Prolific nitrite reoxidation across the Eastern Tropical North Pacific Ocean

Abstract

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−3 isopycnals is reoxidized, whereas 54 ± 2% of the nitrite produced between the 26.7 and 26.9 kg m−3 isopycnals 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.

Rapid Expansion of Fixed Nitrogen Deficit in the Eastern Pacific Ocean Revealed by 50‐Year Time Series

“…(2020a) and Evans et al. (2023). Our selected stoichiometry for fixed nitrogen loss was derived from the same data, then this stoichiometry, water mass endmembers, and weightings were refined by slight adjustments to minimize the sum of squared residuals output from the eOMP (Table S4 in Supporting Information S1).…”

“…In our previous paper, Evans et al (2023), we posited that N* overestimates fixed N loss due to nitrite reoxidation. Instead, it is important to remember that N* includes the loss of both dissolved inorganic nitrogen as well as particulate ammonia (Gruber & Sarmiento 1997).…”

“…Nutrient (PO 4 3− , NO 3 − , and SiO 4 2− ) concentrations for the 13°C Water, NEPIW, and AAIW endmembers were selected by identifying the metabolic switching points for each water mass in the ETNP ODZ (Evans et al., 2023). Parameter weightings were based on Evans et al.…”

“…We grounded our fixed nitrogen loss stoichiometry in the observed stoichiometry for anaerobic remineralization in the ETNP ODZ (Figure 2, values provided in Evans et al. (2023)). With regard to the range of NO 3 − :PO 4 3− stoichiometries observed, we selected −62.1:1 NO 3 − :PO 4 3− as the stoichiometry for fixed nitrogen loss in our eOMP analysis because it had the best fit within the range of NO 3 − :PO 4 3− stoichiometries observed.…”

“…This figure is adapted from Evans et al. (2023), which provides further information about data processing for this data set.…”