A nutrient relay sustains subtropical ocean productivity

The expansive gyres of the subtropical ocean account for a significant fraction of global organic carbon export from the upper ocean. In the gyre interior, vertical mixing and the heaving of nutrient-rich waters into the euphotic layer sustain local productivity, in turn depleting the layers below. However, the nutrient pathways by which these subeuphotic layers are themselves replenished remain unclear. Using a global, eddy-permitting simulation of ocean physics and biogeochemistry, we quantify nutrient resupply mechanisms along and across density surfaces, including the contribution of eddy-scale motions that are challenging to observe. We find that mesoscale eddies (10 to 100 km) flux nutrients from the shallow flanks of the gyre into the recirculating interior, through time-varying motions along density surfaces. The subeuphotic layers are ultimately replenished in approximately equal contributions by this mesoscale eddy transport and the remineralization of sinking particles. The mesoscale eddy resupply is most important in the lower thermocline for the whole subtropical region but is dominant at all depths within the gyre interior. Subtropical gyre productivity may therefore be sustained by a nutrient relay, where the lateral transport resupplies nutrients to the thermocline and allows vertical exchanges to maintain surface biological production and carbon export.

The sinking of particulate organic carbon from the sunlit euphotic zone into the deeper, dark ocean maintains an oceanic reservoir of dissolved inorganic carbon, changes in which can significantly modify atmospheric CO₂ (1). The ocean’s subtropical gyres exhibit low surface concentrations of nutrients and biomass but, due to their very large surface area, may contribute a significant fraction of global export. For example, the North Pacific subtropical gyre is estimated to represent ~20 to 50% of the total North Pacific organic sinking flux (2, 3). The wind forcing over the subtropical basins leads to a downward doming of the density surfaces that contain an extensive volume of low-nutrient waters (4), as revealed in an observed transect from the North Pacific (Fig. 1) (5). The mode of nutrient resupply and the long-term maintenance of biological productivity in subtropical gyres have presented a conundrum for several decades. Inorganic nutrients are incorporated into photosynthetic phytoplankton and pass through the food web, but despite efficient recycling within the sunlit euphotic zone (6, 7), gravitational sinking and the subduction of organic matter deplete the surface nutrients of the subtropical gyres. These surface nutrient losses are largely viewed as being offset by the physical transport of nutrient-enriched, deeper waters back into the sunlit zone, together with smaller contributions from atmospheric deposition and nitrogen fixation. However, the nature of the nutrient pathways recharging the subtropical nutrient reservoir below the euphotic zone remains poorly constrained.Open in a separate windowFig. 1.PO43− concentration (μmol kg⁻¹) in the North Pacific basin obtained from the Global Ocean Data Analysis Project (GLODAP) V2.2 atlas and plotted using Data-Interpolating Variational Analysis (DIVA) gridding in Ocean Data View. (A) Transect along 155 ^∘W, with contours of constant density σ_o (thick dashed lines) and sampling locations (gray points). The solid black line indicates the σ_o = 26.0 surface. (B) PO43− distribution along σ_o = 26.0 with sampling locations indicated by gray points.The sharp vertical gradient of nutrients just below the euphotic layer can sustain the vertical supply of nutrients up to the surface. Transfer of nutrients upward into the subtropical euphotic zone has been attributed to vertical diapycnal mixing (8–11); the passage of mesoscale eddies, which adiabatically lift nutrient-rich, subeuphotic layers into the light (12–19); and submesoscale (1 to 10 km) features that are associated with strong vertical circulations (20–24). However, all of these localized processes deliver nutrients into the euphotic layer while depleting the subeuphotic layers below (17, 25). A long-standing question is how the nutrient inventory of this subeuphotic layer, which fuels the local vertical supplies, is maintained over the longer term (26, 27).At the flanks of the subtropical gyre, wind-driven upwelling brings nutrient-rich waters toward the surface, forming nutrient gradients in the subpolar and equatorial regions, as seen in observed transects (Fig. 1A). Within the frictional boundary layer of the ocean, the wind-driven, meridional Ekman transport acts on these lateral gradients to transfer nutrients into the subtropical gyre, significantly supplementing vertical processes in sustaining local productivity (4, 28, 29). However, this lateral transport contribution is confined to the upper few tens of meters and diminishes away from the gyre margins due to biological consumption (4).Mesoscale eddies can provide a lateral transport of nutrients, which involves both stirring and advective transfer along density surfaces (11, 17, 25, 30, 31). Idealized simulations and theory suggest an important combined nutrient supply to the gyre from lateral eddy diffusion and Ekman transport (30). Diagnostics of a more realistic, global eddy-resolving model show that lateral eddy transfers do provide an important nutrient flux across the boundaries of the subtropical gyre (31). Stimulated by these model-based inferences, an observational field study, measuring microscale turbulence and nutrients, reveal signals of eddy stirring along density surfaces, providing a weak nutrient supply within the thermocline over the center of the North Atlantic subtropical gyre (11). A closure for the nutrient supply then suggests that the nutrient delivery by mesoscale eddy stirring should be one to two orders of magnitude larger over the flanks of the subtropical gyre due to an increase in nutrient gradients and a greater tilt of the density surfaces (11). These enhanced nutrient gradients and isopycnal slopes are evident near the margins of the subtropical gyre, particularly at its southern flank, between σ₀ = 24.0 and 26.5 (Fig. 1). An eddying numerical simulation reveals the associated mesoscale flux of nutrients, visible as streamers of high-phosphate waters emanating from nutrient-rich currents all around the margins of the subtropical gyre (Fig. 2A and Movie S1).Open in a separate windowFig. 2.Snapshot of simulated PO43− concentration (mmol m⁻³) during the month of September and along the σ₀ = 26.0 isopycnal (A) over the north subtropical Pacific basin and (B) in the proximity of the Hawaiian islands. The black line contours in B depict the depth of the isopycnal surface with 10-m contour intervals. Mesoscale features transport nutrients into the North Pacific subtropical gyre through the combined action of eddy stirring that draws out filaments of tracer and advection by coherent eddy structures.In this study, we quantify the nutrient pathways and fluxes of the subtropical North Pacific Ocean, in the context of a global eddy-permitting numerical model with explicit representation of biogeochemical and ecological processes (Materials and Methods). In doing so, we test and illustrate the hypothesis of ref. 11 that the lateral eddy transfer contribution is stronger at the gyre margins and in the thermocline. Complementing previous work (31), we resolve along- and across-density surface fluxes and the depth and intragyre structures in fluxes and balances, as well as the contribution of dissolved organics.We find that a nutrient relay occurs in the subtropical gyre (11), involving the eddying transfer of nutrients from the upwelling flanks of the gyre, downward along sloping isopycnals into the subtropical gyre interior. This nutrient supply into the thermocline offsets the export of organic matter to the deep ocean and fuels upward vertical transfer to the surface, helping to sustain biological production in the euphotic layer. We illustrate this nutrient relay in the following, detailed analysis of the nutrient budget for density layers over the North Pacific subtropical gyre.