Using δ15N of Amino Acids and Nitrate to Investigate Particle Production and Transformation in the Ocean: A Case Study From the Eastern Tropical North Pacific Oxygen Deficient Zone

Abstract

The eastern tropical North Pacific oxygen deficient zone (ETNP-ODZ) exhibits a distinct physical and biological environment compared to other oxygenated water columns, leading to a unique scenario of particulate organic matter (POM) production and vertical transport. To elucidate these biological pump processes, we present the first comparison of δ¹⁵N values of nitrate, phenylalanine (Phe), and glutamic acid (Glu) within two distinct size fractions of particles collected along a productivity gradient in the ETNP-ODZ. Low δ¹⁵N_Phe and δ¹⁵N_Glu values in both particle pools at sites with prominent secondary chlorophyll maximum (SCM), compared to the ambient δ¹⁵N-NO₃⁻, suggest the presence of recycled N-utilizing primary producers distinct from those at the primary chlorophyll maximum and their contribution to export. We observed reduced ¹⁵N enrichment of Phe in small particles and a narrower δ¹⁵N_Phe disparity between the two particle size fractions compared to the results from oxic waters, likely due to slower heterotrophic microbial degradation of small particles. Unique δ¹⁵N_Phe and δ¹⁵N_Glu signatures of particles were found at the lower oxycline, potentially attributable to chemoautotrophic production and zooplankton mediation. These findings underscore the need for further investigations targeting particles generated at the SCM, their subsequent alteration by zooplankton, and the new production by chemoautotrophs. This will allow for a better evaluation of the efficiency of the biological pump in the globally expanding ODZs under contemporary climate change.