Oxygen Deficient Zones (ODZs) are key areas of N loss, a process dependent on organic matter. Understanding the sources of organic matter to the ODZ is necessary to predict how biogeochemical cycles will respond to ocean changes. Size fractionated (5-20, 20-53, 53-180, 180-500, >500 µm) particulate organic C and N (POM) concentration and isotopic composition depth profiles from three stations in the offshore Eastern Tropical North Pacific (ETNP) ODZ were used to create previously missing ETNP specific particle size to carbon and nitrogen relationships for models, while gaining insights into the origins of POM in the ODZ. Since the within-ODZ Prochlorococcus assimilate nitrite, we used the resulting highly depleted d15N signal to trace organic matter of cyanobacterial origin to medium sized particles at the secondary chlorophyll maximum (SCM), and to >500 µm particles directly below the SCM. This organic nitrogen was consumed in the upper ODZ. Other POM maxima were seen at the zooplankton vertical migration maxima with the increase in POM marked in the 5-20 µm fraction. In the deep ODZ, below the zooplankton migration depth, POM concentrations in the 5-20 µm fraction were unusually small, the C:N ratios were extremely high (>20), and d15N was enriched (8-12‰), indicating degraded material. In deep samples, d13C was more depleted in larger particle size and correlated with enriched d15N, indicating increased degradation in 53-500 µm particles. This trend suggests an additional source of small particles, such as from in situ production, rather than just the fragmentation of large particles.

Oxygen Deficient Zones (ODZs) are the largest pelagic sink of N containing nutrients in the ocean. The offshore Eastern Tropical North Pacific ODZ has been shown to be organic matter limited. We propose zooplankton/forage fish as a key source of organic matter for N2 production that has previously been ignored. We examined datasets from four cruises (April 2012, Jan 2017, April 2018, Oct 2019) at a station in the central ETNP. Backscattering data was used to determine zooplankton vertical migration depths (250-450 m, maximum at 270-280 m). Metazoan DNA concentrations, as measured by quantitative PCR, had a reproducible maximum at 270-280 m, confirming that these signals indicate the presence of zooplankton/forage fish. Additionally, a large maximum in sinking pteropod shells was found at 270 m, indicating that pteropods were part of the migrating community. While crustacean zooplankton have been shown to reduce respiration and excretion of ammonium under anoxia, we found intermittently measurable ammonium concentrations at 270 m. Here we show signatures consistent with organic matter of zooplankton/forage fish origin in the C:N and \(\delta\)13C of suspended and sinking organic matter at the vertical migration depth that suggest transportation to these depths by migrating zooplankton/forage fish. Also coincident with the migration maximum was a reproducible-between-years maximum in the biological N2 gas, and a tertiary nitrite maximum, which suggest that the migrating zooplankton are linked to N loss. Thus zooplankton/forage fish appear to be one source of organic matter which can fuel biological N2 production in ODZs.

A document by Jacob Cram. Click on the document to view its contents.

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 also 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.