Sinking/suspended particles and zooplankton interactions in eastern tropical north pacific oxygen deficient zone revealed by δ15n-amino acids

Photosynthesis by phytoplankton at the surface ocean fixes atmospheric carbon dioxide into organic matters that sink to the deep ocean in the form of particles. The composition and flux of particles could be affected by microbial degradation and consumption and repackaging by zooplankton. Within the oxygen deficient zones (ODZs), the zooplankton abundance is greatly reduced and oxygen is absent as electron acceptors for aerobic degradation of organic matters. Elevated biomass of zooplankton was found just below the lower ODZ boundary, presumably due to the higher availability of sinking particles for feeding. However, there is a lack of observations that could establish the link between sinking/suspended particles and zooplankton in the ODZs. A better understanding of sinking/suspended particles and zooplankton dynamics through ODZs is essential for evaluating the carbon export to the deep ocean, as ODZ is expected to expand due to the current warming climate. Sinking/suspended particles and zooplankton samples were collected at the eastern tropical north Pacific ODZ (14N, 104W) at various depths in December 2020. 15N of phenylalanine ( 15NPhe) and glutamic acid ( 15NGlu), the canonical source and trophic amino acid, are analyzed to assess the N isotope baseline and trophic positions (TPGlu-Phe) of these samples. 15NPhe and 15NGlu of surface suspended particles were 3.50‰ and 10.57‰ respectively, yielding a TPGlu-Phe of 1.48 that indicates the input of both phytoplankton and zooplankton materials to the suspended particle pool. Little enrichment in 15N of Glu was found between the suspended particles collected from the surface and below the lower boundary of ODZ (~1‰). This suggests limited trophic processing of sinking particles through the ODZ before zooplankton disaggregates them into suspended particles below the ODZ, which is in agreement with the slower sinking flux attenuation rate previously observed in ODZs.