Conclusions

If ODZs expand in response to the changing climate, larger areas of the ocean are likely to resemble this environment, which is oligotrophic and has an oxygen deficient zone spanning most of the mesopelagic zone. Previous models and observations have suggested that ODZs are sites of efficient carbon transfer to the deep ocean (Cram et al., 2018; Hartnett & Devol, 2003; Van Mooy et al., 2002; Weber & Bianchi, 2020), and our data appear to support this contention. Indeed, the mechanism of efficient transfer appears to be slowing of particle remineralization, presumably from decreased microbial metabolism, with zooplankton playing an important role in both active particle transport and particle disaggregation.
Our data could potentially be used in conjunction with mechanistic models (e.g. Weber & Bianchi, 2020) to constrain the relative carbon oxidation rate by nitrate reduction, denitrification and sulfate reduction processes, which is currently poorly understood (Bristow, 2018). Furthermore, it appears that diel migratory organisms both disaggregate particles and transport carbon throughout the top 500 m of the water column. Day-to-day and within day variability in organic matter transport was evident, though overall patterns in particle size, flux and disaggregation appeared to be consistent over the course of the time-series. The change in particle abundance and size between 500 m and the bottom of the ODZ has implications for the free-living microbes living in this region. These microbes are likely particularly organic matter starved, and so these decaying particles are likely an important energy source for them. Our data highlights the heterogeneous nature of the ETNP ODZ with depth and indicates that more detailed sampling should be performed for rate and microbial measurements to properly extrapolate to the entire ODZ.