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.

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.

The largest volcanic eruption of this century, which was submarine, led to a dramatic phytoplankton bloom north of the island of Tongatapu, in the Kingdom of Tonga. In the absence of shipboard observations, we reconstructed the dynamics of this event by using a suite of satellite observations. Two independent bio-optical approaches confirmed that the phytoplankton bloom was a robust observation and not an optical artifact due to volcanogenic material. Furthermore, the timing, size, and position of the phytoplankton bloom suggest that plankton growth was primarily stimulated by nutrients released from volcanic ash rather than by nutrients upwelled through submarine volcanic activity. The appearance of a large region with high chlorophyll a concentrations less than 48 hours after the largest eruptive phase indicates a fast ecosystem response to nutrient fertilization. However, net phytoplankton growth probably initiated before the main eruption, when weaker volcanism had already fertilized the ocean.

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.

In stratified oligotrophic waters, phytoplankton communities forming the deep chlorophyll maximum (DCM) are isolated from atmospheric iron sources above and remineralized iron below. Reduced supply leads to a minimum in dissolved iron (dFe) near 100 m, but it is unclear if iron limits growth at the DCM. Here, we propose that natural iron addition events occur regularly with the passage of mesoscale eddies, which alter the supply of dFe and other nutrients relative to the supply of light, and can be used to test for iron limitation at the DCM. This framework is applied to two eddies sampled in the North Pacific Subtropical Gyre. Observations in an anticyclonic eddy center indicated downwelling of iron-rich surface waters, leading to increased dFe at the DCM but no increase in productivity. In contrast, uplift of isopycnals within a cyclonic eddy center increased supply of both nitrate and dFe to the DCM, and led to dominance of picoeukaryotic phytoplankton. Iron addition experiments did not increase productivity in either eddy, but did enhance leucine incorporation at ambient light in the cyclonic eddy, a potential indicator of iron stress among Prochlorococcus. Rapid cycling of siderophores and low dFe:nitrate uptake ratios also indicate that a portion of the microbial community was stressed by low iron. However, near-complete nitrate drawdown in this eddy, which represents an extreme case in nutrient supply compared to nearby Hawaii Ocean Time-series observations, suggests that recycling of dFe in oligotrophic ecosystems is sufficient to avoid iron limitation in the DCM under typical conditions.