Stable isotopic evidence for the differential contribution of diazotrophs to the epipelagic grazing food chain in the mid-Pacific Ocean

Aim Biological nitrogen fixation supports primary production in oligotrophic water, but its link to higher trophic levels has not been described fully on a biogeographical basis. Here, we determine the regional patterns of the contribution of the combined nitrogen to biological production within the epipelagic layer of the mid-Pacific Ocean using the isotopic signatures of nitrogen (delta N-15) and carbon (delta C-13) in the biological components. Location Time period The mid-Pacific Ocean along 170 degrees W between the southern subtropical front and the Chukchi Sea. Northern and austral summer in 2013 and 2014. Major taxa studied Methods Planktonic and micronektonic biota in the euphotic layer. We measured the geographical variations in delta N-15 and delta C-13 of the suspended particulate organic matter (POM), mesozooplankton assemblage and micronektonic fish. We analysed the relationships among these values and the environmental variables of temperature, nitrate concentration and biological nitrogen fixation activity along a 12,000-km meridional transect. Results Main conclusions The POM delta N-15 at 0 m was negatively correlated with in situ N-2 fixation activity in the subtropical region, whereas that in the equatorial and high-latitude regions was correlated with the nitrate concentration at 0 m. We found that the ratios of the increase in delta N-15 to delta C-13 along the grazing food chain were consistent throughout the equatorial and subtropical regions. Cluster analyses based on the stable isotopic signatures in the biotic components revealed that the food chains in the stations within the subtropical mid-Pacific Ocean were separated into three groups based on the differential contributions of biological nitrogen fixation. Distinct food chains from primary to tertiary production sustained by different nitrogen sources, nitrate below the euphotic zone, and diazotrophic nitrogen occur within the same biogeographical provinces in the subtropical mid-Pacific Ocean. The diazotroph-dominant community contributes substantially to the apex predators in the central areas of the subtropical gyres.