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Nitrite is a key intermediate during fixed nitrogen loss in the ocean, and it accumulates within marine Oxygen Deficient Zones (ODZ). ODZs are vast subsurface regions where nitrate is the dominant electron acceptor, and these regions host approximately 50% of the fixed nitrogen loss in the world's oceans. Nitrite accumulates in these waters, and recent research has discovered substantial reoxidation of nitrite back to nitrate, a significant process in the global nitrogen cycle. Partitioning between reduction and oxidation determines if marine fixed nitrogen is lost or recycled. Investigations into nitrite oxidation typically rely on results from incubations, which limits the spatiotemporal sampling coverage. Using basin‐scale data, we analyzed the ratios of nutrient regeneration within the three water masses that feed the Eastern Tropical North Pacific (ETNP) ODZ. Deviations in the ratios of nutrient regeneration from Redfield stoichiometry indicated prolific nitrite reoxidation across this region. We estimate that 79 ± 7% of the nitrite produced in the ODZ between the 26.2 and 26.4 kg m⁻³isopycnals is reoxidized, whereas 54 ± 2% of the nitrite produced between the 26.7 and 26.9 kg m⁻³isopycnals is reoxidized. Our analysis also illustrates discrete “metabolic switching points” from primarily aerobic to primary anaerobic processes, which occur in each water mass. We applied water mass analysis to repeat cruises on the WOCE P18 line from Baja California to 10°N, which revealed high spatiotemporal variability in nitrite reoxidation. These results confirm previous measurements of significant fixed nitrogen recycling across the ETNP; however, our analysis enables high‐resolution estimates of this process.

Models and observations suggest that particle flux attenuation is lower across the mesopelagic zone of anoxic environments compared to oxic environments. Flux attenuation is controlled by microbial metabolism as well as aggregation and disaggregation by zooplankton, all of which shape the relative abundance of differently sized particles. Observing and modeling particle spectra can provide information about the contributions of these processes. We measured particle size spectrum profiles at one station in the oligotrophic Eastern Tropical North Pacific Oxygen Deficient Zone (ETNP ODZ) using an underwater vision profiler (UVP), a high‐resolution camera that counts and sizes particles. Measurements were taken at different times of day, over the course of a week. Comparing these data to particle flux measurements from sediment traps collected over the same time‐period allowed us to constrain the particle size to flux relationship, and to generate highly resolved depth and time estimates of particle flux rates. We found that particle flux attenuated very little throughout the anoxic water column, and at some time points appeared to increase. Comparing our observations to model predictions suggested that particles of all sizes remineralize more slowly in the ODZ than in oxic waters, and that large particles disaggregate into smaller particles, primarily between the base of the photic zone and 500 m. Acoustic measurements of multiple size classes of organisms suggested that many organisms migrated, during the day, to the region with high particle disaggregation. Our data suggest that diel‐migrating organisms both actively transport biomass and disaggregate particles in the ODZ core.

Oceanic oxygen deficient zones (ODZs) influence global biogeochemical cycles in a variety of ways, most notably by acting as a sink for fixed nitrogen (Codispoti et al. 2001). Optimum multiparameter analysis of data from two cruises in the Eastern Tropical North Pacific (ETNP) was implemented to develop a water mass analysis for the large ODZ in this region. This analysis reveals that the most pronounced oxygen deficient conditions are within the 13°C water (13CW) mass, which is distributed via subsurface mesoscale features such as eddies branching from the California Undercurrent. Nitrite accumulates within these eddies and slightly below the core of the 13CW. This water mass analysis also reveals that the 13CW and deeper Northern Equatorial Pacific Intermediate Water (NEPIW) act as the two Pacific Equatorial source waters to the California Current System. The Equatorial Subsurface Water and Subtropical Subsurface Water are synonymous with the 13CW and this study refers to this water mass as the 13CW based on its history. Since the 13CW has been found to dominate the most pronounced oxygen deficient conditions within the Eastern Tropical South Pacific ODZ and the Peru‐Chile Undercurrent, the 13CW and the NEPIW define boundaries for oxygen minimum conditions across the entire eastern Pacific Ocean.