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.

A budget approach is used to disentangle drivers of the seasonal mixed layer carbon cycle at Station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the North Pacific Subtropical Gyre (NPSG). The budget utilizes data from the WHOTS (Woods Hole - Hawaii Ocean Time-series Site) mooring, and the ship-based Hawai‘i Ocean Time-series (HOT) in the North Pacific Subtropical Gyre (NPSG), a region of significant oceanic carbon uptake. Parsing the carbon variations into process components allows an assessment of both the proportional contributions of mixed layer carbon drivers, and the seasonal interplay of drawdown and supply from different processes. Annual net community production reported here is at the lower end of previously published data, while net community calcification estimates are 4- to 7-fold higher than available sediment trap data, the only other estimate of calcium carbonate export at this location. Although the observed seasonal cycle in dissolved inorganic carbon (DIC) in the NPSG has a relatively small amplitude, larger fluxes offset each other over an average year, with major supply from physical transport, especially lateral eddy transport throughout the year and entrainment in the winter, and biological carbon uptake in the spring. Gas exchange plays a smaller role, supplying carbon to the surface ocean between Dec-May, and outgassing in Jul-Oct. Evaporation-precipitation (E–P) is variable with precipitation prevailing in the first- and evaporation in the second half of the year. The observed total alkalinity signal is largely governed by E–P, with a somewhat stronger net calcification signal in the wintertime.

Following sea-ice retreat, surface waters of Arctic marginal seas become nutrient-limited and subsurface chlorophyll maxima (SCM) develop below the pycnocline where nutrients and light conditions are favorable. The productivity associated with these “hidden” features has traditionally not been well constrained. Here, we use a unique combination of high-resolution biogeochemical and physical observations collected on the Chukchi shelf in 2017 to constrain the fine-scale structure of nutrients, O2, particles, SCM, and turbulence. We find large O2 excess at mid-depth, identified by positive saturation (∆O2) maxima of 15-20% that unambiguously indicate significant subsurface production. The ∆O2 maxima were situated immediately beneath the pycnocline and coincided with a complete depletion of inorganic nitrogen ([NO3-] + [NH4+]). The complete nutrient drawdown and O2 excess from this horizon is consistent with subsurface production that amounts to 1/3 to 1/2 the total regional primary production. Nitracline depths aligned with both the base of the mid-depth O2 maxima and with SCM depths, suggesting this horizon represents a compensation point for balanced growth and loss. Furthermore, SCM were also associated with turbulence minima and sat just above a high turbidity bottom layer where light attenuation increased significantly due to high particle loads. Spatially, the largest ∆O2 maxima were associated with high nutrient winter-origin water masses, under a shallower pycnocline associated with seasonal melt. These data implicate short-term and long-term control of SCM and associated productivity by stratification, turbulence, light, and seasonal water mass formation, with corresponding potential for climate-related sensitivities.

We report in situ rates of gross oxygen production (GOP), community respiration (R), and net community production (NCP) in the North Pacific Subtropical Gyre derived from mixed layer O/Ar measurements. The measurements were conducted between November 2013 and January 2019 at the site of the Hawaii Ocean Time-series program. Biological O concentration anomalies in the mixed layer showed a consistent diel variation, with values increasing during daytime due to net primary production and decreasing during nighttime due to respiration. In situ mixed layer GOP and R, determined from these variations, co-varied but showed no clear seasonal pattern, averaging 0.9 and 0.8 mmol O m d, respectively. In situ rates of NCP determined from mixed layer O/Ar ranged between -0.7 and 17.6 mmol O m d. Our analyses indicate that at certain times of the year the diapycnal flux of O across the base of the mixed layer may be non-negligible and therefore a fraction of O/Ar-derived NCP may form below the mixed layer. The seasonal climatology of NCP below the mixed layer (down to 150 m) was also estimated using near-monthly changes in dissolved O concentrations. These calculations allowed us to estimate NCP for the entire euphotic zone (0-150 m), which shows pronounced seasonality, with a maximum in May and a minimum in December, when the ecosystem becomes temporarily net heterotrophic. Annual NCP was estimated to be 2.4 ± 0.5 mol O m yr, approximately twice the export of C through sinking particles captured in sediment traps at 150 m.

Parameterizations of algal photosynthesis commonly employed in global biogeochemical simulations generally fail to capture the observed vertical structure of primary production. Here we examined the consequences of decoupling photosynthesis (carbon fixation) and biosynthesis (biomass building) with accumulation or exudation of excess photosynthate under energy rich conditions in both regional and global models. The results show that the decoupling of these two processes improved the simulated vertical profile of primary production, increased modeled global primary production up to ~35%, improved simulated meridional patterns of particulate C:N:P and increased modeled surface pool of semi-labile DOC.

We examined the biogeochemical impact of pairs of mesoscale cyclones and anticyclones in spatial proximity (<200 km apart) in the North Pacific Subtropical Gyre. While previous studies have demonstrated that upwelling associated with the intensification of cyclonic eddies supplies nutrients to the euphotic zone, we find that cyclonic eddies in their mature stage sustain plankton growth by increasing the diapycnal flux of nutrients to the lower portion of the euphotic zone. This increased supply results from enhanced vertical gradients in inorganic nutrients due to erosion of the nutricline that accompanied plankton growth during eddy intensification. From a biological standpoint, increased nutrient flux was linked with expansion of eukaryotic phytoplankton biomass and intensification of the deep chlorophyll maximum layer. This perturbation in the plankton community was associated with increased fluxes of biominerals (opal and calcium carbonate) and isotopically enriched nitrogen in particles exported in the cyclone. The time-integrated effects of thermocline uplifts and depressions were predictable deficits and surpluses of inorganic nutrients and dissolved oxygen in the lower euphotic zone. However, the stoichiometry of changes in oxygen and inorganic nutrients differed from that predicted for production and consumption of phytoplankton biomass, consistent with additional biological processes that decouple changes in oxygen and nutrient concentrations. The dynamics revealed by this study may be a common feature of oligotrophic ecosystems, where mesoscale biogeochemical perturbations are buffered by the deep chlorophyll maximum layer, which limits the ecological impact of eddies in the well-lit, near-surface ocean.