4.
Discussion
In this discussion, we first put our results into the overall cruise
context and then focus on the causes that drive the differences between
the suspended and sinking particles as measured with the MSC.
Specifically, we discuss why – despite all particles being small –
some particles sank while others remained suspended. Furthermore, we
discuss the potential mechanisms that can explain the presence of small,
slow sinking particles in the mesopelagic. We finally address two
aspects of the methodology: (1) comparison of sinking particle fluxes
derived from different methods, and (2) interpretation and implications
of the observed outliers in our data set including the potential role of
‘patchiness’ on ocean particle distributions.
4.1 Comparison with other particulate biochemical
measurements performed during EXPORTS: consistency in responses by
size
A variety of methods and sampling equipment were used to measure the
biogeochemical composition of particulate matter during the first
EXPORTS field campaign (Siegel et al., 2021). Values of POC and PON
measured in the MSC time zero fraction were slightly higher than those
obtained from the 12-L Niskin bottles sampled from the CTD/rosette
system from the same ship (Graff et al., in review). The differences
between the two measurements may be explained by a combination of the
following: (1) under sampling of sinking particles by Niskin bottles
compared to the MSC, (2) slight overestimation of MSC POC values due to
a possibly higher POC blank than the one used to correct the measured
POC masses (see section 2.4), and (3) spatiotemporal differences in
sampling (Graff et al., in review).
Our PIC concentrations were low, but consistent with low productivity
regimes in the global ocean (Mitchell et al., 2017) and complimentary
measurements made at Station P during EXPORTS (Roca-Martí et al., 2021),
suggesting a limited presence of calcifying microorganisms. In situ pump
observations showed low PIC concentrations and small contributions to
particle stocks during EXPORTS (Roca-Martí et al., 2021). Our PIC
estimates were higher than the in situ pump observations, which may be
attributed to the use of larger filter pore size with in situ pumps
(QMA; ~ 1.0 μm nominal pore size) compared to the
smaller pore size of glass fiber filters used with Niskin bottles and
MSC analyses (GF/F; ~ 0.3 µm nominal pore size after
combustion, Nayar and Chou, 2003) (Graff et al., in review). Indeed,
during EXPORTS, most of the PIC was associated with the smallest size
particle fraction measured by in situ pumps (i.e., in 1–5 µm fraction
rather than in the 5–51 µm or > 51 µm fractions;
Roca-Martí et al., 2021).
Our measurements of bSi standing stocks were in good agreement with
measurements performed on seawater collected with Niskin bottles
(Brzezinski et al., 2022) and measurements obtained with in situ pumps
(Roca-Martí et al., 2021), which used similar pore sizes (0.6, 0.6 and
0.8 µm, respectively). Our molar bSi-to-POC ratios associated with
suspended and sinking particles were in the same range as the 1–51 μm
particle size fractions (bSi:POC = 0.03–0.16, Roca-Martí et al., 2021),
except for one outlier (0.35 at 500 m; see Section 4.5). Molar
bSi-to-POC ratios measured from larger sized particles with in situ
pumps and drifting sediment traps were higher (Roca-Martí et al., 2021;
Estapa et al., 2021). This comparison suggests that the MSC sampled on
average small particles (< 51 μm), under-sampled large
particles, and that using GF/F filters allowed us to better represent
particles < 1 μm in respect to in situ pumps. Overall, the
concentrations and flux values (see also section 4.4) obtained by the
MSCs agreed with other measurements obtained during the cruise, and we
are therefore confident that the MSC observations are representative of
the conditions present in situ during our visit.
4.2 Partitioning of TEP and ballast minerals in suspended
and sinking particles
When comparing sinking
to suspended particles, we expected to find either larger particles in
the sinking fraction compared with the suspended fraction, and/or a
higher ratio of ballast materials-to-POC. However, neither expectation
was met (Figure. 1, 3 and 4 ). Both fractions were characterized
by small particles, which - in a simplistic view - would suggest that
sinking particles contained a higher fraction of ballast material than
suspended particles. Contrary to this idea, however, suspended particles
were characterized by a higher ratio of ballast material in the
mesopelagic. This begs the question why the small non-ballasted
particles sank (and, why the ballasted particles did not sink).
