Lateral advection supports nitrogen export in the oligotrophic open-ocean Gulf of Mexico

In contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 identified low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates for diffusive flux of nitrate into the euphotic zone (< 1 µmol N m−2 d−1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2 – 3 orders of magnitude higher (i.e. 462 – 1144 µmol N m−2 d−1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts.


Constraining the Source of Laterally Transported Organic Matter
In this study, we investigated lateral advection using a hydrodynamic model and remote sensing data products and found a significant association between mesoscale circulation and large-scale transport into the central GoM study region. Furthermore, estimated net lateral transport appears to balance observed nitrogen export-an export term otherwise unbalanced by in situ processes. Yet, without additional validation, the remote sensing and model products should not be used to constraint the composition of the source material from which the laterally transported nitrogen is derived. Whether this bioavailable nitrogen is sourced from (1) subsurface nitrate, (2) N2-fixation, or (3) terrestrial sources remains unresolved, yet several patterns are evident through this study and others.
Geographically, the source regions of the laterally advected nitrogen can be broadly identified based on a mean state approximation (Supplemental Figures 3-4). A substantial proportion of lateral transport is carried through the southern boundary of the control box, which is likely derived from entrainment and localized upwelling associated with the interaction of the Campeche Bank and Loop Applying an eddy detection algorithm 3 to NEMURO-GOM, net fluxes associated with eddies were small (mean: 44 µmol N m -2 d -1 ) relative to average net flux (1165 µmol N m -2 d -1 ). This, however, is likely due to our criteria for an eddy, which implies no lateral divergence (i.e. closed stream function) and thus excludes phenomenon such as filaments and jets that often form on the edges of eddies 4 .
Although mesoscale eddies, which are shed by the Loop Current 5,6 , only carry a small proportion (~4%) of the net lateral transport, they likely force surrounding flow fields 7 that may contribute significant flux over short durations (compared to the Loop Current). Nevertheless, additional data are necessary to determine both the delivery mechanisms and sourcing mechanisms responsible for the lateral N transport in the GoM.
Nitrogen isotopic signatures (i.e. δ 15 N) carry with them information about their original sources (e.g. subsurface nitrate: 2 -4 ‰ vs N2-fixation: -2 -0 ‰), albeit continuously modified by biological processes 8 . The mass-balance constraints for nitrogen and 15 N, as presented, are consistent with upwelled, laterally sourced nitrate 9 and the conclusion that N2-fixation is not substantial in these oligotrophic waters 10 . However, it is not possible to positively associate export material (3 -5‰) with a unique combination of end-members due to the wide range of isotopic signatures for riverine (6 -8‰) 11 , atmospheric (-5 -4‰) 12 , and biotic (-2 -0‰) sources of nitrogen, especially for end members with small relative contributions (such as atmospheric deposition in the GoM 9 ). However, without a significant source of sufficiently low δ 15 N (i.e. << 3‰), N from the northern GoM margin is unlikely be a significant source of organic matter to the oligotrophic GoM. Finally, previous studies have come to mixed conclusions on the degree of connectivity between the shelf environment and the pelagic GoM 2,6,13 .
Combined with the present questions regarding the source of the laterally transported N, process studies in the pelagic GoM are necessary to thoroughly investigate these shelf-basin interactions.

Implications for Vertical Connectivity
Given that the LEU was disproportionally responsible for nitrate uptake 14 , although supporting only 21 -38% of NPP, one might conclude that export production is centered within the LEZ and that export production may thus be supported by episodic fluxes of nitrate at depth. However, observations of the export flux strongly refute such a hypothesis. Particle flux out of the UEZ exceeded that of the LEZ implying that the LEZ as a zone of net remineralization and not of particle formation. This vertical partitioning of export production illustrates the potential problems of including the entire euphotic zone into a single mass-budget. Performing an identical mass-budget for the entire euphotic zone slightly reduces the substantial mismatch determined between local in situ nitrogen sources and sinks by averaging across areas of particle production and heterotrophic consumption. However, importantly, it does not change the overall conclusions regarding the role of lateral transport (Supplemental Figure 5).