Inorganic nitrogen is a key nutrient limiting primary production in many marine ecosystems. In the highly productive Humboldt Upwelling System (HUS) off the coast of Peru and Chile, productivity is thought to scale with the availability of inorganic nitrogen (N) brought to the surface via wind-driven upwelling onto the shelf (also see CUSCO project ). However on its way up to the sunlit surface ocean significant bioavailable N is lost through denitrification and anaerobic ammonium oxidation (anammox) in oxygen deficient subsurface waters (OMZ = oxygen minimum zone). To replenish bioavailable nitrogen lost from the water column, N can be gained through biological N2 fixation by diazotrophic microbes.
The highest N2 fixation rates measured so far have been in shallow coastal waters, directly above oxygen deficient waters, suggesting a close spatial link between loss and gain of fixed N (Fig. 1 Pathway 1, Meyer et al. 2017, Paul et al. in prep). Some studies have suggested that a close proximity between N2 fixation fuelling organic matter production in the surface ocean and its remineralisation in the underlying OMZ would lead to a ‘runaway feedback’ in the N cycle. Due to a stoichiometric imbalance, denitrification of newly fixed nitrogen enhances N loss which further stimulates more N2 fixation . This self-sustaining feedback between N2 fixation and denitrification may constitute a tipping point in primary productivity for the Humboldt Upwelling System (Fig. 1).
To allow for stabilising feedbacks to occur, the newly fixed N would need to be stored in less bioavailable forms such as dissolved organic nitrogen (Fig. 1, Pathway 2) or the N2 fixation and associated organic matter production need to be spatially separated from regions of denitrification (Fig. 1, Pathway 3). Due to a scarce dataset on N2 fixation in the HUS it is currently difficult to accurately assess its magnitude, temporal and spatial variability and potential feedbacks on N turnover in the region. Through spatially and temporally resolved surveys of the Humboldt Upwelling system in collaboration with Peruvian scientists we aim to i) better understand the dynamics of N gain processes and the responsible microbes in the region and ii) assess the three possible pathways and the potential biogeochemical and ecological tipping point in the HUS.