Figure 6. A. GAM smoothed, bin-size and volume normalized particle numbers across the particle size spectrum, at ETNP Station P2. Data are from the only cast that traversed the top 2000 m of the water column, collected on January 13 beginning at 06:13. Horizontal blue lines indicate the top and bottom of the ODZ, while the horizontal green line indicates the base of the photic zone. B. Particle size distribution slope. C-D . Estimated biomass of (C)microaggregate (<500 um) and (D) marine snow (≥500 um) particles, normalized to biomass at the base of the photic zone. In these two biomass panels, data from above the base of the photic zone are not shown.
At the oxic site, particle size distributions generally steepened with depth, while both microaggregates (<500 μm) and ≥500 μm estimated particle biomass followed a power law decrease with depth (Figure S10).

Particle number dynamics differ from model expectations

The modified particle remineralization and sinking model predicted particle size distributions at each depth from the particle size distribution one depth-bin shallower and the calculated flux attenuation between the two depths. At the ETNP ODZ site, we found that the observed particle size distributions usually deviated from model expectations (Figure S11). In the model, remineralization rates are optimized, to ensure that the total predicted flux at each depth matches the observed total flux. However, the predicted size spectrum will diverge from the observed spectrum if the assumptions of the model (i.e., sinking and remineralization are the only particle transformations) are violated. The difference between the observed and predicted flux ofmicroaggregate particles (100 - 500 μm), normalized to depth, therefore serves as a metric of observed deviations from the size distribution expected from sinking and remineralization alone. We call this value Deviation from Model (DFM).
\(DFM=\frac{\left(\ <500\ \mu m\,\text{Flux}\,\text{Observed}-\ \,<500\ \mu m\ Flux\,\text{Modeled}\right)}{\mathrm{\text{ΔZ}}}\)(Eqn. 6)
In the above equation ΔZ is the distance, in meters, between the current depth bin and the previous depth bin, whose particle size distribution is fed into the predictive model.
DFM was positive between the base of the photic zone (160 m) and 500 m, meaning that less <500 μm particle flux attenuated than would be expected from the PRiSM model in this region (Figure 7). There was some variability in the DFM parameter between casts. A general additive model (GAM) that showed that the variability in DFM was statistically significantly related to depth (p < 10-5), day of the study (p = 0.002), but not to hour of the day (p  = 0.051), with these factors together explaining 41.6% of the variance, as measured by R2. DFM was highest shallower in the water column (Figure S12A), highest near day 10 and lower at the beginning and end of the study (Figure S12B). A GAM that only explored the effect of depth accounted for 27.4% of the variance. Comparing a GAM that accounted for study day and depth to one that only accounted for depth effects showed an increase in R2 value of 10.4%, suggesting that study day accounts for an additional 10.4% of the variance, after accounting for depth. Comparing the model that accounts for depth, day and hour to one that only accounts for depth and day, suggests that hour of the day, while not statistically significant, could explain an additional 3.4% of the variance. Below 500 m, DFM was negative. There were only two casts that reached below 500 m at this station, and so an analysis of the dynamics of DFM in this region is not possible. At P16 Station 100, DFM was positive between the base of the photic zone and 350 m and negative below 350 m (Figure S9C).