The partitioning of
particles into suspended and sinking pools must have been controlled by
another factor. So far, we looked at the composition in context of the
traditional constituents: POC, PIC and silicate. However, the molecular
makeup of POC can have a large effect on particle density. For example,
diatoms have been shown to be positively buoyant (i.e., float)
(Villareal, 1988; Moore et al., 1996; Woods et al., 2008). However, the
phytoplankton community consistent predominantly of very small
non-diatom cells (McNair et al., 2021). Alternatively, TEP are
positively buoyant and thus could reduce the sinking velocity of
aggregates (Engel and Schartau, 1999; Azetsu-Scott and Passow, 2004).
Overall, we observed low concentrations of water column TEP between 5
and 500 m (17.3–1.2 µg GXeq L-1). This result is in
line with low productivity systems such as the Southern Ocean (from
undetectable values to 48 µg GXeq L-1; Ortega-Retuerta
et al., 2009), the tropical western Pacific Ocean (5.3–40 µg GXeq
L-1; Yamada et al., 2017), the Eastern Mediterranean
Sea (5.7–25.1 µg GXeq L-1; Ortega-Retuerta et al.,
2019) and the Indian and the subtropical South West Pacific (<
20 µg GXeq L-1, Engel et al., 2020). Despite the
observed low TEP concentrations, TEP-C accounted for approx. 15% of the
suspended POC pool. According to theoretical calculations, a 1-mm
aggregate composed solely of TEP and diatoms can sink only if less than
5% of its carbon content consists of TEP (Mari et al., 2017). Here we
show that the relative contribution of TEP-C was on average three times
that threshold (15%) in suspended particles while in sinking particles
on average only 3% of the POC was TEP. While the model used diatom as
primary particle pool, in our case most of the suspended particle pool
consisted of very small cells (Synechococcus spp, picoeukaryotes and
nanoeukaryotes; McNair et al., 2021), and possibly fragments of old
cells, minerals, and detritus. Nonetheless, our data support the model
implication (Mari et al., 2017) and provide evidence that the chemical
composition of particulate organic carbon, and especially the
contribution of exudates, is critical to understand particle sinking
velocity.
Hence, the higher TEP-to-POC ratio in suspended particles compared to
sinking particles may have led to the difference in the partitioning of
the two particle fractions as collected by the MSC by reducing the
sinking velocity of small, ballasted particles that - in the absence of
TEP - would have been part of the collected sinking fraction. Whereas
the suspended POC was associated with ballasting material and consisted
of a large fraction of TEP, the sinking POC consisted of relatively
small fractions of ballasting material and TEP. This result suggests
that the ratio of ballasting material-to-POC cannot be used as a sole
predictor for sinking velocity, but that organic carbon composition
needs to be considered.
The importance of TEP for potentially reducing sinking velocity and thus
flux was first suggested by Mari et al. (2017), and here we present the
first in situ measurements suggesting that high TEP concentrations
reduce particle sinking velocities. Yet, our observations are limited to
one field campaign only, and hence we cannot generalize. We observed
this phenomenon in a low productivity system where particle
concentrations were low and TEP concentrations were high. Our dataset
was collected during late summer, when the system was characterized by
low concentrations of chlorophyll a , nutrients, particles, and
ballast minerals (Siegel et al., 2021; Roca-Martí et al., 2021;
Brzezinski et al., 2022). The TEP-to-particle ratio may have been higher
compared to other seasons, due to stressful conditions for phytoplankton
growth and high light levels (Ortega-Retuerta et al. 2009), implying
that the observed mechanisms could be seasonal (Mari et al., 2017).
Regardless, our data suggests that future biological pump studies should
characterize POC composition, including TEP, to help gain a mechanistic
understanding of how these materials influence particle sinking
velocities and, in turn, the role of particle dynamics in ocean
biogeochemical cycles across different productivity systems and seasons.
