Nitrogen Biogeochemistry of Adjacent Mesoscale Eddies in the North Pacific Subtropical Gyre

Abstract

We examined the nitrogen (N) biogeochemistry of adjacent cyclonic and anticyclonic eddies near Hawai'i in the North Pacific Subtropical Gyre (NPSG) and explored mechanisms that sustain productivity in the cyclone after the initial intensification stage. The top of the nutricline was uplifted into the euphotic zone in the cyclone and depressed in the anticyclone. Subsurface nutrient concentrations and apparent oxygen utilization at the cyclone's inner periphery were higher than expected from isopycnal displacement, suggesting that shallow remineralization of organic material generated excess nutrients in the subsurface. The excess nutrients may provide a supply of subsurface nutrients to sustain productivity in maturing eddies. The shallow remineralization also raises questions regarding the extent to which cyclonic eddies promote deep carbon sequestration in subtropical gyres such as the NPSG. An upward increase in nitrate ¹⁵N/¹⁴N isotope ratios below the euphotic zone, indicative of partial nitrate assimilation, coincided with negative preformed nutrients—potentially signaling heterotrophic bacterial consumption of carbon-rich (nitrogen-poor) organic material. The ¹⁵N/¹⁴N of material collected in shallow sediment traps was significantly higher in the cyclone than in the anticyclone and showed correspondence to the ¹⁵N/¹⁴N ratio of the nitrate supply, which is acutely sensitive to sea level anomaly in the region. A number of approaches were applied to estimate the contribution of N₂ fixation to export production. Results among approaches were inconsistent, which we attribute to non-steady state conditions during our observation period.