Dissolved organic nitrogen (DON) and phosphorus (DOP) are potential nutrient sources to sustain productivity in the oligotrophic ocean where inorganic nutrient concentrations are low. Variations in the carbon(C):nitrogen(N):phosphorus(P) stoichiometry of surface ocean dissolved organic matter (DOM) can trace patterns of DON and DOP production and consumption, however, concurrent dissolved organic carbon (DOC), DON, and DOP concentration observations are limited. Using new global ocean DOM concentration datasets, we develop inverse DOC and DON models to obtain global ocean DOC and DON concentration fields and associated biogeochemical fluxes. Including autotrophic DON uptake improves the model fit to observations. Combining our modeled DOC and DON concentration fields with a global ocean DOP concentration field from our previous inverse DOP model, we obtain a modeled global ocean DOM stoichiometry field. We further evaluate the lateral transport of semi-labile DON (SLDON) and semi-labile DOP (SLDOP) to the oligotrophic low latitudes (15˚to 40˚) and identify the equatorial Pacific and Atlantic as important sources of SLDON and SLDOP. We also quantify the preferential loss of DON and DOP relative to DOC from the surface to 500 m, which, with physical circulation, may retain nutrients in the gyres, further enhancing productivity. Our findings highlight two modes by which DON and DOP serve as organic nutrient sources to sustain productivity in the oligotrophic low latitudes, with lateral transport more important and capable of supporting ~6 to 15% of export production in these regions.

Surface ocean marine dissolved organic matter (DOM) serves as an important reservoir of carbon (C), nitrogen (N), and phosphorus (P) in the global ocean, and is produced and consumed by both autotrophic and heterotrophic communities. While prior work has described distributions of dissolved organic carbon (DOC) and nitrogen (DON) concentrations, our understanding of DOC:DON:DOP stoichiometry in the global surface ocean has been limited by the availability of DOP concentration measurements. Here we estimate mean surface ocean bulk and labile DOC:DON:DOP stoichiometry in biogeochemically and geographically defined regions, using newly available marine DOM concentration databases. Global mean surface ocean bulk (C:N:P = 387:26:1) and labile (C:N:P = 179:20:1) DOM stoichiometries are higher than Redfield stoichiometry, with labile DOM stoichiometry similar to that of global mean surface ocean particulate organic matter (C:N:P = 160:21:1) reported in a recent compilation. DOM stoichiometry varies across ocean basins, ranging from 251:17:1 to 638:43:1 for bulk and 83:15:1 to 414:49:1 for labile DOM C:N:P, respectively. Surface ocean DOP exhibits larger relative changes than DOC and DON, driving surface ocean gradients in DOC:DON:DOP stoichiometry. Inferred autotrophic consumption of DOP helps explain intra- and inter-basin patterns of marine DOM C:N:P stoichiometry, with regional patterns of water column denitrification and iron supply influencing the biogeochemical conditions favoring DOP use as an organic nutrient. Specifically, surface ocean marine DOM exhibits increasingly P-depleted stoichiometries from east to west in the Pacific and from south to north in the Atlantic consistent with patterns of increasing P stress and alleviated iron stress, respectively.

Marine oxygen deficient zones (ODZs) are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of these regions, including production of the greenhouse gas nitrous oxide (N2O). We present here the results of two oceanographic cruises investigating nitrification, nitrifying microorganisms, and N2O production and distribution from the offshore waters of the Eastern Tropical South Pacific (ETSP). On each cruise, high-resolution measurements of ammonium ([NH4+]), nitrite ([NO2-]), and N2O were combined with 15N tracer-based determination of ammonia oxidation, nitrite oxidation, nitrate reduction and N2O production rates. Depth-integrated inventories of NH4+ and NO2- were positively correlated with one another, and with depth-integrated primary production. Depth-integrated ammonia oxidation rates were correlated with sinking particulate organic nitrogen flux but not with primary production; ammonia oxidation rates were undetectable in trap-collected sinking particulate material. Nitrite oxidation rates exceeded ammonia oxidation rates at most mesopelagic depths. We found positive correlations between archaeal genes and ammonia oxidation rates and between -like 16S rRNA genes and nitrite oxidation rates. N2O concentrations in the upper oxycline reached values of greater than 140 nM, even at the western extent of the cruise track, supporting air-sea fluxes of up to 1.71 umol m-2 d-1. Our results suggest that a source of N2O other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore ETSP and that air-sea fluxes of N2O from this region may be higher than previously estimated.

Marine dissolved organic phosphorus (DOP) can serve as an organic nutrient to marine autotrophs, helping to sustain a portion of annual net community production (ANCP). Numerical models of ocean circulation and biogeochemistry have diagnosed the magnitude of this process at regional to global scales but have thus far been validated against DOP observations concentrated within the Atlantic basin. Here we assimilate a new marine DOP dataset with global coverage to optimize an inverse model of the ocean phosphorus cycle to investigate the regionally variable role of marine DOP utilization by autotrophs contributing to ANCP. We find ~25% of ANCP accumulates as DOP with a regionally variable pattern ranging from 8 – 50% across nine biomes investigated. Estimated mean surface ocean DOP lifetimes of ~0.5 – 2 years allow for transport of DOP from regions of net production to net consumption in subtropical gyres. Globally, DOP utilization by autotrophs sustains ~14% (0.9 Pg C yr-1) of ANCP with regional contributions as large as ~75% within the oligotrophic North Atlantic and North Pacific. Shallow export and remineralization of DOP within the ocean subtropics contributes ~30 – 80% of phosphate regeneration within the upper thermocline (< 300 m). These shallow isopycnals beneath the subtropical gyres harboring the preponderance of remineralized DOP outcrop near the poleward edge of each gyre, which when combined with subsequent lateral transport equatorward by Ekman convergence, provide a shallow overturning loop retaining phosphorus within the subtropical biome, likely helping to sustain gyre ANCP over multi-annual to decadal timescales.