22
3 Biology of Fronts
(a ubiquitous feature of fronts) may aid in controlling the size structure of phytoplankton near fronts affecting the structure of the food web (Rodriguez et al. 2001).
Fronts stimulate the growth of large-sized phytoplankton, mainly composed by
fast growing bloom specialists (Dutkiewicz et al. 2009), and support a classical food
chain with the dominance of herbivorous forms like copepods, euphausiids or appendicularians. Away from fronts, there is a tendency for the dominance of mostly carnivorous or omnivorous taxa in the zooplankton assemblages (Ohman et al. 2012).
Thus, vertical flows at fronts provide nutrients and turbulent energy that promote not
only high primary production but also shorter and more efficient food webs in which
a larger proportion of the primary production is channeled to larger organisms.
In most fronts the generated biomass is exploited by a trophic web involving,
at the higher trophic levels, highly mobile organisms (e.g. fish; squids; birds), and
as a consequence of their migrations the biomass produced at the front is exported
to remote oligotrophic areas. In this way, biomass over-accumulation is avoided
in the front and the exploited community remains juvenile and opportunistic.
The interaction of two parts of the system, a productive one and a consumer and
mobile one, may be considered as an exploitation (=biomass exportation) of an
ecosystem by another one. This is why the total effects of biological production at
fronts are hard to estimate, and ecological phenomena occurring at fronts may be
more important in determining the ecological properties of the area than the phenomena occurring inside the two adjacent water masses (Frontier 1986).
Phytoplankton size can also affect food supply to the benthos. Large cells sink
faster than small ones, since sinking rates increase exponentially with the cell size
and because larger cells aggregate more rapidly into sinking flocs than small cells.
Consequently, a greater fraction of the large cells primary production may sink out
of the water column (Kiorboe 1993). The production of large cells (e.g. diatoms)
at fronts could enhance the pelagic-benthic coupling because phytoplankton cells
may drift away from the upwelling core, promoting the arrival of phytoplankton to
the sea bed and increasing the heterotrophic activity of benthic communities.
It is virtually universal that among the plankton and the nekton a predator is
larger than its prey, although this is not so generally true of the benthos. Marine
animals live in a medium that is eight hundred times denser than air, where only
a streamlined morphology allows active and efficient movements (though small
plankton living under low Reynolds numbers are an exception to this (Mann and
Lazier 2006); this is why the development of appendages to handle and capture
large-sized prey is not common. Thus, a pelagic predator must have a jaw large
enough to swallow its prey as a whole. As the size of the jaw is related to the organism’s size, the predation process is believed to be largely determined by the size
ratio between predators and prey (Sheldon et al. 1977). On the other hand, feeding
on too small sized preys could be inefficient unless predators have special structures to concentrate small preys (e.g. the gill rakers of filter feeding fishes such as
sardines and anchovies; or the baleens of true whales), and at most sites predators
select large preys that they can swallow, even when small ones might be present.
Marine fronts may affect predator distribution by augmenting the profitability
of small sized preys. Many predators are threshold foragers, which require prey to
3 Biology of Fronts
(a ubiquitous feature of fronts) may aid in controlling the size structure of phytoplankton near fronts affecting the structure of the food web (Rodriguez et al. 2001).
Fronts stimulate the growth of large-sized phytoplankton, mainly composed by
fast growing bloom specialists (Dutkiewicz et al. 2009), and support a classical food
chain with the dominance of herbivorous forms like copepods, euphausiids or appendicularians. Away from fronts, there is a tendency for the dominance of mostly carnivorous or omnivorous taxa in the zooplankton assemblages (Ohman et al. 2012).
Thus, vertical flows at fronts provide nutrients and turbulent energy that promote not
only high primary production but also shorter and more efficient food webs in which
a larger proportion of the primary production is channeled to larger organisms.
In most fronts the generated biomass is exploited by a trophic web involving,
at the higher trophic levels, highly mobile organisms (e.g. fish; squids; birds), and
as a consequence of their migrations the biomass produced at the front is exported
to remote oligotrophic areas. In this way, biomass over-accumulation is avoided
in the front and the exploited community remains juvenile and opportunistic.
The interaction of two parts of the system, a productive one and a consumer and
mobile one, may be considered as an exploitation (=biomass exportation) of an
ecosystem by another one. This is why the total effects of biological production at
fronts are hard to estimate, and ecological phenomena occurring at fronts may be
more important in determining the ecological properties of the area than the phenomena occurring inside the two adjacent water masses (Frontier 1986).
Phytoplankton size can also affect food supply to the benthos. Large cells sink
faster than small ones, since sinking rates increase exponentially with the cell size
and because larger cells aggregate more rapidly into sinking flocs than small cells.
Consequently, a greater fraction of the large cells primary production may sink out
of the water column (Kiorboe 1993). The production of large cells (e.g. diatoms)
at fronts could enhance the pelagic-benthic coupling because phytoplankton cells
may drift away from the upwelling core, promoting the arrival of phytoplankton to
the sea bed and increasing the heterotrophic activity of benthic communities.
It is virtually universal that among the plankton and the nekton a predator is
larger than its prey, although this is not so generally true of the benthos. Marine
animals live in a medium that is eight hundred times denser than air, where only
a streamlined morphology allows active and efficient movements (though small
plankton living under low Reynolds numbers are an exception to this (Mann and
Lazier 2006); this is why the development of appendages to handle and capture
large-sized prey is not common. Thus, a pelagic predator must have a jaw large
enough to swallow its prey as a whole. As the size of the jaw is related to the organism’s size, the predation process is believed to be largely determined by the size
ratio between predators and prey (Sheldon et al. 1977). On the other hand, feeding
on too small sized preys could be inefficient unless predators have special structures to concentrate small preys (e.g. the gill rakers of filter feeding fishes such as
sardines and anchovies; or the baleens of true whales), and at most sites predators
select large preys that they can swallow, even when small ones might be present.
Marine fronts may affect predator distribution by augmenting the profitability
of small sized preys. Many predators are threshold foragers, which require prey to