46
5 Comparisons of Fronts with Other Boundaries at Sea
determine the rate of upward nutrient transport are found to vary both in space and
time. The boundary layer characteristics depend on the smoothness of the local
and surrounding ice, the regularity of the current regime and other factors related
to the gravitational stability of the surrounding water (Demers et al. 1986).
Melting glacial ice in the ocean may lead to stratification associated with the
freshwater input and is also a source of nutrients, particularly Fe, which is dissolved and may fertilize the adjacent ocean. This process may be significant in the
Southern Ocean, which is characterized by low iron concentrations (Statham et al.
2008).
Herbivore zooplankton can feed on ice algae during springtime when water column productivity is low. These under-ice organisms are believed to represent a link
in the transfer of energy from ice algal production to amphipods to sea-birds and
mammals. Amphipods were observed to swim very close to the underside of the
ice and attach themselves for periods of time, as well as sometimes to enter cracks
and holes in the ice. Copepods and krill larvae are abundant under the ice, which
could be preyed upon by amphipods (Demers et al. 1986; Krapp et al. 2008). Ice
algae accessible from the underside of ice floes constitute an important resource
for larval as well as postlarval Antarctic krill. Observed declines of krill populations in some sectors of the Southern Ocean are presumably linked to decreasing
recruitment success caused by loss of sea ice habitat. The pronounced presence
of Antarctic krill under the ice highlights its potential as an energy transmitter
between the production of ice algae and the pelagic food web (Flores et al. 2012).
5.5 Fronts Contrasted with the Other Interfaces
Although biological properties of the non-frontal boundaries here accounted for
are not homogeneous (they really are different biotopes), they have in common
their large extensions, presenting much higher spatial coverage than fronts, which
are narrow regions. Moreover, non-frontal boundaries are quasi-horizontal instead
of vertically inclined interfaces, that is, they do not develop along the direction
of the gravity force and light that have a central ecological significance at sea
(Margalef 1997). There is a lack of nutrient pumping mechanisms at most nonfrontal boundaries; however some pycnoclines may oscillate due to the passage of
internal waves, producing vertical displacements of nutrients and phytoplankton.
Most of non-frontal boundaries act as a “substrate” that passively may aggregate
or concentrate nutrients and organisms due to physical properties (density, viscosity) or species behavior. However, such concentrations may be locally important.
Due to the lesser spatial scale and the action of mechanisms generating upward
nutrient fluxes, fronts are characterized by much higher concentration of biological productivity than other interfaces. As mentioned above, the mechanical energy
of the ocean becomes available for biological production at fronts. This auxiliary
energy is not directly used by plants, but it is efficient in providing a more or less
constant supply of nutrients for phytoplankton.
5 Comparisons of Fronts with Other Boundaries at Sea
determine the rate of upward nutrient transport are found to vary both in space and
time. The boundary layer characteristics depend on the smoothness of the local
and surrounding ice, the regularity of the current regime and other factors related
to the gravitational stability of the surrounding water (Demers et al. 1986).
Melting glacial ice in the ocean may lead to stratification associated with the
freshwater input and is also a source of nutrients, particularly Fe, which is dissolved and may fertilize the adjacent ocean. This process may be significant in the
Southern Ocean, which is characterized by low iron concentrations (Statham et al.
2008).
Herbivore zooplankton can feed on ice algae during springtime when water column productivity is low. These under-ice organisms are believed to represent a link
in the transfer of energy from ice algal production to amphipods to sea-birds and
mammals. Amphipods were observed to swim very close to the underside of the
ice and attach themselves for periods of time, as well as sometimes to enter cracks
and holes in the ice. Copepods and krill larvae are abundant under the ice, which
could be preyed upon by amphipods (Demers et al. 1986; Krapp et al. 2008). Ice
algae accessible from the underside of ice floes constitute an important resource
for larval as well as postlarval Antarctic krill. Observed declines of krill populations in some sectors of the Southern Ocean are presumably linked to decreasing
recruitment success caused by loss of sea ice habitat. The pronounced presence
of Antarctic krill under the ice highlights its potential as an energy transmitter
between the production of ice algae and the pelagic food web (Flores et al. 2012).
5.5 Fronts Contrasted with the Other Interfaces
Although biological properties of the non-frontal boundaries here accounted for
are not homogeneous (they really are different biotopes), they have in common
their large extensions, presenting much higher spatial coverage than fronts, which
are narrow regions. Moreover, non-frontal boundaries are quasi-horizontal instead
of vertically inclined interfaces, that is, they do not develop along the direction
of the gravity force and light that have a central ecological significance at sea
(Margalef 1997). There is a lack of nutrient pumping mechanisms at most nonfrontal boundaries; however some pycnoclines may oscillate due to the passage of
internal waves, producing vertical displacements of nutrients and phytoplankton.
Most of non-frontal boundaries act as a “substrate” that passively may aggregate
or concentrate nutrients and organisms due to physical properties (density, viscosity) or species behavior. However, such concentrations may be locally important.
Due to the lesser spatial scale and the action of mechanisms generating upward
nutrient fluxes, fronts are characterized by much higher concentration of biological productivity than other interfaces. As mentioned above, the mechanical energy
of the ocean becomes available for biological production at fronts. This auxiliary
energy is not directly used by plants, but it is efficient in providing a more or less
constant supply of nutrients for phytoplankton.