Atlantic Polar Biome
137
times are also characteristic of other arctic biota: chaetognaths (e.g., Sagitta elegans) have
a 2-year life cycle and reach an unusually large size in BPLR. Other polar organisms, such
as the predatory medusa Aglanthe digitale, have an entirely different pattern, reproducing
at depth essentially year-round, even in the Arctic Ocean.
Vertically stratified tows across the Lomonosov Ridge show that Calanus hyperboreus
in summer has a distribution that extends very deep, essentially to the bottom, while
maintaining a zone of maximum abundance in the upper 25 m, just below the ice cover.
Somewhat deeper, with maximum biomass at 200–300 m, we may expect to encounter
Gaetanus tenuispinus, whereas forms like Lucicutia spp. occupy the deepest water masses
(Kosobokova and Hirsche, 2000). Diel migrations are generally thought to be largely
absent in summer, though Groendahl and Hernroth (1986) did find both O. borealis and
M. longa performing short-range diel migrations in the Nansen Basin, to the north of
Spitzbergen, when the daily irradiance cycle became sufficiently strong to induce such
behavior; nevertheless, ontogenetic migrations are the principal determinant of vertical
distribution of herbivores. Conover (1988) described the seasonal succession of copepods
below fast ice as an ontogenetic escalator on which waves of the F0, F1, and F2 generations
(successively, and in that sequence) of C. hyperboreus and C. glacialis rise toward the
surface. Pseudocalanus, in contrast, remains close below the ice surface. With the erosion
of sheets of diatoms from the undersurface of the ice as summer advances, all copepods
come to be concentrated in the upper 20 m of the water column. At this season, depth
partitioning of the near-surface layer occurs between species and their growth stages,
which tend to be differentially distributed with depth.
In Resolute Sound in the Canadian archipelago, under full ice cover in February–
March, hyperiidid amphipods (Themisto libellula), small calanoid copepods (Pseudocalanus acuspes, Acartia spp., Oithona spp.), and naupliar stages of larger species aggregate
at very high concentrations just below the bottom surface of the ice. At this season the
epontic flora is starting to be dispersed, largely in response to tidal currents, and it was
observed experimentally that the copepods could fill their guts in just a few moments
from appropriate in situ suspended cell concentrations (Conover et al., 1991). This fauna,
in the period after light returns but while ice cover is still complete, perform a routine
short-range diel migration, mostly within the upper 10 m below the ice, rising at dusk to
feed and descending later in the night, replete (Hattori and Saito, 1995).
Marginal Ice Zones As Smith (1987) emphasized, ice edges or marginal ice zones
occur in a variety of geographic locations in this province over both deep and shallow
water. These zones are influenced by both physical conditions and biota and either may
form an abrupt edge, or else are represented by zones as much as several hundred
kilometers wide. For these reasons, it is not easy to generalize their ecology. Nevertheless,
we must try, and it may help if we concentrate our attention on the properties of a
receding ice edge in spring.
Seasonal ice cover in the open ocean is necessarily accompanied by a front where
pack ice meets open water. These seasonally migrating zones may extend laterally across
hundreds of kilometers, within which mesoscale variability is imposed by the interaction
of wind, current, and the ice edge (Smith, 1987). Retreat of the ice edge may occur
rapidly (10 km per day in the Bering Sea), and in 3 months the edge moves several
hundred kilometers. Rapid algal growth is induced in a shallow (often <25 m) mixed
layer. Retreating MIZs are then the focus of biological activity at all trophic levels; local
“biological spring” may, of course, occur on almost any date throughout the summer
months. Available nitrate is progressively depleted, though renewal may occur by iceedge upwelling (either density or wind driven); without such input, the algal bloom at a
rapidly retreating ice edge may be sustained for as little as 10 days (Smith and Sakshaug,
1990). There is an important ecological connection between epontic algal communities
137
times are also characteristic of other arctic biota: chaetognaths (e.g., Sagitta elegans) have
a 2-year life cycle and reach an unusually large size in BPLR. Other polar organisms, such
as the predatory medusa Aglanthe digitale, have an entirely different pattern, reproducing
at depth essentially year-round, even in the Arctic Ocean.
Vertically stratified tows across the Lomonosov Ridge show that Calanus hyperboreus
in summer has a distribution that extends very deep, essentially to the bottom, while
maintaining a zone of maximum abundance in the upper 25 m, just below the ice cover.
Somewhat deeper, with maximum biomass at 200–300 m, we may expect to encounter
Gaetanus tenuispinus, whereas forms like Lucicutia spp. occupy the deepest water masses
(Kosobokova and Hirsche, 2000). Diel migrations are generally thought to be largely
absent in summer, though Groendahl and Hernroth (1986) did find both O. borealis and
M. longa performing short-range diel migrations in the Nansen Basin, to the north of
Spitzbergen, when the daily irradiance cycle became sufficiently strong to induce such
behavior; nevertheless, ontogenetic migrations are the principal determinant of vertical
distribution of herbivores. Conover (1988) described the seasonal succession of copepods
below fast ice as an ontogenetic escalator on which waves of the F0, F1, and F2 generations
(successively, and in that sequence) of C. hyperboreus and C. glacialis rise toward the
surface. Pseudocalanus, in contrast, remains close below the ice surface. With the erosion
of sheets of diatoms from the undersurface of the ice as summer advances, all copepods
come to be concentrated in the upper 20 m of the water column. At this season, depth
partitioning of the near-surface layer occurs between species and their growth stages,
which tend to be differentially distributed with depth.
In Resolute Sound in the Canadian archipelago, under full ice cover in February–
March, hyperiidid amphipods (Themisto libellula), small calanoid copepods (Pseudocalanus acuspes, Acartia spp., Oithona spp.), and naupliar stages of larger species aggregate
at very high concentrations just below the bottom surface of the ice. At this season the
epontic flora is starting to be dispersed, largely in response to tidal currents, and it was
observed experimentally that the copepods could fill their guts in just a few moments
from appropriate in situ suspended cell concentrations (Conover et al., 1991). This fauna,
in the period after light returns but while ice cover is still complete, perform a routine
short-range diel migration, mostly within the upper 10 m below the ice, rising at dusk to
feed and descending later in the night, replete (Hattori and Saito, 1995).
Marginal Ice Zones As Smith (1987) emphasized, ice edges or marginal ice zones
occur in a variety of geographic locations in this province over both deep and shallow
water. These zones are influenced by both physical conditions and biota and either may
form an abrupt edge, or else are represented by zones as much as several hundred
kilometers wide. For these reasons, it is not easy to generalize their ecology. Nevertheless,
we must try, and it may help if we concentrate our attention on the properties of a
receding ice edge in spring.
Seasonal ice cover in the open ocean is necessarily accompanied by a front where
pack ice meets open water. These seasonally migrating zones may extend laterally across
hundreds of kilometers, within which mesoscale variability is imposed by the interaction
of wind, current, and the ice edge (Smith, 1987). Retreat of the ice edge may occur
rapidly (10 km per day in the Bering Sea), and in 3 months the edge moves several
hundred kilometers. Rapid algal growth is induced in a shallow (often <25 m) mixed
layer. Retreating MIZs are then the focus of biological activity at all trophic levels; local
“biological spring” may, of course, occur on almost any date throughout the summer
months. Available nitrate is progressively depleted, though renewal may occur by iceedge upwelling (either density or wind driven); without such input, the algal bloom at a
rapidly retreating ice edge may be sustained for as little as 10 days (Smith and Sakshaug,
1990). There is an important ecological connection between epontic algal communities