Chapter 1
41
a more rapid aggregation or by increasing density and in fine, sinking velocity (Armstrong et
al., 2002; Le Moigne et al., 2013b). The transfer efficiency of POC to the deep ocean has been
strongly related to the calcite flux (François et al., 2002) because calcite is denser, more
abundant than opal and clays (Klass and Archer, 2002) and is susceptible to dissolve less than
opal in the upper water column (Nelson and Brzezinski, 1997). A recent study has also
highlighted the importance of calcium carbonate to export POC in the Southern Ocean (Salter
et al., 2014), where the carbon export has usually been attributed to large and heavily silicified
diatoms. Nonetheless, diatoms represent an important vector of carbon export as they carry
out half of the primary production in the world ocean (Buesseler, 1998; De La Rocha and
Passow, 2007; Tréguer et al., 1995).
Finally, recent studies have demonstrated that the effect of ballast minerals on the POC flux
varies regionally (Frédéric A C Le Moigne et al., 2014; Le Moigne et al., 2012). In the high-
latitude North Atlantic, mineral ballasting seems to be important with 60% of the POC flux
associated with minerals (on average, mostly CaCO
3
and BSi) whereas this fraction is only
about 40% in Southern Ocean (on average, mostly BSi). The remainder of the export flux is
unballasted, suggesting that the association between POC and minerals is not always
necessary to promote the export of organic matter. In this case, low export efficiencies due to
the easier degradation of the unballasted POC have been reported (e.g. Le Moigne et al.,
2012). However, some discrepancies have been observed, indicating that other parameters
such as low microbial activity or large zooplankton migration can impact the magnitude of
surface export (Le Moigne et al., 2016).
1.2.2.2.
Biological controls
Spatial variations in phytoplankton size structure are known to exert a control on the magnitude
of the POC export flux (Boyd and Newton, 1999) and high POC exports are usually related to
a greater size of the sinking phytoplankton cells (Alldredge and Silver, 1988; Guidi et al., 2009).
In addition to the size structure, the composition of the phytoplankton community has been
Chapter 1
42
shown to influence the magnitude of the biogeochemical export fluxes. As seen in the previous
section, diatoms and calcifying species are important vectors of carbon export, as their ‘hard
parts’ (BSi and CaCO
3
, respectively) can be incorporated into aggregates and increase the
excess density of suspended particles provoking an increase of the sinking velocity (Honjo,
1996). The life stage of the certain phytoplankton communities can also influence the
magnitude of the export fluxes. For example, the formation of diatom resting spores has been
shown to strongly enhance the deep carbon export (Rembauville et al., 2016b, 2015a;
Rynearson et al., 2013; Salter et al., 2012). The succession of the different phytoplankton
communities as well as the succession of the different life stages can be triggered by a nutrient
limitation (see section 1.2.2.1; Sugie and Kuma, 2008) but also by turbulence (Margalef, 1978)
or light changes (Lasbleiz et al., 2016).
The magnitude and the fate of the primary production exported from surface waters to deeper
depths are also related to remineralization processes that are led by zooplankton and bacteria
activities in the euphotic zone but also in the mesopelagic zone to support their metabolic
demands (Figure 1.9). Modest variations of the remineralization depth have been shown to
impact strongly the air-sea carbon balance. For example, a remineralization depth increasing
by only 24 m would induce a decrease in atmospheric CO
2
concentrations from 10 to 30 ppm,
i.e., ~3 to 9% of the present atmospheric pCO
2
(Kwon et al., 2009).
Chapter 1
43
Figure 1.9: Attenuation of the carbon export flux from the mixed layer into the mesopelagic zone by the
microbial and zooplankton metabolisms (Steinberg et al., 2008).
- Grazing and fecal pellets production by zooplankton
Food sources for micro- and meso-zooplankton are phytoplankton and sinking phytodetrital
aggregates (also known as “marine snow”). Zooplankton communities can fragment large
sinking particles into smaller less-sinking particles, leading to a POC flux decrease (Steinberg
et al., 2008). Once consumed, a fraction of carbon is respired back into DIC or released as
DOC (Allredge and Jackson, 1995). Similarly, nutrients such as N or Fe are released to the
water column after zooplankton regeneration (e.g. Giering et al., 2012). This process can
account for 30 to 100% of total Fe supplied to the euphotic zone (Bowie et al., 2001; Sarthou
et al., 2008; Strzepek et al., 2005) and is also a significant source of bioavailable Fe (Dalbec
and Twining, 2009; Nuester et al., 2014). Zooplankton respiration has been estimated to
account for 7-66% of the loss of sinking POC flux in the bathypelagic (> 1000 m), depending
on region and season (Burd et al., 2010). Another fraction of the carbon consumed by
zooplankton is exported as sinking fecal pellets, packing the phytoplankton cells and
increasing the downward flux (Silver and Gowing, 1991). The fecal pellets can be dense and
fast sinking in the water column and many studies have considered them as a major constituent
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