140
Chapter 9: The Atlantic Ocean
Euphausiids form no more than 0.1–7% of individuals. Recently, it has been found that
protozoan microplankton comprise only about 10–20% of total biomass.
Lipid storage is very important for copepods, especially in diapausing copepodites in
winter, whose ammonium excretion is reduced to very low levels (Head and Harris, 1985),
this being an especially important process in those taxa having multiyear generation times.
Calanus hyperboreus accumulates lipid especially during the C5 stage in preparation for
the adult molt, and for egg production at depth during the last winter of each multiyear
generation cycle (Conover, 1988). Wax ester levels rise from 0.4 to 21 mg ind
−1 between
June and September of their penultimate summer.
In the open-water summer situation there is a strong differentiation in depth selection
by zooplankton genera. Species respond to the three-layer partition of the water column
into (i) warmed surface water to 30–50 m, (ii) cold arctic water from there to 250 m,
and (iii) warmer Atlantic water below that. Zooplankton respond also to the depth of
the maximum aggregation of plant cells, usually at the junction between (ii) and (iii). In
summer, in northern Baffin Bay, most individuals of Gaidius tenuispinus and Metridia
longa occur in the upper part of the Atlantic water, whereas Oithona spp., Parathemisto
libellula, and Pseudocalanus minutus occur preferentially in the upper part of the cold
arctic water mass and just below the depth of maximal algal cell numbers. Limacina
helicina, for its part, is restricted to the warm surface water (Longhurst et al., 1984).
Stage-specific depth selection occurs during development and we must understand the
reproductive status of a population in order sensibly to interpret its vertical distribution.
C1–C4 of Calanus glacialis are restricted almost exclusively to the upper warm layer,
whereas stages 5 and adults are widely distributed within the depth zone of the cold arctic
water at 75–300 m.
As in warmer seas, detailed depth distributions (as are obtained with BIONESS or
LHPR sampling systems) indicate that herbivores tend to select either the depth of
maximum production rate or the slightly deeper chlorophyll maximum. Intrageneric
specialization may occur, because C. glacialis and C. finmarchicus follow the depth of
maximal production rate, whereas layers of C. hyperboreus occur deeper, near the chlorophyll maximum (Herman, 1983). One is not surprised, therefore, by such observations
as that in high summer, at the deep chlorophyll maximum in the MIZ northeast of
Spitzbergen, C. hyperboreus and C. glacialis consume from 65% to 90% of the daily
primary production. At the same time, they support about 35% of the local cell growth
by their own excretion of ammonium (Eilertsen et al., 1989a).
Nevertheless, the primary production/herbivore consumption ratio may takes a positive value in the photic zone (Longhurst and Head, 1989): in late summer, the remnant
herbivore grazers remove less than 1% of daily production in Baffin Bay and the fate
of most algal cells must be to sink through the pycnocline as soon as winter mixing
destroys this feature. Rapid, mass sinking of diatom-dominated algal floc is suspected to
be a feature of the final phase of blooms in this province.
Regional Benthic and Demersal Ecology
The extensive shelf sediments of the continental shelf encircling the Arctic Ocean are
relatively inaccessible to study, but we have sufficient information from parts of it for a
preliminary interpretation. One characteristic that is mentioned by several authors is the
massive end-of-summer sedimentation of phytoplankton cells, resulting in soft organicrich deposits on the floor of the continental shelf where its topography encourages
settlement. The relationships among benthic diversity, benthic biomass, and sediment
type (indicated by the relative content of organic matter and the C/N ratio) have been
investigated in the Chukchi Sea (Feder et al., 1994). There is some indication that
although hydrographic fronts encouraged the sedimentation of organic material and the
Chapter 9: The Atlantic Ocean
Euphausiids form no more than 0.1–7% of individuals. Recently, it has been found that
protozoan microplankton comprise only about 10–20% of total biomass.
Lipid storage is very important for copepods, especially in diapausing copepodites in
winter, whose ammonium excretion is reduced to very low levels (Head and Harris, 1985),
this being an especially important process in those taxa having multiyear generation times.
Calanus hyperboreus accumulates lipid especially during the C5 stage in preparation for
the adult molt, and for egg production at depth during the last winter of each multiyear
generation cycle (Conover, 1988). Wax ester levels rise from 0.4 to 21 mg ind
−1 between
June and September of their penultimate summer.
In the open-water summer situation there is a strong differentiation in depth selection
by zooplankton genera. Species respond to the three-layer partition of the water column
into (i) warmed surface water to 30–50 m, (ii) cold arctic water from there to 250 m,
and (iii) warmer Atlantic water below that. Zooplankton respond also to the depth of
the maximum aggregation of plant cells, usually at the junction between (ii) and (iii). In
summer, in northern Baffin Bay, most individuals of Gaidius tenuispinus and Metridia
longa occur in the upper part of the Atlantic water, whereas Oithona spp., Parathemisto
libellula, and Pseudocalanus minutus occur preferentially in the upper part of the cold
arctic water mass and just below the depth of maximal algal cell numbers. Limacina
helicina, for its part, is restricted to the warm surface water (Longhurst et al., 1984).
Stage-specific depth selection occurs during development and we must understand the
reproductive status of a population in order sensibly to interpret its vertical distribution.
C1–C4 of Calanus glacialis are restricted almost exclusively to the upper warm layer,
whereas stages 5 and adults are widely distributed within the depth zone of the cold arctic
water at 75–300 m.
As in warmer seas, detailed depth distributions (as are obtained with BIONESS or
LHPR sampling systems) indicate that herbivores tend to select either the depth of
maximum production rate or the slightly deeper chlorophyll maximum. Intrageneric
specialization may occur, because C. glacialis and C. finmarchicus follow the depth of
maximal production rate, whereas layers of C. hyperboreus occur deeper, near the chlorophyll maximum (Herman, 1983). One is not surprised, therefore, by such observations
as that in high summer, at the deep chlorophyll maximum in the MIZ northeast of
Spitzbergen, C. hyperboreus and C. glacialis consume from 65% to 90% of the daily
primary production. At the same time, they support about 35% of the local cell growth
by their own excretion of ammonium (Eilertsen et al., 1989a).
Nevertheless, the primary production/herbivore consumption ratio may takes a positive value in the photic zone (Longhurst and Head, 1989): in late summer, the remnant
herbivore grazers remove less than 1% of daily production in Baffin Bay and the fate
of most algal cells must be to sink through the pycnocline as soon as winter mixing
destroys this feature. Rapid, mass sinking of diatom-dominated algal floc is suspected to
be a feature of the final phase of blooms in this province.
Regional Benthic and Demersal Ecology
The extensive shelf sediments of the continental shelf encircling the Arctic Ocean are
relatively inaccessible to study, but we have sufficient information from parts of it for a
preliminary interpretation. One characteristic that is mentioned by several authors is the
massive end-of-summer sedimentation of phytoplankton cells, resulting in soft organicrich deposits on the floor of the continental shelf where its topography encourages
settlement. The relationships among benthic diversity, benthic biomass, and sediment
type (indicated by the relative content of organic matter and the C/N ratio) have been
investigated in the Chukchi Sea (Feder et al., 1994). There is some indication that
although hydrographic fronts encouraged the sedimentation of organic material and the