15
modulates the light intensity experimented by phytoplankton cells. Consequently,
a phytoplankton population can only proliferate if mixing is shallower than such a
critical depth. On the other hand, particulate matter (the source for plant nutrients)
sinks in the sea (Fig. 3.1), and the export of organic matter from the upper ocean
to the deep ocean is an essential mechanism of the cycling of organic matter.
About 1–40 % of the photosynthetically fixed carbon sinks and is remineralized
in the deep ocean at substantially slower rates than in surface waters (Ducklow
et al. 2001). The resultant increase in dissolved inorganic carbon concentrations
towards the interior of the ocean, referred to as the biological pump, is regulated
by food web processes such as grazing. The tendency of particles to form aggregates accelerates the downward movement and the flux increases gradually with
depth. The remineralization of sinking particles by heterotrophic bacteria occurs
mostly at depth, and enriches the deep water masses with nutrients essential for
phytoplankton (Eppley and Peterson 1979; Jackson and Checkley 2011; Tesi et al.
2012). Consequently, most parts of the ocean are characterized by very low concentrations of plant nutrients in the illuminated layer, and abundance of nutrients
in the dark deep levels (Margalef 1978) (Fig. 3.1). Moreover, if the density of phytoplankton cells exceeds that of the surrounding waters they will sink; and if sinking is not countered by an upward current the cells will sink below the illuminated
zone. Hence the organization of the marine ecosystem is approximately defined
by gravity and light directions (Margalef 1997), and the entire living system is
crucially dependent on various processes by which organisms and materials are
transported and redistributed (Bakun 1996). Because marine fronts are characterized by relatively intense vertical circulations (Klein and Lapeyre 2009) and are
frequently associated with flow convergence, they provide mechanical energy that
contributes to the trophic energy balance of the biological community (Margalef
1978; Legendre et al. 1986; Bakun 1996).
Early observations of fronts emphasized the convergence of surface waters and
the associated downwelling, because any organism buoyant enough to resist the
downwelling would be expected to aggregate at the front (Fig. 3.2). Thus, it was
thought that passive advection was a possible explanation for the concentration of
planktonic organisms found at fronts. An alternative explanation invoked in situ
production, made possible by particularly favorable conditions of light and nutrients (Mann and Lazier 2006). There are a number of processes (all linked to the
secondary 1 circulation and hence not so easily distinguishable from each other) that
lead to the enrichment of nutrients for plants in the vicinity of fronts. The strong
horizontal pressure gradients generated at fronts are often balanced by the Coriolis
force, leading to strong along-front currents. When these two forces are not perfectly balanced, vertical circulations can be generated at the front. These vertical
circulations have the potential of carrying deep, nutrient-rich waters into the welllit surface layer, and stimulate phytoplankton growth (Sournia 1994). Both
1 Secondary circulation refers to the motion relative to a basic flow (geostrophic and hydrostatic
balanced).
3.1 Biological Production
modulates the light intensity experimented by phytoplankton cells. Consequently,
a phytoplankton population can only proliferate if mixing is shallower than such a
critical depth. On the other hand, particulate matter (the source for plant nutrients)
sinks in the sea (Fig. 3.1), and the export of organic matter from the upper ocean
to the deep ocean is an essential mechanism of the cycling of organic matter.
About 1–40 % of the photosynthetically fixed carbon sinks and is remineralized
in the deep ocean at substantially slower rates than in surface waters (Ducklow
et al. 2001). The resultant increase in dissolved inorganic carbon concentrations
towards the interior of the ocean, referred to as the biological pump, is regulated
by food web processes such as grazing. The tendency of particles to form aggregates accelerates the downward movement and the flux increases gradually with
depth. The remineralization of sinking particles by heterotrophic bacteria occurs
mostly at depth, and enriches the deep water masses with nutrients essential for
phytoplankton (Eppley and Peterson 1979; Jackson and Checkley 2011; Tesi et al.
2012). Consequently, most parts of the ocean are characterized by very low concentrations of plant nutrients in the illuminated layer, and abundance of nutrients
in the dark deep levels (Margalef 1978) (Fig. 3.1). Moreover, if the density of phytoplankton cells exceeds that of the surrounding waters they will sink; and if sinking is not countered by an upward current the cells will sink below the illuminated
zone. Hence the organization of the marine ecosystem is approximately defined
by gravity and light directions (Margalef 1997), and the entire living system is
crucially dependent on various processes by which organisms and materials are
transported and redistributed (Bakun 1996). Because marine fronts are characterized by relatively intense vertical circulations (Klein and Lapeyre 2009) and are
frequently associated with flow convergence, they provide mechanical energy that
contributes to the trophic energy balance of the biological community (Margalef
1978; Legendre et al. 1986; Bakun 1996).
Early observations of fronts emphasized the convergence of surface waters and
the associated downwelling, because any organism buoyant enough to resist the
downwelling would be expected to aggregate at the front (Fig. 3.2). Thus, it was
thought that passive advection was a possible explanation for the concentration of
planktonic organisms found at fronts. An alternative explanation invoked in situ
production, made possible by particularly favorable conditions of light and nutrients (Mann and Lazier 2006). There are a number of processes (all linked to the
secondary 1 circulation and hence not so easily distinguishable from each other) that
lead to the enrichment of nutrients for plants in the vicinity of fronts. The strong
horizontal pressure gradients generated at fronts are often balanced by the Coriolis
force, leading to strong along-front currents. When these two forces are not perfectly balanced, vertical circulations can be generated at the front. These vertical
circulations have the potential of carrying deep, nutrient-rich waters into the welllit surface layer, and stimulate phytoplankton growth (Sournia 1994). Both
1 Secondary circulation refers to the motion relative to a basic flow (geostrophic and hydrostatic
balanced).
3.1 Biological Production