Chapter 1
32
1.2. The biological
carbon pump
1.2.1. General view
The first step of the BCP occurs in the surface ocean, where phytoplankton species, which are
photoautotrophic organisms, use energy from sunlight to synthesize complex organic
compounds such as carbohydrates, lipids and proteins. This is known as primary production.
Phytoplankton can be eukaryotes (the cell contains a nucleus and other organelles enclosed
within membranes) such as the opal-secreting diatoms, prymnesiophytes (including the
CaCO
3
-secreting coccolithophores) and dinoflagellates; or prokaryotes (the cell lacks a distinct
nucleus and membrane bound organelles) such as cyanobacteria (Sigman and Hain, 2012).
In
addition to carbon, phytoplankton require
a wide range of nutrients, many of which become
limiting in certain regions of the ocean, exerting a significant control on surface ocean
production and ultimately the efficiency of the BCP (Moore et al., 2013). These elements are
presented in section 1.2.2.1. Other controls of the primary production in surface waters include
the dissolved CO
2
concentrations, temperature, stratification of the water column, and grazing.
The second step of the BCP is the export of organic matter from the sunlit surface ocean to
depth through different types of vertical transfer: 1) the passive sinking of particles and
aggregates, 2) the physical advection and diffusion between the surface and the deep ocean
and 3) the daily vertical migration of zooplankton, grazing phytoplankton, ingesting thus the
associated-chemical elements in the surface ocean and then excreting fecal pellets after their
descent to mesopelagic depths (100-1000 m). Sinking particles transport carbon but also all
the other elements composing POM, previously incorporated as nutrients by phytoplankton.
The export occurs when there is an excess density of the POM, i.e. when the density of the
POM is greater than the density of seawater. This generally occurs at the death of organisms
but most phytoplankton cells are too small to sink individually, so sinking occurs once they
aggregate into larger particles or are repackaged into fecal pellets by zooplankton. The
biogenic minerals (BSi and CaCO
3
), as well as the lithogenic minerals (dust, clay particles),
can be incorporated into aggregates, thereby increasing the excess density of settling particles
Chapter 1
34
Figure 1.6: Schematic view of the decrease of the downward POC flux with depth (Lampitt et al., 2008).
Therefore, it is important to determine POC fluxes from the surface to the deep sea in order to
assess the efficiency of the BCP. This efficiency will depend on the amount of primary
production in surface waters, itself depending on nutrient and light availability; the rate at which
the particles sink, depending on their size and density; and the degradation rate of organic C
and other elements
composing the particles, following grazing and remineralisation.
1.2.2. Influencing factors
1.2.2.1.
Chemical controls
-
Macro- and micro-nutrients availability
To grow, phytoplankton requires chemical elements, which are typically
identified as nutrients
(Sigman and Hain, 2012). Nutrients can be divided in two categories: macro- and micro-
nutrients, this distinction being made by their concentration in seawater (µmol.L
-1
and
nmol.L
-1
to pmol.L-
1
, respectively). In the following, the cycle of the elements of interest are
briefly described.