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.