136
Chapter 9: The Atlantic Ocean
the Arctic Ocean found these colored patches along most of the track from 83
N to
the pole. In some places in the central basin, >10% of the snow surface was discolored.
Estimates of their potential gross production vary greatly, but are significant in relation
to the equally variable estimates for total primary production below the ice of the Arctic
Ocean (Gradinger and Nürnberg, 1996).
It is also necessary to recognize that the ice itself or, rather, cavities within the ice
along its lower surface are an important habitat for many species of planktonic algae.
These epontic cells, although they occur in a low-light environment, are not an obligate
shade flora and their population growth is limited—as in the water column—by both
light and inorganic nutrient supply: Melosira arctica and loosely attached mats of both
centric and pennate diatoms are characteristic of this flora. As Smith et al. (1987) commented, ice algae are a case study in photoautotrophic growth and of metabolism under
chronically low temperature and irradiance: they display marked shifts in metabolism
that are consistent with changes in light and nutrient supply. The growth of epontic algae
below sea ice begins as soon as sunlight returns in spring. On the underside of ice 1–2 m
thick the light regime resembles that of the lower photic zone in open water, receiving
<2% of incident illumination. The photosynthetic efficiency of epontic cells is unusually
great so that they are able to photosynthesize down to 0.01% of surface illumination.
Nutrient supply and demand are more complex than in the water column and involve
nutrients dissolved in brine rejected from the ice, whereas external supply depends on
hydrodynamic forcing of wind and tide, and the turbulent water flow along the lower
surface of the ice (Cota et al., 1990). Some local heterotrophic regeneration occurs, just
as in the water column. The epontic ice flora is intolerant of low salinities and collapses
rapidly when ice melts during the arctic summer.
Phytoplankton blooms under the ice may occur at times and places where the sea
ice becomes snow free, albeit at rates of about one-third of the water column over the
surrounding continental shelves and MIZs (Legendre et al., 1992; English, 1961).
The water column below the Arctic Ocean ice is inhabited by a sparse but permanent
zooplankton community, higher biomass being located in the central basin, where ice
cover is not so thick as around the margins; on the 1994 polar transect, highest biomass
occurred at 87
N, in the Amundsen Basin, and here it represented—because of very high
individual lipid content—40% of the total POC in the upper 100 m of the water column.
These organisms, during the daylight period, were ingesting <30% of their body carbon
daily as phytoplankton (Thibault et al., 1999).
Zooplankton biomass (0–200 m) below ice cover is dominated, as elsewhere in the Arctic, by calanoid copepods: Calanus glacialis, C. hyperboreus, and Metridia longa and larger
numbers, but smaller biomass, of Pseudocalanus, Oithona, Microcalanus, and Oncaea. In
the Nansen Basin, Oncaea borealis forms 40–80% of the individuals. It may be noted
that the geographical distribution of C. glacialis approximately matches the boundaries
of the BPLR province (Conover, 1988) and this species is often associated with shallow
water over very high latitude shelf regions, having a preferred water temperature of about
−05
C (Longhurst et al., 1984). In the Labrador Sea region, it is restricted essentially to
this habitat, being replaced by C. hyperboreus and C. finmarchicus over deep water (Head
et al., 2003). The small Pseudocalanus minutus is distributed similarly in this province
and the even smaller Microcalanus pygmaeus is the most abundant copepod of the Arctic
Ocean proper. Metridia longa is also distributed in such a manner as to be consistent
with having an Arctic source, compared with the Atlantic M. lucens.
Calanus hyperboreus requires 3 years to complete its life cycle in the Arctic Ocean. In
the first year it achieves growth to C2, in the second year it reaches C4 or C5, and in
the third year becomes adult and reproduces (Conover, 1988; Smith and Schnack-Schiel,
1990). Of course, there is some effect of latitude on generation time, so that in the North
Water, many individuals become adult during their second year of life. Long generation
Chapter 9: The Atlantic Ocean
the Arctic Ocean found these colored patches along most of the track from 83
N to
the pole. In some places in the central basin, >10% of the snow surface was discolored.
