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1. Multiple metabolisms constrain the anaerobic nitrite budget in the Eastern Tropical South Pacific: Nitrogen Dynamics in the Eastern Tropical South Pacific

   https://doi.org/10.1002/2016GB005407  

   Babbin, Andrew R. ; Peters, Brian D. ; Mordy, Calvin W. ; Widner, Brittany ; Casciotti, Karen L. ; Ward, Bess B. ( January 2017 , Global Biogeochemical Cycles)
2. Resurfacing of South Pacific Tropical Water in the Equatorial Pacific and Its Variability Associated with ENSO

   https://doi.org/10.1175/JPO-D-16-0078.1  

   Qu, Tangdong ; Gao, Shan ( May 2017 , Journal of Physical Oceanography)
3. Role of Stochastic Atmospheric Forcing from the South and North Pacific in Tropical Pacific Decadal Variability

   https://doi.org/10.1175/JCLI-D-18-0536.1  

   Sun, Tianyi ; Okumura, Yuko M. ( July 2019 , Journal of Climate)

   Stochastic variability of internal atmospheric modes, known as teleconnection patterns, drives large-scale patterns of low-frequency SST variability in the extratropics . To investigate how the decadal component of this stochastically driven variability in the South and North Pacific affects the tropical Pacific and contributes to the observed basinwide pattern of decadal variability, a suite of climate model experiments was conducted . In these experiments, the models are forced with constant surface heat flux anomalies associated with the decadal component of the dominant atmospheric modes, particularly the Pacific–South American (PSA) and North Pacific Oscillation (NPO) patterns . Both the PSA and NPO modes induce basinwide SST anomalies in the tropical Pacific and beyond that resemble the observed interdecadal Pacific oscillation . The subtropical SST anomalies forced by the PSA and NPO modes propagate to the equatorial Pacific mainly through the wind–evaporation–SST feedback . This atmospheric bridge is stronger from the South Pacific than the North Pacific due to the northward displacement of the intertropical convergence zone and the associated northward advection of momentum anomalies. The equatorial ocean dynamics is also more strongly influenced by atmospheric circulation changes induced by the PSA mode than the NPO mode. In the PSA experiment, persistentmore »and zonally coherent wind stress curl anomalies over the South Pacific affect the zonal mean depth of the equatorial thermocline and weaken the equatorial SST anomalies resulting from the atmospheric bridge. This oceanic adjustment serves as a delayed negative feedback and may be important for setting the time scales of tropical Pacific decadal variability.

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4. Dynamics and controls of heterotrophic prokaryotic production in the western tropical South Pacific Ocean: links with diazotrophic and photosynthetic activity

   https://doi.org/10.5194/bg-15-2669-2018  

   Van Wambeke, France ; Gimenez, Audrey ; Duhamel, Solange ; Dupouy, Cécile ; Lefevre, Dominique ; Pujo-Pay, Mireille ; Moutin, Thierry ( January 2018 , Biogeosciences)

   Abstract. Heterotrophic prokaryotic production (BP) was studied in the western tropical South Pacific (WTSP) using the leucine technique, revealing spatial and temporal variability within the region. Integrated over the euphotic zone, BP ranged from 58 to 120mg Cm⁻²d⁻¹ within the Melanesian Archipelago, and from 31 to 50mg Cm⁻²d⁻¹ within the western subtropical gyre. The collapse of a bloom was followed during 6 days in the south of Vanuatu using a Lagrangian sampling strategy. During this period, rapid evolution was observed in the three main parameters influencing the metabolic state: BP, primary production (PP) and bacterial growth efficiency. With N₂ fixation being one of the most important fluxes fueling new production, we explored relationships between BP, PP and N₂ fixation rates over the WTSP. The contribution of N₂ fixation rates to bacterial nitrogen demand ranged from 3 to 81%. BP variability was better explained by the variability of N₂ fixation rates than by that of PP in surface waters of the Melanesian Archipelago, which were characterized by N-depleted layers and low DIP turnover times (T_DIP<100h). This is consistent with the fact that nitrogen was often one of the main factors controllingmore »BP on short timescales, as shown using enrichment experiments, followed by dissolved inorganic phosphate (DIP) near the surface and labile organic carbon deeper in the euphotic zone. However, BP was more significantly correlated with PP, but not with N₂ fixation rates where DIP was more available (T_DIP>100h), deeper in the Melanesian Archipelago, or within the entire euphotic zone in the subtropical gyre. The bacterial carbon demand to gross primary production ratio ranged from 0.75 to 3.1. These values are discussed in the framework of various assumptions and conversion factors used to estimate this ratio, including the methodological errors, the daily variability of BP, the bacterial growth efficiency and one bias so far not considered: the ability for Prochlorococcus to assimilate leucine in the dark.

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5. Aphotic N₂ fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean

   https://doi.org/10.5194/bg-15-3107-2018  

   Benavides, Mar ; Shoemaker, Katyanne M. ; Moisander, Pia H. ; Niggemann, Jutta ; Dittmar, Thorsten ; Duhamel, Solange ; Grosso, Olivier ; Pujo-Pay, Mireille ; Hélias-Nunige, Sandra ; Fumenia, Alain ; et al ( January 2018 , Biogeosciences)

   Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N₂) fixation. Here, as in other marine environments across the oceans, N₂ fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N₂ fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N₂ fixation in the WTSP, we measured N₂ fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N₂ fixation in the aphotic ocean. N₂ fixation rates were low (average 0.63±0.07nmolNL⁻¹d⁻¹) but consistently detected across all depths and stations, representing ∼ 6–88% of photic N₂ fixation. N₂ fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subclustermore »1G dominated at 650dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N₂ fixation activity at all depths sampled during our study suggest that aphotic N₂ fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.

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