Particle size and abundance measurements suggest decreased particle attenuation and disaggregation in the Eastern Tropical North Pacific Oxygen Minimum Zone

Models and observations suggest that that particle flux attenuation is lower across the mesopelagic zone of anoxic environments compared to oxic ones. While decreased microbial metabolism under anoxia is likely an important component of this difference, particle aggregation and disaggregation may also contribute. In situ measurements of particle properties, in particular optical measurements of particle size spectra, and trap based measurements of particle flux can inform us about the dynamics particle characteristics with depth. Furthermore, they inform about the relative roles of particle remineralization, aggregation and disaggregation.

We measured particle size profiles at a station in the core of the Eastern Tropical North Pacific Oxygen Minimum Zone (ETNP OMZ) using an underwater vision profiler (UVP) multiple times of day, at different times of day, over the course of a week. We normalized our UVP measurements by comparing them to particle flux measurements measured by sediment traps, and we compared our observations to UVP measurements from similar latitudes but with higher oxygen concentrations.

Particle flux attenuated more slowly in the ETNP OMZ than in a similar latitude oxic environment transect, and there were fewer small particles, relative to large particles, in the oxygen minimum zone. Below the oxygen minimum zone, at 1000m, there was an increase both in flux attenuation and the abundance of small particles. These particle size dynamics were constant across multiple casts, and did not appear to co-vary with timing of migration of micronekton swarms, which we observed acoustically.

These patterns suggest that the low oxygen environment not only suppressed particle remineralization but also particle disaggregation. This may reflect decreased interactions between particles and grazers, as postulated previously, or different physical processes. Alternatively, differential remineralization rates, with larger but not smaller particles breaking down more slowly than in other environments could also explain this pattern. Understanding particle dynamics in the anoxic zone will necessitate exploring potential biological and physical mechanisms that may be responsible for either of these processes.