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).