4.3 Potential mechanisms explaining small particles at
depth
Our observations suggest that suspended particles were increasingly
reworked with depth, e.g., a larger fraction of POC was remineralized
with depth, leading to the relative increase in the ratio between
ballasting material and POC with depth. Sinking POC did not decrease
with depth, suggesting that either the attenuation of the sinking
particle population was negligible, or an input of sinking material
balanced the expected losses. If we assume that sinking particles
collected at 500 m were produced at 30 m (within the mixed layer) and
sank at 18 m d-1, those particles would require 26
days to reach 500 m. We can estimate the loss rate assuming a
temperature-dependent remineralization rate of 0.04
d-1 measured experimentally using a temperature of 7
°C (average temperature between 30 and 500 m from CTD) and a
Q10 of 3.5 (Iversen and Ploug, 2013; Giering et al.,
2017). If particles were to sink undisturbed, 30% of the POC would have
been remineralized by bacteria during that time window (26 days). Yet,
such decrease was not observed.
One theoretical explanation for a lack of flux attenuation of sinking
particles would be rapid physical transport processes (Omand et al.,
2015; Giering et al., 2016; Siegel et al., 2023). Particle subduction or
convective mixing can, however, be excluded as during our visit density
stratification beneath the mixed layer was strong and consistent and
lateral density gradients were weak (Siegel et al., 2021).
Alternatively, an input of sinking particles at depth may
have balanced their loss. Large sinking phytodetrital aggregates were
rare, implying that disaggregation of sinking aggregates was not the
prevailing source of small sinking particles. However, if suspended POC
existed as loosely packed aggregates within a buoyant TEP matrix (e.g.,
low-density aggregates; Mari et al., 2017), their disaggregation would
allow dense components to sink, albeit at a relatively slow velocity.
Such disaggregation could be caused by zooplankton (Passow and
Alldredge, 1999) or by microbially mediated degradation processes
(Passow, 2002). During microbially mediated degradation, lighter
C-components may be consumed preferentially, leaving denser and more
recalcitrant material in the sinking fraction (Hamanaka et al., 2002).
In fact, extracellular particulate carbohydrates released by
phytoplankton (i.e., TEP) are remineralized by bacteria at a faster rate
than non-TEP carbon (i.e., 0.53 and 0.21 d-1, respectively; Harvey et
al., 1995; Mari et al., 2017).
Particle flux was, in fact, dominated by fecal pellets
during EXPORTS (~80%; Durkin et al., 2021; Steinberg et
al., 2023). Specifically, mini-pellets (ESD < 100 μm)
contributed up to 46% of the total carbon flux in the upper mesopelagic
(Durkin et al., 2021), with mesozooplankton and salp pellets
contributing the majority of the sinking flux (Durkin et al., 2021;
McNair et al., 2023). Fragmentation of fecal pellets, e.g., due to
sloppy feeding or swimming motions (Steinberg and Landry, 2017) could
have produced small and dense particles, which may have constituted the
bulk of the small sinking particle pool we collected. Such pellet
fragments would likely contain a high percentage of organic carbon and a
low percentage of TEP, similar to our sinking particle pool. Though we
did not observe large fast sinking pellets in the MSCs, this absence
could have been caused by an overall low abundance of pellets or because
the MSCs were deployed during the daytime when a large fraction of the
zooplankton community likely resided at depth (e.g., Steinberg et al.,
2023).
Microzooplankton grazing of suspended POC, and the resulting
production of mini-pellets (Stemmann et al., 2004), may have also
contributed to the production of small, dense sinking particles.
Consumption of small particles by zooplankton and incorporation of their
carbon and biogenic silica into sinking particles (Dagg et al., 2003) is
especially important for carbon flux if it occurs within relatively
short-path food webs (Richardson, 2019). This mechanism could also have
been important during our study: Compound-specific isotope analysis
(CSIA-AA) performed during our field campaign (Connor Shea, 2023, pers.
commun.) found that the mesopelagic zooplankton food web at Station P
was mainly (72–96 %) based on small particles (< 6
µm).
Although we cannot say with certainty why sinking flux of POC did not
decrease with depth, likely a series of biologically mediated
aggregation or fragmentation processes led to a replenishment of the
small sinking particle pool with depth. The described mechanisms are
potentially significant contributors in dictating the efficiency of the
biological carbon pump, especially in stable low-productivity systems,
characterized by small particles in low concentrations.
