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 may 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 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.

Climate change is expected to alter the input of nitrogen (N) sources in the Eastern Canadian Arctic Archipelago (ECAA) and Baffin Bay due to increased discharge from glacial meltwater and permafrost thaw. Since dissolved inorganic N is generally depleted in surface waters, dissolved organic N (DON) could represent a significant N source fueling phytoplankton activity in Arctic ecosystems. Yet, few DON data for this region exist. We measured concentrations and stable isotope ratios (δ15N and δ18O) of DON and nitrate (NO3−) to investigate the sources and cycling of dissolved nitrogen in regional rivers and at the sea surface from samples collected in the ECAA and Baffin Bay during the summer of 2019. The isotopic signatures of NO3- in rivers could be reproduced in a steady state isotopic model by invoking mixing between atmospheric NO3- and nitrified ammonium as well as NO3- assimilation by phytoplankton. DON concentrations were low in most rivers (≤4.9 µmol L−1), whereas the concentrations (0.54–12 µmol L−1) and δ15N of DON (−0.71–9.6 ‰) at the sea surface were variable among stations, suggesting dynamic cycling and/or distinctive sources. In two regions with high chl-a, DON concentrations were inversely correlated with chlorophyll‐an and the d15N of DON, suggesting net DON consumption in localized phytoplankton blooms. We derived an isotope effect of −6.9‰ for DON consumption. Our data helps establish a baseline to assess future change in nutrient regime for this climate sensitive region.