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

Abstract

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 (δ15N) and carbon (δ13C) in the biological components. Location The mid‐Pacific Ocean along 170° W between the southern subtropical front and the Chukchi Sea. Time period Northern and austral summer in 2013 and 2014. Major taxa studied Planktonic and micronektonic biota in the euphotic layer. Methods We measured the geographical variations in δ15N and δ13C 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 The POM δ15N at 0 m was negatively correlated with in situ N2 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 δ15N to δ13C 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. Main conclusions 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.

Heterogeneous nitrogen fixation rates confer energetic advantage and expanded ecological niche of unicellular diazotroph populations

“…itrogen (N 2 ) fixing microorganisms (diazotrophs) are critical suppliers of bioavailable nitrogen (N) in the world's oceans. The N 2 fixed by these organisms supports cell growth, but also enters the food web through grazing by zooplankton and excretion of ammonium (NH þ 4 ) or other dissolved nitrogenous compounds [1][2][3][4][5] . All diazotrophs have a N 2 fixing enzyme complex, nitrogenase.…”

Vertical eddy diffusivity in the subsurface pycnocline across the Pacific

“…The diffusivity levels and geographical distributions presented in this study improve estimations of the material transport into oligotrophic euphotic layers and will thus contribute to better understanding biogeochemical cycles. As for the other sources of the nitrogen for the new production, the role of biological nitrogen fixation has increasingly been highlighted (e.g., Horii et al 2018). An evaluation of the vertical turbulent nutrient transport from the lower layer will be conducted in a future study, which will refine the definition of ocean biogeography based on the source type of the nitrogen for the new production.…”

Spatial and Seasonal Variations of Stable Isotope Ratios of Particulate Organic Carbon and Nitrogen in the Surface Water of the Kuroshio

“…Carbon and nitrogen stable isotope ratios also help track nitrogen sources in marine ecosystems (Wada et al, 2012;Zanden & Rasmussen, 2001) and assess animal movements (Rubenstein & Hobson, 2004). However, nitrogen isotope ratios in marine ecosystems change both spatially and temporally with the nitrogen sources of primary producers (Choy et al, 2012;Horii et al, 2018;Shiozaki et al, 2014). Therefore, the stable isotope ratios of phytoplankton can also be used to understand the grazing food chain, and subsequently the marine ecosystem.…”