4.4 Comparison of fluxes derived from different methods
during
EXPORTS
Our background POC fluxes compared to the cruise mean POC fluxes
obtained using both neutrally buoyant and surface-tethered sediment
traps (Estapa et al., 2021) and water column Thorium-234 assessment
(Buesseler et al., 2020) were similar in the upper 100 m, but larger
below 100 m (Figure 6 ). These differences between the estimates
for depths > 100 m existed even when the lower-bound
estimate (assuming a sinking velocity of 18 m d-1;
Section 3.4) was used. In contrast, bSi flux estimates obtained from the
MSC were in good agreement with those derived from the traps, but lower
than the Thorium-234-based values (Roca-Martí et al., 2021)
(Figure 6 ).
These three approaches for determining sinking particle fluxes differ in
the design, spatiotemporal resolutions, and size of collected particles;
thus, may not be directly comparable and comparisons should be made
cautiously. The approaches mainly diverge in: (1) Temporal coverage: the
MSC provided virtually instantaneous estimates, whereas drifting
sediments trap (Gardner, 1977) and water column Thorium-234 carbon flux
determinations (Buesseler et al., 1992) integrate over periods of 3–6
days (time to collect sinking particles) and 24 days (the half-life of234Th), respectively. (2) Spatial coverage: the MSC
deployments have the smallest spatial resolution among these methods
with 39 discrete deployments as each MSC deployment assesses the flux
from a single ~ 100 L sample. Drifting sediment traps
sampled sinking particles from an area of ≤ 10 km2(Siegel et al., 2008). Due to the long integration times and abundance
of samples (nearly 1000), 234Th particle fluxes
average over even larger spatial scales. (3) Minimum size of collected
particles: differences in pore sizes of filters can lead to differences
in measured biogeochemical concentrations. Here we measured POC using
filters with a pore size of ~ 0.3 μm, while POC from
sediment traps (Estapa et al., 2021) and water column Thorium-234
samples (Buesseler et al., 2020) were determined using filters with a
pore size of ~ 1 μm. The filter pore sizes used to
estimate bSi were 0.6 μm, ~ 1 μm and 0.8 μm for MSC,
sediment traps and Thorium-234, respectively. Additionally, drifting
sediment traps often underestimate small particles due to hydrodynamics
(Buesseler et al., 2007). (4) Maximum size of collected particles: the
MSC likely under samples rare particles, such as large particles, as it
collects a limited amount of seawater. During EXPORTS, sediment trap
samples were highly impacted by zooplankton “swimmers” and fluxes were
corrected to account for the removed biomass (Estapa et al., 2021).
The potential sources of discrepancies between fluxes measured from
sediment traps and water column Thorium-234 were addressed in Estapa et
al. (2021). Briefly, Thorium-234 fluxes measured in sediment traps were
roughly three-fold smaller than water column Thorium-234 fluxes. The
different temporal coverages did not play a major role due to the
stability in production and exports before the EXPORTS cruise (McNair et
al., 2023). The mismatch was largely attributed to under-sampling of
both small particles (< 32 μm) due hydrodynamic biases and
rare, large particles (> 1 mm) and zooplankton active
migrant flux by the sediment traps (Estapa et al., 2021). The size of
particles collected in the polyacrylamide gel traps, which ranged from
75 to 1461 μm (median of 282 μm), supported this hypothesis (Estapa et
al., 2021). Estimates from the MSC account for the contribution of those
small particles close to ~ 0.3 μm. Thus, the discrepancy
between MSC and sediment traps may be partially explained by an
under-characterization of very small (< 1 µm) particles by the
traps. This statement is also valid when comparing Thorium-234-based POC
fluxes if we consider the pore size of the filters used and that
Thorium-based fluxes were estimated assuming the POC/Th ratio of
mid-sized particles (5–51 μm) (Buesseler et al., 2020).
However, the MSCs did not capture large, rare particles such as
aggregates, salp fecal pellets nor the active flux due to migration. We
would therefore have expected to see the opposite trend: higher flux
measured using Thorium-234 and traps, if large, rare particles played an
important role in overall flux at the site. This comparison suggests
that POC flux from the MSCs is complementary to those measured from
Thorium-234 and traps. Most importantly, it suggests that small sinking
particles were a relevant part of the sinking POC flux in the upper
mesopelagic zone, likely exceeding the contribution of rare, large
sinking particles. The fact that the MSCs and sediment traps produced
similar estimates of bSi fluxes reinforced our finding that the
population of sinking particles collected by the MSCs included small
sinking POC missed by the other methods, whereas bSi, which was mostly
associated with slightly larger particles (Roca-Martí et al., 2021;
Estapa et al., 2021; Brzezinski et al., 2022), was equally sampled by
all three methods.
