1.
Introduction
The biological carbon pump (BCP) comprises the processes that mediate
the transfer of organic carbon from the euphotic zone, where it is
produced, to the deep ocean (Volk and Hoffert, 1985). Without the BCP
the concentrations of atmospheric CO2 would be
~ 200 ppm higher than present (Parekh et al., 2006). The
efficiency of the BCP depends primarily on the gravitational settling of
particles, physical advection and mixing of particles, and the active
vertical transport of particles due to migrating zooplankton and fish
(Boyd et al., 2019). These processes shape the flux of carbon and other
elements through the mesopelagic and abyssopelagic zones, where carbon
can be locked away from the atmosphere, impacting atmospheric
CO2 concentrations over climatologically relevant
timescales (Kwon et al., 2009; DeVries et al., 2012). Accurately
predicting the response of the ocean carbon storage to already underway
and future climate changes requires a mechanistic knowledge of the
processes making up the BCP (e.g., Siegel et al., 2023).
The largest component of the BCP is settling particles (Boyd et al.,
2019; Nowicki et al., 2022). Particles in the open ocean are mainly
produced in the surface ocean and consist of living and dead
phytoplankton cells, detritus, carcasses, fecal material, and minerals.
Their size range spans between less than a micrometer (typical threshold
defined as 0.7 µm in diameter) to many millimeters, with those with a
diameter of > 0.5 mm being referred to as “marine snow”
(Alldredge and Silver, 1988). The fate and distribution of particles are
influenced by the transformation processes that change a particle’s
size, composition and, consequently, sinking velocity. Particles can
aggregate, disaggregate, solubilize or be remineralized back to
inorganic forms via mechanical forcing, bacterioplankton activity, and
interaction with zooplankton (Stemmann et al., 2004; Burd et al., 2010;
Giering et al., 2014; Collins et al., 2015). When particles aggregate or
are repackaged into fecal matter, they have the potential to sink
rapidly (~50 to ≥ 2,000 m d-1) and
hence more easily escape consumption, fragmentation, and dissolution at
shallow depths; thus, bringing organic matter to the deep ocean more
efficiently than smaller particles (Alldredge and Silver, 1988;
Ebersbach and Trull, 2008).
Particle sinking velocities are typically thought to be largely
determined by size. Stokes’ Law, which quantifies the sinking velocity
of spherical solid particles under laminar flow conditions, assumes that
particle sinking velocity increases as the product of the spherical
particle’s diameter squared and the excess density with respect to
seawater. The implication for sinking particles in the ocean is that
large particles should sink fast and are hence effective vectors for
carbon transport to depth, whereas small particles sink slow and are
remineralized within the upper mesopelagic, contributing little to
BCP-mediated ocean carbon storage (Kriest, 2002; Marsay et al., 2015;
Cavan et al., 2017). Nevertheless, the presence of small particles
(0.2–20 μm) has been observed at great depths (> 1000 m)
(e.g., Dall’Olmo and Mork, 2014; Briggs et al., 2020). Furthermore,
recent studies have shown that the downward flux of particulate organic
carbon (POC) via small particles (< 100 μm) can be significant
in specific ecosystems and seasons, at times constituting the bulk of
the total POC flux through the mesopelagic (e.g., Durkin et al., 2015;
Giering et al., 2016; Bisson et al., 2020; Dever et al., 2021).
The presence of ballast minerals (biogenic silica from diatoms,
particulate inorganic carbon from coccolithophores and foraminifera, and
lithogenic material from aeolian and riverine inputs) is also thought to
increase particle sinking velocity by increasing particle excess density
in respect to seawater (Armstrong et al., 2002; Passow and De La Rocha,
2006; Laurenceau-Cornec et al., 2019; Iversen and Lampitt, 2020;
Iversen, 2023). However, cause and effect in the relationship between
organic matter and minerals are not clear, and sinking aggregates
originating from biological activity in the mixed layer could scavenge
and subsequently transport small, suspended mineral particles to depth
(Passow, 2004).
The presence of transparent exopolymer particles (TEP; Alldredge et
al.,1993) also has the potential to influence the sinking of particles.
TEP are largely composed of polysaccharides released by phytoplankton
and bacteria as extracellular surface-active exopolymers (Passow, 2002),
especially under nutrient-limited conditions (Obernosterer and Herndl,
1995). TEP may act as biological glue and thus enhance particle
coagulation rate by increasing particle ”stickiness” (Passow et al.,
1994; Jackson, 1995). However, by being positively buoyant, TEP may also
reduce aggregates’ sinking velocities especially when aggregates are
characterized by high TEP-to-solid particles ratio (Engel and Schartau,
1999; Azetsu-Scott and Passow, 2004). Although TEP has the potential to
play an important role in controlling the downward transport of
particles, we lack a robust mechanistic understanding of this process
(e.g., Mari et al., 2017; Nagata et al., 2021).
Our hypothesis is that TEP play a critical role in determining the
sinking velocity of particles, potentially outcompeting the role of
ballast minerals. We assess how particle size and composition regulates
the partitioning between sinking and suspended particles within the
upper mesopelagic in an iron-limited region of the Northeast Pacific
Ocean. Particles were collected during the EXPORTS (Export Processes in
the Ocean from RemoTe Sensing) field campaign using Marine Snow Catchers
(MSC). Finally, we discuss the possible mechanisms driving the formation
of small sinking particles in the mesopelagic and highlight the
importance of studying small particle characteristics and particle
patchiness to enhance our understanding of the functioning of the
biological carbon pump.