Linkage Between Dinitrogen Fixation and Primary Production in the Oligotrophic South Pacific Ocean

Abstract

AbstractThe import of nitrogen via dinitrogen fixation supports primary production, particularly in the oligotrophic ocean; however, to what extent dinitrogen fixation influences primary production, and the role of specific types of diazotrophs, remains poorly understood. We examined the relationship between primary production and dinitrogen fixation together with diazotroph community structure in the oligotrophic western and eastern South Pacific Ocean and found that dinitrogen fixation was higher than nitrate‐based new production. Primary production increased in the middle of the western subtropical region, where the cyanobacterium Trichodesmium dominated the diazotroph community and accounted for up to 7.8% of the phytoplankton community, and the abundance of other phytoplankton taxa (especially Prochlorococcus) was high. These results suggest that regenerated production was enhanced by nitrogen released from Trichodesmium and that carbon fixation by Trichodesmium also contributed significantly to total primary production. Although volumetric dinitrogen fixation was comparable between the western and eastern subtropical regions, primary production in the western waters was more than twice as high as that in the eastern waters, where UCYN‐A1 (photoheterotroph) and heterotrophic bacteria were the dominant diazotrophs. This suggests that dinitrogen fixed by these diazotrophs contributed relatively little to primary production of the wider community, and there was limited carbon fixation by these diazotrophs. Hence, we document how the community composition of diazotrophs in the field can be reflected in how much nitrogen becomes available to the wider phytoplankton community and in how much autotrophic diazotrophs themselves fix carbon and thereby influences the magnitude of local primary production.