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1. Dissolved Organic Nitrogen Concentration and d15N Distribution along a Zonal Transect in the South Pacific

   Liang, Z.; Marconi, D; Sigman, D.; Knapp, A. (April 2022, 2022 Ocean Sciences Meeting)

   Dissolved organic nitrogen (DON) is the dominant form of bioavailable nitrogen in the euphotic zone of subtropical gyres, where nitrate (NO3-) concentrations are low. However, the spatial distribution of DON production and consumption in the surface ocean remains poorly resolved due to the relatively narrow range in euphotic zone DON concentrations. Recently, the stable isotopic composition (d15N) of DON has been used to identify DON production and consumption in the surface ocean, making isotopic measurements a more sensitive indicator of DON cycling than concentration measurements alone. Here we report DON concentration and d15N measurements in the upper ~300 m from a zonal transect along ~30˚S in the South Pacific (GO-SHIP P06-2017), including samples in the Western South Pacific (154˚E-170˚W), in the oligotrophic South Pacific Subtropical Gyre (110˚W -170˚W), and overlying the Oxygen Deficient Zone (ODZ) in the east (78˚W-110˚W). We observed small variations in surface DON concentrations. Surface DON in Western South Pacific, oligotrophic South Pacific Subtropical Gyre and above the ODZ are 4.6±1.0 µM, 4.3±0.7 µM, and 4.8±0.5 µM, respectively. d15N of DON in the euphotic zone is lower in the west and higher in the east, consistent with distributions of nitrogen fixation and denitrification, respectively, in the South Pacific. Similar decreasing trend in DON d15N in the euphotic zone and subsurface nitrate d15N was observed from the east to the west in the South Pacific, suggesting the d15N in subsurface nitrate could be imprinted in the DON d15N in the euphotic zone. Low surface ocean DON d15N in the Western South Pacific (2.4±1.8 ‰) and oligotrophic South Pacific Subtropical Gyre (2.6±1.6 ‰) compared with surface ocean DON d15N above ODZ (5.4±2.3 ‰) infer significant low-d15N nitrogen is added to the western South Pacific and oligotrophic South Pacific Subtropical Gyre, potentially from N2 fixation. Additionally, high DON d15N at ~180˚ was consistent with entrainment of subsurface NO3- into surface waters due to shallow bathymetry. Together, these observations suggest that DON production and consumption processes operate on timescales adequately fast to produce isotopic gradients across the South Pacific. Comparisons of surface ocean DON d15N with subsurface nitrate d15N constrain the locations and timescales of these processes. 

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2. Constraining the sources of nitrogen fueling export production in the Gulf of Mexico using nitrogen isotope budgets

   https://doi.org/10.1093/plankt/fbab049  

   Knapp, Angela N; Thomas, Rachel K; Stukel, Michael R; Kelly, Thomas B; Landry, Michael R; Selph, Karen E; Malca, Estrella; Gerard, Trika; Lamkin, John (August 2021, Journal of Plankton Research)

   Moisander, Pia (Ed.)

   Abstract The availability of nitrogen (N) in ocean surface waters affects rates of photosynthesis and marine ecosystem structure. In spite of low dissolved inorganic N concentrations, export production in oligotrophic waters is comparable to more nutrient replete regions. Prior observations raise the possibility that di-nitrogen (N2) fixation supplies a significant fraction of N supporting export production in the Gulf of Mexico. In this study, geochemical tools were used to quantify the relative and absolute importance of both subsurface nitrate and N2 fixation as sources of new N fueling export production in the oligotrophic Gulf of Mexico in May 2017 and May 2018. Comparing the isotopic composition (“δ15N”) of nitrate with the δ15N of sinking particulate N collected during five sediment trap deployments each lasting two to four days indicates that N2 fixation is typically not detected and that the majority (≥80%) of export production is supported by subsurface nitrate. Moreover, no gradients in upper ocean dissolved organic N and suspended particulate N concentration and/or δ15N were found that would indicate significant N2 fixation fluxes accumulated in these pools, consistent with low Trichodesmium spp. abundance. Finally, comparing the δ15N of sinking particulate N captured within vs. below the euphotic zone indicates that during late spring regenerated N is low in δ15N compared to sinking N. 

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3. Dissolved organic phosphorus (DOP) distributions in the Eastern Indian Ocean and subtropical South Pacific Ocean

   Liang, Z.; Letscher, R.T.; McCabe, K.; Marconi, D.; Sigman, D.M.; Knapp, A.N. (February 2020, 2020 Ocean Sciences Meeting)

