Figure 7. Deviation from Model (DFM) indicates the difference between the observed flux of small particles (< 500 μm), and the flux of small particles that would be estimated by a model, which assumes that particles in the depth bin above only remineralized and sank, following the PRiSM model. Values are normalized to the change in depth and are in units of μ mol Carbon m-3d-1. This value serves as a metric of processes that cannot be captured by a null model, which assumes that particles only sink and remineralize. Positive values suggest an excess of <500 μm particles, suggesting disaggregation or advection of small particles, while negative values suggest a dearth of small particles, suggesting repackaging or aggregation. DFM is only reported for <500 μm particles, because it is the inverse of the deviation from expected flux of ≥500 μm particles. DFM is reported for all casts at ETNP Station P2. Horizontal blue lines indicate the top and bottom of the ODZ, while the horizontal green line indicates the base of the photic zone.

ETNP particle dynamics differ from those seen at an oxic site

The ODZ data were compared to an oxic water column in order to identify the spectral signatures that are particular to oxygen deficient waters. The oxic site, P16 Station 100, was characterized by a more gradually sloping pycnocline, and an oxygen minimum at 500 m of 19.7 μM, which is hypoxic (Figure S1B). There was no working fluorescence sensor on that cruise, but data from World Ocean Atlas (Boyer et al., 2018) suggest that the photic zone is characterized by a single fluorescence peak with a maximum at 110 m and which disappeared at 200 m (Figure S1C). Thus, we define the mesopelagic as beginning at 200 m at the oxic site. Turbidity followed chlorophyll concentration and did not have a peak in the mesopelagic (Figure S1D), unlike the ODZ site. There was a salinity peak at 150 m (Figure S1B).
Particle numbers were higher between the base of the photic zone through 1000 m at the ETNP ODZ site, than at the same-latitude, oxygenic, P16 Station 100 (Figure S7A). Particle size distributions were similar between the two sites above 500 m, being characterized by overlapping confidence intervals generated by a general additive model. From 500 m to 1000 m, particle size distributions were flatter at the ETNP site, being characterized by a smaller proportion of smaller particles, relative to larger ones (Figure S7B).
Microaggregate particles (100 μm - 500 μm) at the ETNP ODZ site were about two orders of magnitude more common than marine snow particles ( ≥ 500 μm) (Figure S8). ≥500 μm particle numbers appeared to attenuate more quickly than <500 μm particles, and more generally follow a power law decrease, while <500 μm particles appeared to increase around 500 m depth. Flux was predicted to be predominantly from <500 um, rather than ≥500 μm particles, at all depths except the shallowest depth bin in the surface of the photic zone. The particle size distribution, calculated only on ≥500 μm particles, was more variable between depths than calculated for <500 μm particles. Data from the oxic P16 Station 100 suggested more particles, steeper particle size distribution, and more flux at this station than at the ETNP station. They also suggested that differences between <500 μm and ≥ 500 μm particles, with respect to number, flux and size distribution that were broadly similar to the ones seen at ETNP Station P2. In contrast to the anoxic station, at the oxic station flux always decreased with depth (Figure S9A+B).