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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. Nitrification and Nitrous Oxide Production in the Offshore Waters of the Eastern Tropical South Pacific

   https://doi.org/10.1029/2020GB006716  

   Santoro, Alyson E. ; Buchwald, Carolyn ; Knapp, Angela N. ; Berelson, William M. ; Capone, Douglas G. ; Casciotti, Karen L. ( February 2021 , Global Biogeochemical Cycles)

   Marine oxygen deficient zones 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. On each cruise, high‐resolution measurements of ammonium ([NH₄⁺]), nitrite ([NO₂⁻]), and N₂O were combined with¹⁵N 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 archaealamoAgenes and ammonia oxidation rates and betweenNitrospina‐like 16S rRNA genes and nitrite oxidation rates. N₂O concentrations in the upper oxycline reached values of >140 nM, even at the western extent of the cruise track, supporting air‐sea fluxes of up to 1.71 μmol m⁻² day⁻¹. Our results suggest that a source of NO₂⁻other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore Eastern Tropical South Pacific and that air‐sea fluxes of N₂O from this region may be higher than previously estimated.

    

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3. 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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4. Nitrogen and Isotope Flows Through the Costa Rica Dome Upwelling Ecosystem: The Crucial Mesozooplankton Role in Export Flux

   https://doi.org/10.1029/2018GB005968  

   Stukel, Michael R. ; Décima, Moira ; Landry, Michael R. ; Selph, Karen E. ( December 2018 , Global Biogeochemical Cycles)

   The Costa Rica Dome (CRD) is an open‐ocean upwelling ecosystem, with high biomasses of picophytoplankton (especiallySynechococcus), mesozooplankton, and higher trophic levels. To elucidate the food web pathways supporting the trophic structure and carbon export in this unique ecosystem, we used Markov Chain Monte Carlo techniques to assimilate data from four independent realizations of δ¹⁵N and planktonic rate measurements from the CRD into steady state, multicompartment ecosystem box models (linear inverse models). Model results present well‐constrained snapshots of ecosystem nitrogen and stable isotope fluxes. New production is supported by upwelled nitrate, not nitrogen fixation. Protistivory (rather than herbivory) was the most important feeding mode for mesozooplankton, which rely heavily on microzooplankton prey. Mesozooplankton play a central role in vertical nitrogen export, primarily through active transport of nitrogen consumed in the surface layer and excreted at depth, which comprised an average 36–46% of total export. Detritus or aggregate feeding is also an important mode of resource acquisition by mesozooplankton and regeneration of nutrients within the euphotic zone. As a consequence, the ratio of passively sinking particle export to phytoplankton production is very low in the CRD. Comparisons to similar models constrained with data from the nearby equatorial Pacific demonstrate that the dominant role of vertical migrators to the biological pump is a unique feature of the CRD. However, both regions show efficient nitrogen transfer from mesozooplankton to higher trophic levels (as expected for regions with large fish, cetacean, and seabird populations) despite the dominance of protists as major grazers of phytoplankton.

    

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5. Quantifying Nitrous Oxide Cycling Regimes in the Eastern Tropical North Pacific Ocean With Isotopomer Analysis

   https://doi.org/10.1029/2020GB006637  

   Kelly, Colette L. ; Travis, Nicole M. ; Baya, Pascale A. ; Casciotti, Karen L. ( February 2021 , Global Biogeochemical Cycles)

   Nitrous oxide (N₂O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N₂O cycling in an ODZ, we analyzed N₂O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N₂O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 ± 20.5 nM at the coast. Above theσ_θ = 25.0 kg/m³isopycnal, Keeling plot analysis revealed two sources to the near‐surface N₂O maximum, with different δ¹⁵N₂O^αand δ¹⁵N₂O^βvalues, but each with a site preference (SP) of 6‰–8‰. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%–26% nitrification (bacterial or archeal), and 74%–83% denitrification (or nitrifier‐denitrification). Below theσ_θ = 25.0 kg/m³isopycnal, box model analysis indicated that the observed 46‰–50‰ SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N₂O production by denitrification, and may indicate instead a transient net consumption of N₂O. Furthermore, time‐dependent model results indicated that while δ¹⁵N₂O^αand δ¹⁸O‐N₂O reflect both N₂O production and consumption in the anoxic core of the ODZ, δ¹⁵N₂O^βpredominantly reflects N₂O sources. Finally, we infer that the high (N₂O) observed at some stations derive from a set of conditions supporting high rates of N₂O production that have not been previously encountered in this region.

    

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