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

Biological N-2 fixation rates were quantified in the Eastern Tropical South Pacific (ETSP) during both El Nino (February 2010) and La Nina (March-April 2011) conditions, and from Low-Nutrient, Low-Chlorophyll (20 degrees S) to High-Nutrient, Low-Chlorophyll (HNLC) (10 degrees S) conditions. N-2 fixation was detected at all stations with rates ranging from 0.01 to 0.88nmolNL(-1) d(-1), with higher rates measured during El Nino conditions compared to La Nina. 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 mu molL(-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 mu molNm(-2) d(-1) at northern stations, and from 0 to 148 mu molm(-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.