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

Abstract

Abstract Biological N 2 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 2 fixation was detected at all stations with rates ranging from 0.01 to 0.88 nmol N L −1 d −1 , with higher rates measured during El Niño conditions compared to La Niña. High N 2 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 −1 , indicating that inputs of new N can occur in parallel with N loss processes in OMZs. Water‐column integrated N 2 fixation rates ranged from 4 to 53 µmol N m −2 d −1 at northern stations, and from 0 to 148 µmol m −2 d −1 at southern stations, which are of the same order of magnitude as N 2 fixation rates measured in the oligotrophic ocean. N 2 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 2 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 2 fixation.