Evidence of active dinitrogen fixation in surface waters of the eastern tropical south pacific during El ni o and La ni a events and evaluation of its potential nutrient controls
Biological N2 fixation rates were quantified in the Eastern Tropical South Pacific (ETSP) during both El Ni o (February 2010) and La Ni a (March-April 2011) conditions, and from Low-Nutrient, Low-Chlorophyll (20 S) to High-Nutrient, Low-Chlorophyll (HNLC) (10 S) conditions. N2 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 Ni o conditions compared to La Ni a. High N2 fixations rates were reported at northern stations (HNLC conditions) at the oxycline and in the oxygen minimum zone (OMZ), despite nitrate (NO3-) concentrations up to 30 mol L-1, indicating that inputs of new N can occur in parallel with N loss processes in OMZs. Water-column integrated N2 fixation rates ranged from 4 to 53 mol N m-2 d-1 at northern stations, and from 0 to 148 mol m-2 d-1 at southern stations, which are of the same order of magnitude as N2 fixation rates measured in the oligotrophic ocean. N2 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 N2 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 N2 fixation.