3. Results
3.1 Marine snow and abiotic aggregation
potential
No visible sinking marine snow particles (diameter > 0.5
mm) were found in the tray from any of the MSCs deployments. This
observation suggests that the concentration of sinking marine-snow-sized
particles was less than 0.02 per L. This finding is consistent with
particle size distribution data from the UVP (pers. comm. Andrew
McDonnell, 2021). Furthermore, during the four aggregation potential
experiments, no visible marine snow-sized aggregates formed in the
rolling tanks regardless of depth (55 and 95 m) and sample fraction
(T0, top or base ).
3.2 Particle concentrations and size
Most of the imaged particles (were smaller than 16 μm in diameter (ESD),
whether suspended or sinking (Figure 1 ). Furthermore, particle
concentrations measured in the tray were equal or lower than the ones
measured in the base for 20 out of 30 MSC deployments. These
observations suggest that the concentration differences betweenbase and tray particle fractions were too small to be
detected with this method. We hence summed the concentrations of slow-
and fast-sinking particles into a single sinking particle fraction. This
decision is additionally supported by the lack of marine snow, which
would have contributed appreciably to the fast-sinking particle
fraction.
Our results, based on the analysis of particles with diameters (ESD)
between 4 and 128 μm, suggest that > 70% of suspended and
sinking particles had a diameter of 4–8 μm, and that particles larger
than 32 μm were at the detection limit of our method (i.e., too rare to
be quantified). The partitioning of particles across the size bins was
consistent among particle fractions (paired t-test, p = 0.33–0.63, n =
17), indicating that suspended and sinking particles were similarly
sized (Figure 1 and Table S4 ).
Additionally, we calculated the particle size distribution (PSD, 4–128
μm) of suspended particles and of sinking (‘tray ’) particles
(uncorrected fraction) to assess differences in slopes among fractions
with depth. Overall, throughout the entire water column, PSD slopes of
suspended particles were similar (-3.3 ± 0.3) to those of sinking
particles (-3.5 ± 0.2; Figure S1 ). Yet below 100 m, PSDs
of sinking particles were significantly steeper than those of suspended
particles (paired t-test, p = 0.03, n = 5; Figure S1 ).
Furthermore, the slope of suspended PSD became significantly shallower
with depth between 20 and 500 m (R2 = 0.3, p
< 0.02, n = 17), while the slopes of sinking PSDs remained
constant (R2 = 0; p = 0.79, n = 17; Figure
S1 ).
It is important to note that the determination of particle size using
this approach is associated with uncertainties due to samples handling.
We cannot exclude the possibility that fragile and larger aggregates
(ESD < 0.5 mm) may have broken up due to physical disturbance
developed during the sampling and partitioning of the three particle
fractions and during injection of samples in the FlowCam flowcell used
in combination with the x40 objective (minimum size of 300 μm).
Therefore, the size characterization performed with the FlowCam must be
taken with caution. Lastly, particle type and shape could not be
reliably resolved with the FlowCam due to the small size of the
particles.
3.3 Biogeochemical partitioning by particle sinking
fraction
The background suspended POC concentrations decreased significantly with
depth in the euphotic zone between 20 and 65 m (R2 =
0.43, p = 0.03, n = 11) and in the upper mesopelagic between 95 and 500
m (R2 = 0.59, p = 0.001, n = 14) (Figure 2A ),
whereas background sinking POC concentrations did not significantly
attenuate with depth (R2 = 0, p = 0.61, n = 24). The
relative contribution of background suspended POC to total background
POC decreased significantly in the upper mesopelagic
(R2 = 0.70, p < 0.001, n = 13), resulting in
a concomitant increase in the contribution of the background sinking POC
(R2 = 0.70, p < 0.001, n = 13)
(Figure 2A ). Nearly all the background POC was found in
suspended particles, whose contribution to the total POC was on average
88 ± 9%. In respect to outliers, in five out of 29 MSC deployments, the
measured POC of either suspended or sinking particle fractions was at
least twice higher than POC concentrations measured at other stations in
the same depth layer (Table S5 ).
Patterns of PON (Figure S2 ) were similar to those of POC, with
molar POC-to-PON ratios of the suspended fraction on average slightly
higher than the Redfield ratio of 6.6 (average: 7.9 ± 3.9). The molar
POC-to-PON ratios did not show any trend with depth
(R2 = 0.06, p = 0.25, n = 24) and were in general
agreement with molar POC-to-PON ratios measured during the EXPORTS field
campaign (Graff et al., in review). Molar POC-to-PON for sinking
particles varied between 3 and 16 (average: 7.2 ± 3.8) and did not show
any significant trend with depth (R2= 0.11, p = 0.1, n
= 26).
PIC was below detection in 20% of the suspended particle fractions, and
in 60% of sinking particle fractions analyzed. Suspended PIC
concentrations increased significantly with depth (R2= 0.40, p = 0.05, n = 10) (Figure 2B ). Background
sinking PIC was on average 0.4 ± 0.3 µg C L-1 and
remained constant with depth (R2 = 0.05, p = 0.59, n =
8). Background molar PIC-to-POC ratios calculated for suspended
particles increased significantly with depth (R2 =
0.80, p < 0.001, n = 10) and were significantly higher than
PIC-to-POC ratios of sinking particles in the mesopelagic (paired
t-test, p = 0.01, n = 5) (Figure 3A ).
