Coastal N2 Fixation Rates Coincide Spatially With Nitrogen Loss in the Humboldt Upwelling System off Peru

• Published in: Global biogeochemical cycles Vol. 37; no. 2
• Main Authors: Kittu, Leila R., Paul, Allanah J., Fernández‐Méndez, Mar, Hopwood, Mark J., Riebesell, Ulf
• Format: Journal Article
• Language: English
• Published: 01.02.2023
• Subjects: Eastern Tropical South Pacific; marine nitrogen cycling; nitrogen fixation
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2022GB007578

Marine nitrogen (N2) fixation supports significant primary productivity in the global ocean. However, in one of the most productive regions of the world ocean, the northern Humboldt Upwelling System (HUS), the magnitude and spatial distribution of this process remain poorly characterized. This study presents a spatially resolved data set of N2 fixation rates across six coastal transects of the northern HUS off Peru (8°S–16°S) during austral summer. N2 fixation rates were detected throughout the waters column including within the Oxygen Minimum Zone (OMZ) between 12°S and 16°S. N2 fixation rates were highest where the subsurface OMZ (O2 < 20 μmol L−1) was most intense and estimated nitrogen (N) loss was highest. There, rates were measured throughout the water column. Hence, the vertical and spatial distribution of rates indicates a colocation of N2 fixation with N loss in the coastal productive waters of the northern HUS. Despite high phosphate and total dissolvable iron (TdFe) concentrations throughout the study area, N2 fixation was still generally low (1.19 ± 3.81 nmol L−1 d−1) and its distribution could not be directly explained by these two factors. Our results suggest that the distribution was likely influenced by a complex interplay of environmental factors including phytoplankton biomass and organic matter availability, and potentially iron, or other trace metal (co)‐limitation of both N2 fixation and primary production. In general, our results support previous conclusions that N2 fixation in the northern HUS plays a minor role as a source of new N and to replenish the regional N loss. Plain Language Summary High phytoplankton productivity in the Humboldt Upwelling System (HUS) is underpinned by a rich supply of nutrients, such as nitrate, brought to shelf surface waters from depth. However, marine microbes use up some of the nitrate in low‐oxygen waters, converting it back to nitrogen gas (N2). Future climate change projections indicate that nitrate availability may decline in the surface ocean. Less phytoplankton growth in the HUS due to reduced nitrate supply could impact ocean services such as fish production and biological carbon drawdown. Previous studies hypothesize that biological nitrogen fixation by microbes could supply nitrogen and is present in the HUS. Here, we studied the distribution and amount of nitrogen fixation in this region in relation to environmental conditions to better understand if the inputs and losses of nitrogen are balanced. Our results indicate that nitrogen fixation rates are too low to counterbalance the local nitrogen loss and do not contribute significantly to the nitrogen supply for phytoplankton growth. The availability of phosphate and iron is thought to control biological N2 fixation rates on a global scale, but within the HUS, we are unable to find evidence for this suggesting that other environmental factors, such as organic matter availability, control biological nitrogen fixation. Key Points A north‐to‐south pattern in N2 fixation rates was observed, implying increased N turnover between 12°S and 16°S where N loss was pronounced The highest N2 fixation rates were measured in coastal productive waters above and within the Oxygen Minimum Zone, showing no clear relationship with Fe or P The magnitude of N2 fixation was low compared to predictions, estimated to account for ∼0.3% of primary production and <2% of local N loss