17
In late winter in mid to high latitudes, when phytoplankton growth is limited
by lack of sufficient light intensity the upper water column tends to be well mixed.
Under these conditions the onset of stratification leading to the spring bloom is
thought to be triggered by net heat flux through the sea surface. The lack of vertical stratification required to support phytoplankton close to the illuminated surface layers, however, can also be overcome by lateral mixing across fronts. Recent
observations from the subpolar North Atlantic and biophysical models indicate
that the initial stratification and resulting bloom can be caused by eddy-driven
slumping of the cross-front density gradient (Mahadevan et al. 2012). These analyses show that frontal eddies exchange cold-dense water equatorward and warmlight water poleward creating shallow mixed-layers about 20–30 days earlier than
would occur by surface warming. The combination of very weak vertical stratification and turbulent regime that characterizes subpolar waters limits the growth
of phytoplankton regardless of the relatively high nutrient concentrations, creating the so-called high-nutrient low-chlorophyll (HNLC) environments. At the
transition between oligotrophic subtropical and HNLC subpolar waters, however,
small-scale cross-frontal mixing can create layers with sufficient nutrients and
stratification to promote phytoplankton growth (Brandini et al. 2000).
Secondary circulations in frontal areas not only promote fertilization by nutrients, but also may result in downward export of particles and organisms toward
subsurface layers and account for the persistence of large populations of both
invertebrate and vertebrate species at depth (Sournia 1994). Pelagic productivity
and physical processes largely determine the quantity and quality of organic matter reaching the seafloor, which can be derived from a variety of sources, including phyto- and zooplankton remains, crustacean molts, macro-aggregates (‘marine
snow’), and fecal pellets; all originating in the primary productivity by phytoplankton (Berkenbusch et al. 2011). In general terms, the flux of organic carbon
is directly linked to surface-water productivity and the supply of organic matter
generally decreases with increasing water depth. The importance of fronts for benthic communities arises not only from the high primary production but is also due
to the vertical fluxes that transport food particles to the seafloor, and due to the
weakening of the vertical stratification that increases the sinking rates of particulate matter. Once on the bottom, the relatively un-degraded material is rapidly consumed and incorporated into benthic biomass.
Enhanced biological production occurs at fronts as a consequence of the matching or resonance of physical scales with biological scales (Legendre et al. 1986).
The length of the organism’s generation relative to the persistence and predictability
of a front will determine whether its response is by population growth or behavioral mechanisms (Angel 1986). Plankton generation time is of the order of days or
weeks (Legendre et al. 1986) consequently their abundances at fronts is partly due to
population growth (in some fronts concentration by convergence is also important).
Medium and larger predators actively seek fronts, showing a behavioral response.
High food availability at fronts attract nekton organisms (e.g. fish, squids) transferring the energy to higher trophic levels. The response of organisms to fronts integrates simultaneous reactions to main physical, chemical and biological gradients,
3.1 Biological Production
In late winter in mid to high latitudes, when phytoplankton growth is limited
by lack of sufficient light intensity the upper water column tends to be well mixed.
Under these conditions the onset of stratification leading to the spring bloom is
thought to be triggered by net heat flux through the sea surface. The lack of vertical stratification required to support phytoplankton close to the illuminated surface layers, however, can also be overcome by lateral mixing across fronts. Recent
observations from the subpolar North Atlantic and biophysical models indicate
that the initial stratification and resulting bloom can be caused by eddy-driven
slumping of the cross-front density gradient (Mahadevan et al. 2012). These analyses show that frontal eddies exchange cold-dense water equatorward and warmlight water poleward creating shallow mixed-layers about 20–30 days earlier than
would occur by surface warming. The combination of very weak vertical stratification and turbulent regime that characterizes subpolar waters limits the growth
of phytoplankton regardless of the relatively high nutrient concentrations, creating the so-called high-nutrient low-chlorophyll (HNLC) environments. At the
transition between oligotrophic subtropical and HNLC subpolar waters, however,
small-scale cross-frontal mixing can create layers with sufficient nutrients and
stratification to promote phytoplankton growth (Brandini et al. 2000).
Secondary circulations in frontal areas not only promote fertilization by nutrients, but also may result in downward export of particles and organisms toward
subsurface layers and account for the persistence of large populations of both
invertebrate and vertebrate species at depth (Sournia 1994). Pelagic productivity
and physical processes largely determine the quantity and quality of organic matter reaching the seafloor, which can be derived from a variety of sources, including phyto- and zooplankton remains, crustacean molts, macro-aggregates (‘marine
snow’), and fecal pellets; all originating in the primary productivity by phytoplankton (Berkenbusch et al. 2011). In general terms, the flux of organic carbon
is directly linked to surface-water productivity and the supply of organic matter
generally decreases with increasing water depth. The importance of fronts for benthic communities arises not only from the high primary production but is also due
to the vertical fluxes that transport food particles to the seafloor, and due to the
weakening of the vertical stratification that increases the sinking rates of particulate matter. Once on the bottom, the relatively un-degraded material is rapidly consumed and incorporated into benthic biomass.
Enhanced biological production occurs at fronts as a consequence of the matching or resonance of physical scales with biological scales (Legendre et al. 1986).
The length of the organism’s generation relative to the persistence and predictability
of a front will determine whether its response is by population growth or behavioral mechanisms (Angel 1986). Plankton generation time is of the order of days or
weeks (Legendre et al. 1986) consequently their abundances at fronts is partly due to
population growth (in some fronts concentration by convergence is also important).
Medium and larger predators actively seek fronts, showing a behavioral response.
High food availability at fronts attract nekton organisms (e.g. fish, squids) transferring the energy to higher trophic levels. The response of organisms to fronts integrates simultaneous reactions to main physical, chemical and biological gradients,
3.1 Biological Production