138
Chapter 9: The Atlantic Ocean
and the phytoplankton. As frazil ice forms in the fall, crystals floating upward scavenge
algal cells and incorporate them in the forming ice pack, and algae released from the
melting ice in spring may either sink as large flocs to the sediments or seed the burgeoning
phytoplankton community.
It is principally the effects of the freeze-thaw cycle of seawater that set the ecological
stage in the MIZ by determining the strength of near-surface stratification. Thawing sea
ice in spring releases freshwater to create strong density gradients, whereas freezing in
the autumn partitions seawater between freshwater ice and strong brine that is rejected
and sinks, causing instability and deep mixing. In late winter (March–April), as daylight
returns, the surface water column is deeply mixed, nutrient levels are high, and net carbon
fixation is not yet established (Smith and Brightman, 1991). It is precisely the effect of a
thawing, receding ice edge in spring that stabilizes the water column and gives conditions
likely to lead to phytoplankton bloom, especially where (as expected) cell growth will be
principally light limited. Where a spring bloom in an MIZ reduces nutrient levels below
optimal, further enhancement of algal growth may occur by ice-edge upwelling forced
by wind-driven transport of surface water away from the ice and its replacement from
below. For all these reasons, we may expect that MIZs should be a focus for biological
activity. Indeed, this is seen in the satellite imagery for the Greenland and the Labrador
Currents, both characterized by a receding ice edge in summer, and each of which tends
to be located in satellite chlorophyll fields as an area of enhanced chlorophyll. The Kara
Sea, east of Novaya Zemlya, has higher surface chlorophyll in summer than the Barents
Sea to the west, which has little winter ice cover (see the later section on the Atlantic
Subarctic Province).
Marginal ice zones develop from west Spitzbergen through the northern and eastern
Barents Sea with the vernal retreat of the ice edge. These induce upwelling, which may
be brief, wind-induced events, or longer density-induced events (Johannessen, 1986).
The low-salinity surface layer is on the order of 10–30 m deep at its most shoal position
near the ice edge when density-driven or wind-forced upwelling occurs. The MIZ of
the Chukchi Sea appears only in June, but only a small part of the sea becomes icefree; extremely strong stratification occurs at the ice edge, and reported rates of primary
production seem improbably high (<7 gC m
−2 d
−1 ). At any rate, a strong chlorophyll
maximum occurs at the base of the pycnocline, and nitrate levels are reduced to undetectable. In the Barents Sea, the MIZ retreats very rapidly northward across shallow water
in spring, whereas in the deep Fram Strait it remains close to the boundary between the
northward flow of Atlantic water and the southward flow of polar water. In both regions,
nitrate is reduced to low values after the summer bloom occurs.
In the shelf polynyas of the Arctic Ocean, as in the Beaufort Sea, a second phytoplankton bloom may occur toward the end of summer; this may be more intense
than the initial bloom and appears after some summer stratification of surface water
has occurred (Arrigo and van Dijken, 2004). The timing and intensity of blooms varies
strongly between years, responding to anomalous early warming and stratification (as in
1998) or a later summer ice melt and resulting stratification (as in 2002). In general, it
is found that nutrient limitation occurs over the deeper parts of the polynya, whereas
light limitation obtains near the coast below land-fast ice that delays the bloom by about
1 month. Somewhat similar regional characteristics force earlier and more intense algal
blooms on the Greenland side of the North Water in the early part of the season than
on the west (Odate et al., 2002).
A special case of MIZ occurs at glacier fronts in the Canadian archipelago and around
Greenland where diatom blooms occur, often an order of magnitude denser than elsewhere in the region; these blooms utilize the increased nutrient levels associated with
upwelling on the underwater ice cliffs. This process is forced within meltwater-driven
convection cells at the face of the glacier accompanied by caballing (mixing of waters of
Chapter 9: The Atlantic Ocean
and the phytoplankton. As frazil ice forms in the fall, crystals floating upward scavenge
algal cells and incorporate them in the forming ice pack, and algae released from the
melting ice in spring may either sink as large flocs to the sediments or seed the burgeoning
phytoplankton community.
It is principally the effects of the freeze-thaw cycle of seawater that set the ecological
stage in the MIZ by determining the strength of near-surface stratification. Thawing sea
ice in spring releases freshwater to create strong density gradients, whereas freezing in
the autumn partitions seawater between freshwater ice and strong brine that is rejected
and sinks, causing instability and deep mixing. In late winter (March–April), as daylight
returns, the surface water column is deeply mixed, nutrient levels are high, and net carbon
fixation is not yet established (Smith and Brightman, 1991). It is precisely the effect of a
thawing, receding ice edge in spring that stabilizes the water column and gives conditions
likely to lead to phytoplankton bloom, especially where (as expected) cell growth will be
principally light limited. Where a spring bloom in an MIZ reduces nutrient levels below
optimal, further enhancement of algal growth may occur by ice-edge upwelling forced
by wind-driven transport of surface water away from the ice and its replacement from
below. For all these reasons, we may expect that MIZs should be a focus for biological
activity. Indeed, this is seen in the satellite imagery for the Greenland and the Labrador
Currents, both characterized by a receding ice edge in summer, and each of which tends
to be located in satellite chlorophyll fields as an area of enhanced chlorophyll. The Kara
Sea, east of Novaya Zemlya, has higher surface chlorophyll in summer than the Barents
Sea to the west, which has little winter ice cover (see the later section on the Atlantic
Subarctic Province).
Marginal ice zones develop from west Spitzbergen through the northern and eastern
Barents Sea with the vernal retreat of the ice edge. These induce upwelling, which may
be brief, wind-induced events, or longer density-induced events (Johannessen, 1986).
The low-salinity surface layer is on the order of 10–30 m deep at its most shoal position
near the ice edge when density-driven or wind-forced upwelling occurs. The MIZ of
the Chukchi Sea appears only in June, but only a small part of the sea becomes icefree; extremely strong stratification occurs at the ice edge, and reported rates of primary
production seem improbably high (<7 gC m
−2 d
−1 ). At any rate, a strong chlorophyll
maximum occurs at the base of the pycnocline, and nitrate levels are reduced to undetectable. In the Barents Sea, the MIZ retreats very rapidly northward across shallow water
in spring, whereas in the deep Fram Strait it remains close to the boundary between the
northward flow of Atlantic water and the southward flow of polar water. In both regions,
nitrate is reduced to low values after the summer bloom occurs.
In the shelf polynyas of the Arctic Ocean, as in the Beaufort Sea, a second phytoplankton bloom may occur toward the end of summer; this may be more intense
than the initial bloom and appears after some summer stratification of surface water
has occurred (Arrigo and van Dijken, 2004). The timing and intensity of blooms varies
strongly between years, responding to anomalous early warming and stratification (as in
1998) or a later summer ice melt and resulting stratification (as in 2002). In general, it
is found that nutrient limitation occurs over the deeper parts of the polynya, whereas
light limitation obtains near the coast below land-fast ice that delays the bloom by about
1 month. Somewhat similar regional characteristics force earlier and more intense algal
blooms on the Greenland side of the North Water in the early part of the season than
on the west (Odate et al., 2002).
A special case of MIZ occurs at glacier fronts in the Canadian archipelago and around
Greenland where diatom blooms occur, often an order of magnitude denser than elsewhere in the region; these blooms utilize the increased nutrient levels associated with
upwelling on the underwater ice cliffs. This process is forced within meltwater-driven
convection cells at the face of the glacier accompanied by caballing (mixing of waters of