranging from 0.01 to 0.88 nmol N L<sup>−1</sup> d<sup>−1</sup>, with higher rates measured during El Niño conditions compared to La Niña. High N<sub>2</sub> fixations rates were reported at northern stations (HNLC conditions) at the oxycline and in the oxygen minimum zone (OMZ), despite nitrate concentrations up to 30 µmol L<sup>−1</sup>, indicating that inputs of new N can occur in parallel with N loss processes in OMZs. Water‐column integrated N<sub>2</sub> fixation rates ranged from 4 to 53 µmol N m<sup>−2</sup> d<sup>−1</sup> at northern stations, and from 0 to 148 µmol m<sup>−2</sup> d<sup>−1</sup> at southern stations, which are of the same order of magnitude as N<sub>2</sub> fixation rates measured in the oligotrophic ocean. N<sub>2</sub> fixation rates responded significantly to Fe and organic carbon additions in the surface HNLC waters, and surprisingly by concomitant Fe and N additions in surface waters at the edge of the subtropical gyre. Recent studies have highlighted the predominance of heterotrophic diazotrophs in this area, and we hypothesize that N<sub>2</sub> fixation could be directly limited by inorganic nutrient availability, or indirectly through the stimulation of primary production and the subsequent excretion of dissolved organic matter and/or the formation of micro‐environments favorable for heterotrophic N<sub>2</sub> fixation.</p> </abstract>