Figure 4. A. Observed, volume normalized total
particle numbers from 9 casts taken at different times of the day at
ETNP station P2. B. Calculated particle size distribution
slopes of those particles. These data have not been binned by depth in
order to better show sample to sample variability. Horizontal blue lines
indicate the top and bottom of the ODZ, while the horizontal green line
indicates the base of the photic zone. Hour corresponds to local,
Mexican General Standard, time. Particles are binned into 5 m depth
increments.
Estimated particle flux sometimes increases with depth in
the ODZ
core
Optimization found best agreement between particle flux measured by
traps, and UVP estimated particle flux when per particle flux is fit by
the equation
Flux = (133 μ mol C / m^2/day) = 133 * Size (mm) ^2.00 (Eqn 5)
This equation represents an empirical relationship between particle flux
from traps and particle size measured by UVP. Applying this fit to the
UVP data resulted in a UVP predicted flux profile that broadly fit the
expected trap observed flux profiles (Figure 3).
Particle flux profiles, predicted from the above particle size
abundances and fit, varied between casts between the base of the photic
zone and 500 m (Figure 5A-5B). To examine the rate of change of flux and
to identify regions and time points where flux appeared to increase with
depth, we examined the rate of change of flux. This rate of change was
fifth root transformed to normalize the data and to allow us to focus on
the cases where flux attenuation varied about zero, since we were
interested in identifying factors that related to whether flux was
positive or negative. Between 250 m and 500 m, particle flux appeared to
increase on some, but not all, casts, while attenuating slowly on the
other casts (Figure 5C). Below 500 m, there were not enough casts to
measure variability between casts.
The general additive model that quantified how the of change of flux
between 250 m and 500 m varied with depth, decimal study day and decimal
hour found that depth (p = 0.061) and hour of the day (p =
0.196) did not statistically associate with the fifth root transformed
rate of change of flux while day of study did (p = 0.019,
R2 = 0.264, Figure S6). There were generally increases
in flux over this region towards the beginning and end of the sampling
period and decreases in flux nearer to day 10 (Figure S6B). A general
additive model that looked only at the relationship between study day
and rate of change of flux (fifth root transformed) in this region
suggested that day accounted for 14% of the variance in this value, as
determined by adjusted R2 (p = 0.040). If the
fifth root transformation was not applied to the rate of change of flux,
there was a statistically significant relationship between depth and
rate of change (p = 0.001), but not study day (p = 0.062) or hour
(p = 0.719, R2 = 0.341). This pattern indicated
that, without the transformation, any temporal signal is swamped by the
substantial changes in rate of change in depth, with shallower depths
losing flux faster than deeper ones.