4.5 Outliers – Patchiness or artifacts?
The dataset obtained from the MSC holds the potential of highlighting
the ecosystem’s patchiness through the collection of spatially
heterogeneous features. Patchiness in the ocean exists both vertically
and horizontally spanning from the microscale to the mesoscale (Cassie,
1962; Siegel, 1998; Robinson et al., 2021) and is in general difficult
to map with most oceanographic approaches (McNair et al., 2023). Several
of our biogeochemical measurements resulted in values considerably
higher than average concentrations for their collection depth
(Table S5 ). These “outliers” may be attributed to patchiness
or could be due to methodological artifacts. Methodological artifacts
could be caused by misfiring of the MSCs at a shallower depth than
intended or by contamination of the filter. The latter is improbable as
replicate filters and comparisons between time zero and suspended plus
sinking particle fractions would have indicated “contamination”.
Additionally, filters were always visually inspected for visible
particles, such as macrozooplankton, which, if present, were removed.
These outliers were specific to the MSC, no outliers were detected by
the 12-Liter Niskin bottles of the CTD on board R/V Roger
Revelle .
Three of the five outliers in
POC were observed on August 16, at 55 m, 95 m, 195 m, one on August 21
at 195 m, and one on September 1 at 350 m. The outlier concentrations of
sinking POC measured at 55 and 95 m, which were an order of magnitude
higher than any other sinking POC values, are likely reflective of in
situ patchiness: the respective time zero and suspended POC
concentrations were also within the upper range of the values measured
at those depths. The POC measured in the time zero fraction at 55 m
equals the sum of POC measured in the suspended and sinking particle
fraction, confirming that the MSC samples were uncontaminated.
Associated sinking PON values were also marked as outliers and the molar
C-to-N ratios were reasonable (10.4 and 12.5 at 55 and 95 m,
respectively), albeit somewhat higher than those of the time zero and
suspended fractions (7.3 and 6.4 and 8.3 and 9.7 at 55 and 95 m,
respectively). Lastly, while the associated sinking TEP were high (with
the observation at 95 m being an outlier), suspended TEP concentrations
agreed with TEP measured from water collected with Niskin bottles at
those depths (Figure S3 ). These observations and the fact that
the MSC deployment depths were chosen to specifically target
“interesting” areas as seen by the CTD or UVP profiles, make us
believe that the 100-L sample we collected may have included patches of
unusually high biochemical concentrations.
The three POC outlier concentrations of suspended POC at 195, 195 and
350 m were associated with outlier values of PON, and could reflect POC
and PON concentrations measured at 50–65 m. However, the associated
suspended TEP concentrations (2.8, and 3.1 µg GXan equiv
L-1) suggest samples stemmed from below 50–65 m
(Figure S3 ). Although uncertain, these too could reflect
patchiness.
One value of biogenic silica (176 nmol Si L-1, 500 m)
was more than a factor of 3 higher than the bSi measured on other days
at that depth either with the Niskin bottles or the MSC (57 and 45 nmol
Si L-1, respectively). However, associated POC
concentrations were consistent with the deployment depth and the molar
bSi-to-C ratio of this observation (0.35) matched the one measured for
particles > 5 μm (bSi:POC = 0.16–0.94 ) collected with in
situ pumps (Roca-Martí et al., 2021). Thus, we assume that this sample
included a higher-than-average presence of diatoms or rhizaria. Hot
spots of bSi accumulation have previously been found in low productivity
systems (Crombet et al., 2010) and this depth was targeted because the
UVP profile directly before the MSC deployment indicated a Rhizaria
maximum. Similarly, the elevated PIC concentration associated with
sinking particles (21.5 μg PIC L-1) could potentially
be attributed to the presence of foraminifera test fragments or
coccoliths.
In summary, the “outliers” measured with the MSC are likely, at least
in part, reflective of patchiness in concentrations of small particles
in this low productivity system. Patchiness in the distribution of salp
swarms was also a prevalent feature during this study (Steinberg et al.,
2023). The sporadic nature of patchiness is challenging to quantify and
its importance with respect to standing stocks and fluxes of an entire
ecosystem hard to assess, especially in a low productivity system.