Effects of Mesoscale Eddies on the Contribution of N2 Fixation to Export Production in the North Pacific Subtropical Gyre

Mesoscale eddies in subtropical gyres play an important role in ocean primary production, modulating nutrient delivery to the surface ocean. Biological N₂ fixation is also a means of supplying biologically available nitrogen to the surface of subtropical gyres. However, whether eddy features influence the contribution of N₂ fixation to export production remains unclear. Here we quantified the contribution of N₂ fixation to export production in an open-ocean cyclonic and anticyclonic eddy in the North Pacific Subtropical Gyre from a N isotope mass balance of the euphotic zone, comparing values of subsurface nitrate δ¹⁵N to those of material collected in sediment trap at 150 m depth. Hydrographic data and water samples were collected in the north of Hawai’i in 2017 summer during the MESO-SCOPE cruise, which traversed an adjacent cyclonic and anticyclonic eddy. Isopycnal heaving was evident in the cyclonic eddy, whereas isopycnals deepened in the anticyclonic eddy. The δ¹⁵N of subsurface nitrate was lower in the anticyclonic eddy. Concurrently the δ¹⁵N of sinking PON was proportionally lower in the anticyclonic eddy. Respective estimates of the contribution of newly fixed N to the sinking PON flux in the cyclonic and anticyclonic eddies differed substantially as a function of the assumed depth range from which nutrients are upwelled to the surface. No coherent difference in the fraction of newly fixed N to the exported PON was evident between the cyclonic and anticyclonic eddy, regardless of the assumed depth range of nutrient supply. The results indicate that the δ¹⁵N value of nitrate upwelled to the surface of subtropical Pacific may be modulated by eddies. Our analysis further suggests that this spatial heterogeneity in subsurface nitrate δ¹⁵N adds considerable uncertainty to estimates of N₂ fixation from 1-D N isotope mass balance in this region.