Background suspended and sinking bSi concentrations and their
contribution to total bSi concentrations did not significantly change
with depth between 50 and 500 m (R2 = 0.06–0.18, p =
0.17–0.49, n = 8–11) (Figure 2C ). Suspended particles stored
on average 83 ± 9% of the total background bSi pool. Molar bSi-to-POC
ratios associated with background suspended particles increased
significantly with depth between 50 and 500 m (R2 =
0.84, p < 0.001, n = 8), whereas ratios of sinking particles
remained constant (R2 = 0.16, p = 0.32, n = 8).
Furthermore, molar bSi-to-POC ratios of sinking particles were on
average higher than the ratios associated with suspended particles above
95 m (n = 3), and lower between 350 and 500 m (n = 4). Ratios above and
below 95 m were statistically different (paired t-test, p = 0.01 and p =
0.008, respectively) (Figure 3B ).
Suspended and sinking lSi concentrations and relative contribution to
the total lSi pool did not display a significant trend with depth and
showed high variability (R2 = 0.004–0.07 p =
0.39–0.85, n = 11-13) (Figure 2D ). Suspended lSi accounted, on
average, for 83 ± 19% of the total lSi pool. Molar lSi-to-POC
ratios associated with suspended particles increased significantly with
depth between 50 and 350 m depth (R2 = 0.78, p = 0.02,
n = 6). For sinking particles, molar lSi-to-POC ratios displayed the
tendency of decreasing with depth between 60 and 500 m, although the
trend was not statistically significant (R2 = 0.05, p
= 0.58, n = 9) (Figure 3C ). Ratios between suspended and
sinking particles were statistically different below the euphotic zone
(paired t-test, p < 0.01) with suspended particles
characterized by consistently higher ratios than sinking particles.
Suspended TEP decreased significantly with depth (R2 =
0.53, p =0.001, n = 16) and like POC, displayed the sharpest decrease
between 50 and 95 m. On average, 95 ± 4% of TEP was suspended, and
sinking TEP was often at or near detection (average = 0.3 ± 0.2 µg GXeq
L-1, n = 15) (Figure 2E ). TEP concentrations
measured in the MSCs were in line with the concentrations measured in
the water collected with the CTD rosette (Figure S3 ). After
converting TEP to carbon concentration of TEP, we estimated, in 11 out
of 15 deployments, that 15 ± 5% of suspended POC was TEP and only 3 ±
1% of sinking POC was TEP. In 4 observations collected above 100 m,
TEP-C accounted for 13% of the sinking POC (Figure 3D ).
TEP-C-to-POC ratios associated with suspended and sinking particles were
statistically different (paired t-test, p < 0.001, n = 15).
Background POC, TEP, bSi and lSi concentrations in the suspended and
sinking particle pools did not display a significant variability over
time at 50–65 m, 95 m or 300–500 m (t-test, 95% confidence). Temporal
trend for PIC could not be tested owing to the limited number of
observations available.
To provide an overall assessment of the relative changes in the measured
biogeochemical composition of suspended and sinking particles with
depth, we summed the background cruise-wide averaged concentrations of
organic TEP-C, POC and PON and ballast minerals and evaluated the
differences in their relative contributions (Figure 4 ). We
found that the composition of suspended particles was dominated by POC
(64%) in the euphotic zone and by POC (34 ± 11%), lSi (30 ± 7%) and
PIC (16 ± 8%) in the upper mesopelagic, whereas sinking particles were
on average composed mainly by POC (50 ± 10%), followed by lSi (20 ±
7%), bSi (14 ± 7%) and PON (9 ± 3%) throughout the water column. The
relative contribution of TEP-C in suspended particles was on average
twice the one in sinking particles (Figure 4B ).
3.4 Constraining the particle sinking
velocity
The settling time experiments found no consistent differences in the
fraction of sinking particles to the whole particle population
(suspended plus sinking) as a function of settling time (t-test, 95%
confidence). After 1, 2 and 4 hours of settling, sinking POC made up 10,
11 and 10% and sinking PON 6, 7 and 8% of the whole particle
population (Table S6 ). Sinking bSi made up 23, 25 and 25%,
whereas sinking TEP made up 6, 3 and 5%. However, our experiment was
limited to only two casts within the euphotic zone; hence, we cannot
assume that all sinking particles collected had the potential to sink
within one hour at 36 m d-1. We therefore present
fluxes calculated assuming the standard sinking velocity of 18 m
d-1, and we reinforce that these fluxes represent a
lower bound as suggested by our settling time experiments
(Figure 5 ). Background POC fluxes averaged to 4.2 ± 2.6, 3.1 ±
1.5 and 3.1 ± 1.1 mmol C m-2 d-1 at
50–65 m, 95 m and 300–500 m, respectively. When including the
outliers, the estimates increased up to 7 ± 10, 9 ± 18 and 4 ± 2 mmol C
m-2 d-1, at these same depth
intervals. Fluxes of bSi were on average 0.2, 0.2 and 0.1 mmol Si
m-2 d-1 and lSi fluxes were 0.3, 0.4
and 0.2 mmol Si m-2 d-1 at 50–65 m,
95 m and 300–500 m, respectively.