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).