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Abstract 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.
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Diel variations in the estimated refractive index of bulk oceanic particles
The index of refraction (n) of particles is an important parameter in optical models that aims to extract particle size and carbon concentrations from light scattering measurements. An inadequate choice ofncan critically affect the characterization and interpretation of optically-derived parameters, including those from satellite-based models which provide the current view of how biogeochemical processes vary over the global ocean. Yet, little is known about hownvaries over time and space to inform such models. Particularly, in situ estimates ofnfor bulk water samples and at diel-resolving time scales are rare. Here, we demonstrate a method to estimatenusing simultaneously and independently collected particulate beam attenuation coefficients, particle size distribution data, and a Mie theory model. We apply this method to surface waters of the North Pacific Subtropical Gyre (NPSG) at hourly resolution. Clear diel cycles innwere observed, marked by minima around local sunrise and maxima around sunset, qualitatively consistent with several laboratory-based estimates ofnfor specific phytoplankton species. A sensitivity analysis showed that the daily oscillation innamplitude was somewhat insensitive to broad variations in method assumptions, ranging from 11.3 ± 4.3% to 16.9 ± 2.9%. Such estimates are crucial for improvement of algorithms that extract the particle size and production from bulk optical measurements, and could potentially help establish a link betweennvariations and changes in cellular composition of in situ particles.
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- Award ID(s):
- 1756517
- PAR ID:
- 10380712
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Express
- Volume:
- 30
- Issue:
- 24
- ISSN:
- 1094-4087; OPEXFF
- Format(s):
- Medium: X Size: Article No. 44141
- Size(s):
- Article No. 44141
- Sponsoring Org:
- National Science Foundation
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