Estimates of their potential gross production vary greatly, but are significant in relation
to the equally variable estimates for total primary production below the ice of the Arctic
Ocean (Gradinger and Nürnberg, 1996).
It is also necessary to recognize that the ice itself or, rather, cavities within the ice
along its lower surface are an important habitat for many species of planktonic algae.
These epontic cells, although they occur in a low-light environment, are not an obligate
shade flora and their population growth is limited—as in the water column—by both
light and inorganic nutrient supply: Melosira arctica and loosely attached mats of both
centric and pennate diatoms are characteristic of this flora. As Smith et al. (1987) commented, ice algae are a case study in photoautotrophic growth and of metabolism under
chronically low temperature and irradiance: they display marked shifts in metabolism
that are consistent with changes in light and nutrient supply. The growth of epontic algae
below sea ice begins as soon as sunlight returns in spring. On the underside of ice 1–2 m
thick the light regime resembles that of the lower photic zone in open water, receiving
<2% of incident illumination. The photosynthetic efficiency of epontic cells is unusually
great so that they are able to photosynthesize down to 0.01% of surface illumination.
Nutrient supply and demand are more complex than in the water column and involve
nutrients dissolved in brine rejected from the ice, whereas external supply depends on
hydrodynamic forcing of wind and tide, and the turbulent water flow along the lower
surface of the ice (Cota et al., 1990). Some local heterotrophic regeneration occurs, just
as in the water column. The epontic ice flora is intolerant of low salinities and collapses
rapidly when ice melts during the arctic summer.
Phytoplankton blooms under the ice may occur at times and places where the sea
ice becomes snow free, albeit at rates of about one-third of the water column over the
surrounding continental shelves and MIZs (Legendre et al., 1992; English, 1961).
The water column below the Arctic Ocean ice is inhabited by a sparse but permanent
zooplankton community, higher biomass being located in the central basin, where ice
cover is not so thick as around the margins; on the 1994 polar transect, highest biomass
occurred at 87
N, in the Amundsen Basin, and here it represented—because of very high
individual lipid content—40% of the total POC in the upper 100 m of the water column.
These organisms, during the daylight period, were ingesting <30% of their body carbon
daily as phytoplankton (Thibault et al., 1999).
Zooplankton biomass (0–200 m) below ice cover is dominated, as elsewhere in the Arctic, by calanoid copepods: Calanus glacialis, C. hyperboreus, and Metridia longa and larger
numbers, but smaller biomass, of Pseudocalanus, Oithona, Microcalanus, and Oncaea. In
the Nansen Basin, Oncaea borealis forms 40–80% of the individuals. It may be noted
that the geographical distribution of C. glacialis approximately matches the boundaries
of the BPLR province (Conover, 1988) and this species is often associated with shallow
water over very high latitude shelf regions, having a preferred water temperature of about
−05
C (Longhurst et al., 1984). In the Labrador Sea region, it is restricted essentially to
this habitat, being replaced by C. hyperboreus and C. finmarchicus over deep water (Head
et al., 2003). The small Pseudocalanus minutus is distributed similarly in this province
and the even smaller Microcalanus pygmaeus is the most abundant copepod of the Arctic
Ocean proper. Metridia longa is also distributed in such a manner as to be consistent
with having an Arctic source, compared with the Atlantic M. lucens.
Calanus hyperboreus requires 3 years to complete its life cycle in the Arctic Ocean. In
the first year it achieves growth to C2, in the second year it reaches C4 or C5, and in
the third year becomes adult and reproduces (Conover, 1988; Smith and Schnack-Schiel,
1990). Of course, there is some effect of latitude on generation time, so that in the North
Water, many individuals become adult during their second year of life. Long generation