Assessing the biogeochemical role and controls of nitrogen fixation in the Eastern Tropical South Pacific

The Eastern Tropical South Pacific (ETSP), hosts an extensive Oxygen Minimum Zone (OMZ) in the water column which has a major imprint on local and global marine biogeochemistry. Due to the low oxygen conditions within the OMZ, microbial processes of nitrogen (N) loss, such as anammox and denitrification are sustained in the water column. These processes result in a pronounced N deficit which reduces bioavailable N for primary productivity and thus influences fisheries production in the region. To maintain a balanced marine N inventory regionally in ETSP, the N deficit would have to be compensated by N inputs via upwelling or N2 fixation. A classical assumption is that N2 fixation is favoured by iron (Fe) availability and a surplus of inorganic phosphate relative to inorganic nitrogen (this relativity is defined as P*), both conditions are present in the ETSP. Over the past decades, this assumption has been integrated into most coupled circulation and N-cycle biogeochemical models. These models indicate that there is a close spatial link between areas of high N loss, generally confined to OMZs and N2 fixation. On the contrary, other biogeochemical models have revealed that a close spatial link between N loss and N2 fixation in OMZ areas may give rise to run-away loss of fixed N in the ETSP, ultimately destabilizing the regional marine N inventory. While N loss processes are relatively well understood in the ETSP, the lack of a comprehensive dataset that resolves N2 fixation rates in both space and time constraints an accurate assessment of the regional marine N inventory, potential feedback mechanisms, and their impact on N turnover and productivity. Therefore, the main objective of this doctoral dissertation was to investigate the spatial distribution of N2 fixation relative to N loss in the ETSP, in order to understand potential feedbacks in the regional N cycle.

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