14
3 Biology of Fronts
3.1 Biological Production
Phytoplankton supports most of the trophic webs in the ocean (Smetacek 1999),
but major resources for those single-celled plants are heterogeneously distributed.
One of the problems confronting phytoplankton is their requirement of light and
nutrients for growth and reproduction, but the source of light is above, while the
source of nutrients is at depth (Margalef 1997). Phytoplankton inhabits the welllit upper few meters of the ocean, called the euphotic zone; the remainder of the
ocean is too dark to support net photosynthesis. Ambient light in the sea is logarithmically attenuated by scattering and absorption of seawater which affect various parts of the spectrum differentially with depth (Fig. 3.1). The euphotic zone
has a variable depth depending on several factors but it is always very thin relative
to the total depth of the ocean, extending hardly beyond 100 m in depth (Margalef
1997; Reynolds 2006). Turbulence within the surface layer moves phytoplankton cells up and down, thus plants carried by vertical fluxes experiment different
light intensities while travelling at different depths. The depth to which plants can
be mixed and at which the total photosynthesis for the water column is equal to
the total respiration is known as the “critical depth” (Sverdrup 1953). Turbulence
Fig. 3.1 Light intensity decay exponentially with depth; consequently the illuminated zone
(euphotic layer) results in a thin stratum as compared to the total ocean depth. Organic matter
sinks in the oceans; most of remineralization occurs in deep and dark waters, modified from Lalli
and Parsons (1997)
3 Biology of Fronts
3.1 Biological Production
Phytoplankton supports most of the trophic webs in the ocean (Smetacek 1999),
but major resources for those single-celled plants are heterogeneously distributed.
One of the problems confronting phytoplankton is their requirement of light and
nutrients for growth and reproduction, but the source of light is above, while the
source of nutrients is at depth (Margalef 1997). Phytoplankton inhabits the welllit upper few meters of the ocean, called the euphotic zone; the remainder of the
ocean is too dark to support net photosynthesis. Ambient light in the sea is logarithmically attenuated by scattering and absorption of seawater which affect various parts of the spectrum differentially with depth (Fig. 3.1). The euphotic zone
has a variable depth depending on several factors but it is always very thin relative
to the total depth of the ocean, extending hardly beyond 100 m in depth (Margalef
1997; Reynolds 2006). Turbulence within the surface layer moves phytoplankton cells up and down, thus plants carried by vertical fluxes experiment different
light intensities while travelling at different depths. The depth to which plants can
be mixed and at which the total photosynthesis for the water column is equal to
the total respiration is known as the “critical depth” (Sverdrup 1953). Turbulence
Fig. 3.1 Light intensity decay exponentially with depth; consequently the illuminated zone
(euphotic layer) results in a thin stratum as compared to the total ocean depth. Organic matter
sinks in the oceans; most of remineralization occurs in deep and dark waters, modified from Lalli
and Parsons (1997)