Atlantic Polar Biome
153
Progressively during the summer, the deep chlorophyll maximum deepens to come
to lie on the upper slope of the nitracline. Very high sedimentation rates of algal cells
may occur during and just after the spring bloom, especially if this is dominated by
Phaeocystis. Rates of up to 1 g C m
−2 d
−1 have been recorded, though the average rate
in spring is about one-third of this value (Wassman et al., 1991). Part of this pulse of
sedimentation will be the result of zooplankton grazing and the release of rapidly sinking
fecal pellets, and there is some evidence that interannual variability extends also to the
percentage of total seasonal primary production consumed by primary herbivores, mainly
C. finmarchicus. In the summer of 1980, the calanoid herbivores consumed about 70%
of the total primary production, though they took a much smaller percentage in 1983
and 1984, when massive sedimentation occurred. Eilertsen et al. (1989b), on the other
hand, suggest that the greater part (80–95%) of the phytoplankton biomass produced in
spring settles unconsumed to the sea floor, though later in the season they find much
closer coupling between production and consumption. This rain of organic material to
the sediments, whether directly as aggregates or indirectly as fecal pellets, is surely the
key to the rich benthic fauna and demersal fish stocks of the Barents Sea (see later
discussion).
The species of herbivorous plankton in the SARC part of the Barents Sea and their
life cycles are similar to those of the Norwegian Sea and at OWS I. The principal
difference must be (though this seems not to have been investigated) that since their
normal overwintering depths are not available in the Barents Sea, the population probably
aggregates into the few deep basins, such as the Bear Island Channel. This is how similar
species manage their affairs in the Gulf of Maine, where they encounter the same problem
(see NWCS). Of course, copepods are only a part of the herbivore biomass and larger krill
are also important: Meganyctiphanes norvegicus in the Atlantic water, Thysanoessa inermis
in more coastal regions, and T. longicaudatus in Atlanto-Arctic water. All perform diel
migration, but Meganyctiphanes spends much of the year in the benthic habitat, swarming
near the surface only to reproduce. The filtering mechanism of these organisms restricts
their intake to the larger autotrophic cells.
Studies of the balance between production of zooplankton and its consumption by
fish, and the dynamic balance between different fish species biomasses, have been well
developed in this province and will repay a brief review. Apart from herring and blue whiting, the significant species are cod (Gadus morhua), mackerel (Scomber scombrus), horse
mackerel (T. trachurus), and capelin (M. mallotus) as well as a range of small mesopelagic
species. During summer, the feeding migrations of oceanic herring (and of more coastal
mackerel) involve a substantial intake of zooplankton food. It is especially the larger,
mature herring that perform the most extensive feeding migrations. The relationship
between Calanus and herring is close. Kaartvedt (2000) computes the balance between
the total energy requirement of the Atlanto-Scandia herring and the total production of
Calanus, one of its principal food items, in the Norwegian Sea. The 10–15 × 10
6 t of
herring require 4–7 times their own weight of food annually, and this consumption is
not distant from the 60–75 × 10
6 t annual production of Calanus, suggested from various
sources.
Associated with changes in the primary production cycle, there is also strong betweenyear variability in the primary herbivore population and hence in herring food; in
1980–1982 there were 2–5 × 10
5 ind m
−2 of C. finmarchicus in the central Barents Sea.
In 1983 and 1984, there were just 1 × 10
4 ind m
−2 . The same is seen in the OWS I data:
Irigoien’s analysis of these shows that adult Calanus were late in surfacing in spring 1972
and were very sparse, but in 1973 were abundant and early. Conversely, the OWS I data
for fish larvae show great differences in between-year abundance: in 1971 and 1972 they
were greatly more abundant than in the two subsequent summers.
153
Progressively during the summer, the deep chlorophyll maximum deepens to come
to lie on the upper slope of the nitracline. Very high sedimentation rates of algal cells
may occur during and just after the spring bloom, especially if this is dominated by
Phaeocystis. Rates of up to 1 g C m
−2 d
−1 have been recorded, though the average rate
in spring is about one-third of this value (Wassman et al., 1991). Part of this pulse of
sedimentation will be the result of zooplankton grazing and the release of rapidly sinking
fecal pellets, and there is some evidence that interannual variability extends also to the
percentage of total seasonal primary production consumed by primary herbivores, mainly
C. finmarchicus. In the summer of 1980, the calanoid herbivores consumed about 70%
of the total primary production, though they took a much smaller percentage in 1983
and 1984, when massive sedimentation occurred. Eilertsen et al. (1989b), on the other
hand, suggest that the greater part (80–95%) of the phytoplankton biomass produced in
spring settles unconsumed to the sea floor, though later in the season they find much
closer coupling between production and consumption. This rain of organic material to
the sediments, whether directly as aggregates or indirectly as fecal pellets, is surely the
key to the rich benthic fauna and demersal fish stocks of the Barents Sea (see later
discussion).
The species of herbivorous plankton in the SARC part of the Barents Sea and their
life cycles are similar to those of the Norwegian Sea and at OWS I. The principal
difference must be (though this seems not to have been investigated) that since their
normal overwintering depths are not available in the Barents Sea, the population probably
aggregates into the few deep basins, such as the Bear Island Channel. This is how similar
species manage their affairs in the Gulf of Maine, where they encounter the same problem
(see NWCS). Of course, copepods are only a part of the herbivore biomass and larger krill
are also important: Meganyctiphanes norvegicus in the Atlantic water, Thysanoessa inermis
in more coastal regions, and T. longicaudatus in Atlanto-Arctic water. All perform diel
migration, but Meganyctiphanes spends much of the year in the benthic habitat, swarming
near the surface only to reproduce. The filtering mechanism of these organisms restricts
their intake to the larger autotrophic cells.
Studies of the balance between production of zooplankton and its consumption by
fish, and the dynamic balance between different fish species biomasses, have been well
developed in this province and will repay a brief review. Apart from herring and blue whiting, the significant species are cod (Gadus morhua), mackerel (Scomber scombrus), horse
mackerel (T. trachurus), and capelin (M. mallotus) as well as a range of small mesopelagic
species. During summer, the feeding migrations of oceanic herring (and of more coastal
mackerel) involve a substantial intake of zooplankton food. It is especially the larger,
mature herring that perform the most extensive feeding migrations. The relationship
between Calanus and herring is close. Kaartvedt (2000) computes the balance between
the total energy requirement of the Atlanto-Scandia herring and the total production of
Calanus, one of its principal food items, in the Norwegian Sea. The 10–15 × 10
6 t of
herring require 4–7 times their own weight of food annually, and this consumption is
not distant from the 60–75 × 10
6 t annual production of Calanus, suggested from various
sources.
Associated with changes in the primary production cycle, there is also strong betweenyear variability in the primary herbivore population and hence in herring food; in
1980–1982 there were 2–5 × 10
5 ind m
−2 of C. finmarchicus in the central Barents Sea.
In 1983 and 1984, there were just 1 × 10
4 ind m
−2 . The same is seen in the OWS I data:
Irigoien’s analysis of these shows that adult Calanus were late in surfacing in spring 1972
and were very sparse, but in 1973 were abundant and early. Conversely, the OWS I data
for fish larvae show great differences in between-year abundance: in 1971 and 1972 they
were greatly more abundant than in the two subsequent summers.