On the following subject: "Fate of nitrogen fixation and carbon export in the south-western tropical Pacific Ocean".
Friday 21 December 2018 at 9:00 am; Amphithéâtre OCEANOMED, Bâtiment 26M, 163 avenue de Luminy, 13009 Marseille
Thesis supervisor: Sophie Bonnet, Director of Research IRD/MIO
Thesis co-supervisor: Thierry Moutin, Professor AMU/MIO
This thesis work, carried out as part of the OUTPACE project, focuses on the quantification of diazotrophy and its influence on biogeochemical cycles in the south-western tropical Pacific surface ocean, a region that has been particularly under-sampled to date. The objectives of this work were (1) to quantify N2 fixation and identify the main players in diazotrophy in this region, (2) to assess the influence of N2 fixation on primary production and carbon export, and (3) to identify the transfer pathways for fixed nitrogen in the planktonic trophic chain.
It has been shown that the south-western tropical Pacific region is a hot spot for N2 fixation, with average rates of 570 µmol N m-2 d-1. In the region, diazotrophy could represent a source of 16 Tg N yr-1, thus contributing around 15 % of the nitrogen input by diazotrophy on a global scale. Two distinct sub-regions were identified during the OUTPACE campaign (transect from New Caledonia to French Polynesia). To the west, the oligotrophic waters of the Melanesian archipelagos had high N2 fixation rates (600 µmol N m-2 d-1) and the diazotrophic community was dominated by Trichodesmium. To the east, the ultra-oligotrophic waters of the South Pacific gyre had lower N2 fixation rates ( 90 µmol N m-2 d-1) and the diazotrophic community was dominated by UCYN-B. Along the OUTPACE transect, N2 fixation rates were positively correlated with primary production, bacterial production, picoplankton abundance and heterotrophic bacteria.
Nitrogen balances associated with a Lagrangian strategy carried out at three stations representative of the region show that N2 fixation contributed more than 90 % of new nitrogen inputs to the euphotic layer, greater than nitrate inputs by turbulent vertical diffusion (1-8 %) and atmospheric inputs (< 1.5 %). N2 fixation therefore supported almost all new primary production, 13-18 % of total primary production in the Melanesian archipelago region and 3 % west of the South Pacific gyre. The e-ratio (particulate carbon export / primary production) measured in the region (up to 9.7 %) is much higher than those measured in other oligotrophic regions impacted by diazotrophy, and demonstrates the region's significant export capacity. Diazotrophic organisms present in the water column were not efficiently exported to the deep ocean, suggesting that most of the export was indirect, following the transfer of fixed nitrogen to non-diazotrophic planktonic organisms and their subsequent export.
A study of the transfer pathways for fixed nitrogen shows that between 7 and 15 % of total N2 fixation was transferred to non-diazotrophic organisms. The main beneficiaries of this nitrogen were autotrophic picoplankton (Synechococcus, Prochlorococcus) followed by heterotrophic bacteria. In addition, the transfer of fixed nitrogen to zooplankton, which has been little studied to date, represented 8±5 % of total N2 fixation when Trichodesmium dominated and 28±7 % when UCYN-B dominated.
This thesis work demonstrates that diazotrophy supports the biological pump in the south-western tropical Pacific Ocean, and that it can play a determining role in the structure of planktonic communities and the biogeochemical cycles of carbon and nitrogen in oligotrophic regions.
Key words: Nitrogen fixation, release, transfer, particulate export, biological pump, biogeochemical cycles