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1. Geochemical signatures of nitrogen fixation in the western subtropical South Pacific: d15N budgets and low-d15N DON

   Forrer, H. ; Bonnet, S. ; Thomas, R. ; Grosso, O. ; Guieu, C. ; Knapp, A.N. ( April 2022 , 2022 Ocean Sciences Meeting)

   The western subtropical South Pacific (WSSP) has recently been found to support high rates of di-nitrogen (N2) fixation in association with shallow hydrothermal iron fluxes. While previous 15N2 uptake and short-term d15N budgets have found that high rates of N2 fixation contribute significantly to export production, no longer-term evaluations of N2 fixation’s role in supporting the regional ecosystem were available. Here we present results of an annual d15N budget using the d15N of sinking particles captured in a moored sediment trap deployed at 1000 m from Nov 2019 - Nov 2020. We compare the d15N of the particles collected over this annual cycle with the d15N of subsurface nitrate to evaluate the seasonal and annual importance of N2 fixation for supporting export production. The results indicate that N2 fixation supported up to ~20% of annual export and that N2 fixation was most important during the summer. Notably, the d15N of subsurface nitrate at the trap station was low, 2 to 3 per mil compared to stations further from the vents. We also present some of the region’s first dissolved organic nitrogen (DON) d15N data. The DON samples collected in Nov 2019 and Nov 2020 show similar DON concentrations and d15N between years. However, while DON concentrations in the WSSP, 5 +/- 1 uM, were similar to the eastern tropical South Pacific (ETSP), the d15N of DON in the upper 100 m in the WSSP was between 2 to 4 per mil, which is lower than the ETSP, where DON d15N was between 4 to 6 per mil. Together, the results of the annual d15N budget as well as the low-d15N DON provide a longer-term perspective on the significance of N2 fixation in the WSSP. Additionally, the results suggest that N2 fixation in the WSSP introduces significant low-d15N N to the ocean, offsetting the elevated d15N generated in the oxygen deficient zones of the eastern tropical Pacific. 

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

   https://doi.org/10.1038/s41467-021-23678-9  

   Kelly, Thomas B. ; Knapp, Angela N. ; Landry, Michael R. ; Selph, Karen E. ; Shropshire, Taylor A. ; Thomas, Rachel K. ; Stukel, Michael R. ( June 2021 , Nature Communications)

   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⁻²d⁻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⁻²d⁻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.

    

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3. 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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4. 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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5. Dissolved Organic Nitrogen Cycling in the South China Sea From an Isotopic Perspective

   https://doi.org/10.1029/2020GB006551  

   Zhang, Run ; Wang, Xingchen T. ; Ren, Haojia ; Huang, Jie ; Chen, Min ; Sigman, Daniel M. ( December 2020 , Global Biogeochemical Cycles)

   Dissolved organic nitrogen (DON) is the dominant form of fixed nitrogen in most low and middle latitude ocean surface waters. Here, we report measurements of DON isotopic composition (δ¹⁵N) from the west South China Sea (SCS), with the goal of providing new insight into DON cycling. The concentration of DON in the surface ocean is correlated (r = 0.70,p < 0.0001) with chlorophyllaconcentration, indicating DON production in these surface waters. The concentration and δ¹⁵N of DON fall in a relatively narrow range in the surface ocean (4.6 ± 0.1 μM and 4.3 ± 0.2‰ vs. air, respectively; ±SD), similar to other ocean regions. The mean DON δ¹⁵N above 50 m (4.5 ± 0.3‰) is similar to the δ¹⁵N of nitrate in the “shallow subsurface” (i.e., immediately below the euphotic zone; 4.6 ± 0.2‰) but is higher than the δ¹⁵N of suspended particles in the surface ocean (~2.3‰). This set of isotopic relationships has been observed previously (e.g., in the oligotrophic North Atlantic and North Pacific) and can be explained by the cycling of N between particulate organic nitrogen (PON), DON, and ammonium, in which an isotope effect associated with DON degradation preferentially concentrates¹⁵N in DON. Consistent with this view, a negative correlation (r = 0.70) between the concentration and the δ¹⁵N of DON is observed in the upper 75 m, suggesting an isotope effect of ~4.9 ± 0.4‰ for DON degradation. Comparing the DON δ¹⁵N data from the SCS with other regions, we find that the δ¹⁵N difference between euphotic zone DON and shallow subsurface nitrate δ¹⁵N (Δδ¹⁵N_(DON‐NO3)) rises from ocean regions of inferred net DON production to regions of net DON consumption, with the SCS representing an intermediate case.

    

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