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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 nitrate¹⁵N/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. The¹⁵N/¹⁴N of material collected in shallow sediment traps was significantly higher in the cyclone than in the anticyclone and showed correspondence to the¹⁵N/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 N₂fixation to export production. Results among approaches were inconsistent, which we attribute to non‐steady state conditions during our observation period. 

The index of refraction (n) of particles is an important parameter in optical models that aims to extract particle size and carbon concentrations from light scattering measurements. An inadequate choice ofncan critically affect the characterization and interpretation of optically-derived parameters, including those from satellite-based models which provide the current view of how biogeochemical processes vary over the global ocean. Yet, little is known about hownvaries over time and space to inform such models. Particularly, in situ estimates ofnfor bulk water samples and at diel-resolving time scales are rare. Here, we demonstrate a method to estimatenusing simultaneously and independently collected particulate beam attenuation coefficients, particle size distribution data, and a Mie theory model. We apply this method to surface waters of the North Pacific Subtropical Gyre (NPSG) at hourly resolution. Clear diel cycles innwere observed, marked by minima around local sunrise and maxima around sunset, qualitatively consistent with several laboratory-based estimates ofnfor specific phytoplankton species. A sensitivity analysis showed that the daily oscillation innamplitude was somewhat insensitive to broad variations in method assumptions, ranging from 11.3 ± 4.3% to 16.9 ± 2.9%. Such estimates are crucial for improvement of algorithms that extract the particle size and production from bulk optical measurements, and could potentially help establish a link betweennvariations and changes in cellular composition of in situ particles. 

Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation ( ${\text{c}}_{\text{p}}$) and backscattering ( $b_{\text{bp}}$) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in ${\text{c}}_{\text{p}}$and ${\text{b}}_{\text{bp}}$to daytime particle growth and division of cells, with particles $<<\#comment/>7\text{µ<\#comment/>}\mathrm{m}$driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacteriumCrocosphaera( $\sim <\#comment/>4-<\#comment/>7\text{µ<\#comment/>}\mathrm{m}$) to be an important driver of ${\text{c}}_{\text{p}}$at the time of sampling, whereasProchlorococcusdynamics ( $\sim <\#comment/>0.5\text{µ<\#comment/>}\mathrm{m}$) were essential to reproducing temporal variability in ${\text{b}}_{\text{bp}}$. This study is a step towards improved characterization of the particle size range represented byin situbulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.