Chapter 1
44
of the downward flux with the large organic aggregates (De La Rocha and Passow, 2007;
Ebersbach and Trull, 2008; Laurenceau-Cornec et al., 2015a; Turner, 2015). However,
zooplankton itself can cause the degradation of the downward flux of fecal pellets through
coprophagy (ingestion of fecal pellets), coprorhexy (fragmentation of fecal pellets), and
coprochaly (removal of fecal pellets
membrane; Belcher et al., 2016).
Generally, zooplankton communities consume organic particles in surface waters during the
night and produce fecal pellets at depth during the day (Longhurst and Glen Harrison, 1988).
This vertical migration contributes from a few to 70% of POC flux depending on the season,
the region and the species composition (Steinberg et al., 2000). For example, in the Southern
Ocean, recent studies indicate that despite the low primary production, high zooplankton-
mediated export and low microbial abundance could explain the high export efficiency (Cavan
et al., 2015; Le Moigne et al., 2016).
Finally, under high grazing pressure and in order to avoid predation, diatoms are able to
change their density, by increasing their silica content (Pondaven et al., 2007). The cell wall
silicification directly impacts the efficiency of the export by promoting resistance to grazing and
the subsequent remineralization and by increasing the export fluxes through fast-sinking
diatoms (Quéguiner, 2013).
-
Microbial activity
Bacterial remineralization is the main mechanism of particle degradation in seawater (Herndl
and Reinthaler, 2013), in particular within the mesopelagic zone, where microbes are
responsible for 70 to 92% of the remineralization (Giering et al., 2014). This microbial loop can
be stimulated by the release of DOC from zooplankton activities, enzymatic solubilization or
mechanical disaggregation (Collins et al., 2015; Giering et al., 2014; Steinberg et al., 2008).
Temperature seems to also regulate the heterotrophic respiration with shallower
remineralization in warm waters (Belcher et al., 2016; Laws et al., 2000; Marsay et al., 2015).
Bacteria associated to particles solubilize POC into DOC which is respired directly or used and
respired by free-living bacteria in the mesopelagic zone (Figure 1.9).
Chapter 1
46
1.3. The different approaches to study the
biological carbon pump
1.3.1. Particle
export measurement
There are several methods to estimate the export fluxes. Eppley (1989) described four major
approaches: nutrient uptakes, geochemical budgets, sediment traps and naturally occurring
particle-reactive radionuclides. The measurement of nutrient uptake rates using
15
N in bottle
incubation experiments allow the estimation of the new production via the
f-ratio concept
(Dugdale and Goering, 1967; Eppley and Peterson, 1979). The geochemical budgets (of
nutrient or oxygen) is based on the fact that the nutrient supply in the euphotic zone is balanced
by the downward export of organic matter (Eppley and Peterson, 1979). The naturally occurring
particle-reactive radionuclides and the sediment traps methods are broadly used and are
explained in more details thereafter. Other approaches, recently developed thanks to
technological advances, can be used:
in-situ optical methods allowing the measurement of the
particle size distribution and abundance (Boss et al., 2015); satellite-based models (Siegel et
al., 2014).
-
The naturally occurring particle-reactive radionuclides
The naturally occurring particle-reactive radionuclides are tracers for particle sinking and are
of specific interest to the present work. This method uses the different properties in seawater
and the different half-lives of a radioisotope pair to estimate the export fluxes. The
234
Th:
238
U
pair is well-known and has been used extensively to study the particle export fluxes in the
upper ocean (e.g., Buesseler et al., 1992; Coale and Bruland, 1985; Cochran and Masqué,
2003). Besides, the
210
Pb:
210
Po pair has been also used to determine export fluxes but in a
more limited number of studies (e.g., Le Moigne et al., 2013; Roca-Marti et al., 2016b; Stewart
et al., 2007).
In order to quantify elemental export fluxes, these indirect methods need to determine the ratio
of the element of interest to
234
Th or
210
Po in sinking particles that are usually collected with
in-
situ pumps (ISP). ISP are large volume filtration systems, that allow the collection of adequate