   Significant rates of export production and nitrogen fixation occur in oligotrophic gyres in spite of low inorganic nutrient concentrations in surface waters. Prior work suggests that dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are important nutrient sources when inorganic nutrients are scarce. In particular, DOP has been shown to be an important P source for diazotrophs, which may be better suited to using low concentrations of organic vs. inorganic P. Prior modeling work has also suggested that DOP is important for supporting export production in oligotrophic gyres. However, validation of such models is limited by the number of upper ocean DOP concentration measurements, especially in the South Pacific and Indian Oceans. Here, we present measurements of DOP concentration from the 2016 GO-SHIP I08S and I09N meridional transect in Eastern Indian Ocean, and DON and DOP concentration measurements from the 2017 GO-SHIP P06 zonal transect in the subtropical South Pacific Ocean. Together with DOC and DON concentration measurements from prior occupations of the same GO-SHIP lines we evaluated changes in euphotic zone DOC:DON:DOP stoichiometry. Stoichiometry changes across these two transects are used to infer regions of preferential DON and/or DOP production and consumption. Specifically, north of 36 S in the Indian Ocean an increase in DOC:DON and DOC:DOP concentration ratios, from 11:1 to 14:1 and 118:1 to 190:1, respectively, are observed. Similarly, west of 136 W in the South Pacific Ocean significant increases in DOC:DOP and DON:DOP concentration ratios are observed, from 224:1 to 398:1 and 21:1 to 39:1, respectively. These stoichiometric shifts in upper ocean DOC:DON:DOP concentration ratios are considered in the context of ocean circulation, especially upwelling patterns in the Indian and eastern Pacific Oceans, as well as prior observations of the distribution of nitrogen fixation, especially in the western tropical South Pacific. 

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4. Quantifying N2 fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets

   https://doi.org/10.3389/fmars.2023.1249115  

   Forrer, Heather J.; Bonnet, Sophie; Thomas, Rachel K.; Grosso, Olivier; Guieu, Cecile; Knapp, Angela N. (October 2023, Frontiers in Marine Science)

   The spatial distribution of marine di-nitrogen (N₂) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived δ¹⁵N budgets, we quantified rates of N₂fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (<300 m) hydrothermal vents located along the arc provide significant dissolved iron to the euphotic zone, stimulating N₂fixation. Here we compare measurements of water column δ¹⁵N_NO3+NO2with sinking particulate δ¹⁵N collected by short-term sediment traps deployed at 170 m and 270 m at stations in close proximity to subsurface hydrothermal activity, and the δ¹⁵N of N₂fixation. Results from the δ¹⁵N budgets yield high geochemically-based N₂fixation rates (282 to 638 µmol N m^-2d^-1) at stations impacted by hydrothermal activity, supporting 64 to 92% of export production in late spring. These results are consistent with contemporaneous¹⁵N₂uptake rate estimates and molecular work describing highTrichodesmiumspp. and other diazotroph abundances associated with elevated N₂fixation rates. Further, the δ¹⁵N of sinking particulate N collected at 1000 m over an annual cycle revealed sinking fluxes peaked in the summer and coincided with the lowest δ¹⁵N, while lower winter sinking fluxes had the highest δ¹⁵N, indicating isotopically distinct N sources supporting export seasonally, and aligning with observations from most other δ¹⁵N budgets in oligotrophic regions. Consequently, the significant regional N₂fixation input to the late spring/summer Western Tropical South Pacific results in the accumulation of low-δ¹⁵N_NO3+NO2in the upper thermocline that works to lower the elevated δ¹⁵N_NO3+NO2generated in the oxygen deficient zones in the Eastern Tropical South Pacific. 

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5. On the Variability of Equatorial Pacific Nitrate and Iron Utilization

   https://doi.org/10.5670/oceanog.2024.411  

   Rafter, Patrick (January 2024, Oceanography)

   The tropical Pacific is one of the largest ocean regions on Earth where the trace element iron limits new primary production and therefore the efficiency of carbon export to the deep sea. Although there is a long history of marine biogeochemical research in the tropical Pacific, recent advancements using GEOTRACES key parameters such as iron and nitrate isotopes (nitrate δ15N and δ18O) make this a good time to review the current understanding of tropical Pacific nitrate dynamics—how both regional subsurface nitrate characteristics and surface ocean nitrate utilization change with time. While this article provides a comprehensive overview of the biological, chemical, and physical processes shaping equatorial Pacific subsurface-to-surface nutrients, it principally explores the findings from the first nitrate isotope time series in iron-limited high nutrient, low chlorophyll waters. Results indicate that the preferential recycling of bioavailable iron within the euphotic zone is required to explain even the lowest observed nitrate utilization in the eastern equatorial Pacific (EEP). Furthermore, because seasonal-to-interannual nitrate utilization variability in the EEP cannot be driven by changes in iron supply, this work argues that iron recycling (and therefore bioavailable iron) is modulated by upwelling rate changes, creating a predicted and recently observed spectrum of iron limitation in the iron-limited EEP surface waters. In other words, upper ocean physics overwhelmingly dominates seasonal-to-interannual nitrate utilization in the iron-​limited EEP. This new understanding of nitrate utilization in iron-limited waters helps to explain long-term changes in past equatorial Pacific nitrate utilization obtained via sedimentary proxy records and potentially complicates the efficacy of future iron fertilization of the equatorial Pacific. 

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