152
Chapter 9: The Atlantic Ocean
unless they descend after sampling at OWS I ended in October. These genera are, in any
event, strong diel migrant genera wherever they occur.
Individuals of Thysanoessa longicaudata comprised >99% of the eight euphausiid
species that occurred in the OWS I profiles and were found to have a vertical migration
and reproduction strategy markedly different from that of the copepods just discussed.
The main cohorts of eggs are produced in March–April at from 100 to 800 m, with >90%
of nauplii subsequently occurring at >500 m. The first-feeding stage of euphausiids is the
calyptopa, and these are strongly biased surfaceward, with highest concentrations in the
upper 200 m in the period May–June, though with remnant individuals down to 500 m.
In June, furcilia larvae are restricted to the upper 200 m, and most occur in the upper
50 m. This, as Williams and Lindley (1982) suggested, is an ontogenetic migration that
represents the consequence of the progressive maturation of the larvae. Adults appear in
significant numbers in the profiles only from July until October, during which period
their distribution is, again, biased surfaceward, almost entirely within the upper 100 m.
Juveniles of the hyperiid amphipod Parathemisto gaudichaudi occur only within the upper
50 m, but the adults make significant diel migrations within the upper 200 m.
At OWS I, the near-weekly sampling schedule gave the somewhat surprising information that for many other planktonic organisms the generation time was relatively short:
6–8 weeks for amphipods and only 6 weeks for coelenterates.
Turning now to the southern Barents Sea, three areas having special characteristics
take our attention: (i) adjacent to the MIZ bordering BPLR to the north and east;
(ii) adjacent to the northern coast of Scandinavia, with its narrow shelf region; and
(iii) the main central area. Processes affecting stability of the water column are not
identical and, hence, the timing of the spring bloom differs in each area (Loeng, 1991)
and, as already noted, there is significant between-year variability in the location of the ice
edge, which is more consistent in the western than in the eastern Barents Sea. Thus, down
the western coast of Spitzbergen and around the south of Bear Island, this variability is
on the order of only a few tens of kilometers, but in the eastern end of the Barents Sea
it increases to about 500 km. In 1984, the ice front lay zonally along 75
N to encounter
the northwest coast of Novaya Zemyla; in 1979, it lay southeasterly from Hope Island to
the White Sea, a full 5
of latitude further south. Adjacent to the MIZ, a surface layer
of low-salinity water floods south and west above the more saline Atlantic water. This
creates sufficient near-surface stratification to sustain a very early spring bloom as soon
as sun angle increases sufficiently, perhaps as early as mid-March as in other seasonally
ice-covered seas. Beyond the influence of surface meltwater, thermal heating of the central
regions that are mixed to the sea bottom in winter may not produce sufficient stability
for a bloom to be initiated until much later in the summer—even as late as mid-June.
Coastal water near and above the continental shelf retains some stability even during
winter and here also a spring bloom begins as soon as local irradiance is sufficiently
strong. Though blooms in ice-melt surface water toward the boundary with BPLR rapidly
form a strong deep chlorophyll maximum at close to 50 m depth, the bloom in coastal
water often has uniform chlorophyll throughout the mixed layer (Mitchell et al., 1991a).
The Barents Sea bloom is well described by Skjoldal and Rey (1989), who suggest that it
may be initiated either by diatoms or by P. pouchetti, although small flagellates dominate
postbloom conditions in the classical manner. Winter nitrate in the photic zone is lower
than in other polar seas (12–14 M) and may be reduced to low or even undetectable
levels after the bloom. A deep chlorophyll maximum develops by midsummer within
the upper 50 m, with great between-year variability, forced by changes in stability and
pycnocline formation in the surface layer. In June 1982 there was no deep chlorophyll
maximum and residual nitrate was about 15 M, whereas in 1980 at the same place and
in the same month, nitrate was undetectable and there was a strong DCM at 30 m. In
1981, the situation was intermediate.
Chapter 9: The Atlantic Ocean
unless they descend after sampling at OWS I ended in October. These genera are, in any
event, strong diel migrant genera wherever they occur.
Individuals of Thysanoessa longicaudata comprised >99% of the eight euphausiid
species that occurred in the OWS I profiles and were found to have a vertical migration
and reproduction strategy markedly different from that of the copepods just discussed.
The main cohorts of eggs are produced in March–April at from 100 to 800 m, with >90%
of nauplii subsequently occurring at >500 m. The first-feeding stage of euphausiids is the
calyptopa, and these are strongly biased surfaceward, with highest concentrations in the
upper 200 m in the period May–June, though with remnant individuals down to 500 m.
In June, furcilia larvae are restricted to the upper 200 m, and most occur in the upper
50 m. This, as Williams and Lindley (1982) suggested, is an ontogenetic migration that
represents the consequence of the progressive maturation of the larvae. Adults appear in
significant numbers in the profiles only from July until October, during which period
their distribution is, again, biased surfaceward, almost entirely within the upper 100 m.
Juveniles of the hyperiid amphipod Parathemisto gaudichaudi occur only within the upper
50 m, but the adults make significant diel migrations within the upper 200 m.
At OWS I, the near-weekly sampling schedule gave the somewhat surprising information that for many other planktonic organisms the generation time was relatively short:
6–8 weeks for amphipods and only 6 weeks for coelenterates.
Turning now to the southern Barents Sea, three areas having special characteristics
take our attention: (i) adjacent to the MIZ bordering BPLR to the north and east;
(ii) adjacent to the northern coast of Scandinavia, with its narrow shelf region; and
(iii) the main central area. Processes affecting stability of the water column are not
identical and, hence, the timing of the spring bloom differs in each area (Loeng, 1991)
and, as already noted, there is significant between-year variability in the location of the ice
edge, which is more consistent in the western than in the eastern Barents Sea. Thus, down
the western coast of Spitzbergen and around the south of Bear Island, this variability is
on the order of only a few tens of kilometers, but in the eastern end of the Barents Sea
it increases to about 500 km. In 1984, the ice front lay zonally along 75
N to encounter
the northwest coast of Novaya Zemyla; in 1979, it lay southeasterly from Hope Island to
the White Sea, a full 5
of latitude further south. Adjacent to the MIZ, a surface layer
of low-salinity water floods south and west above the more saline Atlantic water. This
creates sufficient near-surface stratification to sustain a very early spring bloom as soon
as sun angle increases sufficiently, perhaps as early as mid-March as in other seasonally
ice-covered seas. Beyond the influence of surface meltwater, thermal heating of the central
regions that are mixed to the sea bottom in winter may not produce sufficient stability
for a bloom to be initiated until much later in the summer—even as late as mid-June.
Coastal water near and above the continental shelf retains some stability even during
winter and here also a spring bloom begins as soon as local irradiance is sufficiently
strong. Though blooms in ice-melt surface water toward the boundary with BPLR rapidly
form a strong deep chlorophyll maximum at close to 50 m depth, the bloom in coastal
water often has uniform chlorophyll throughout the mixed layer (Mitchell et al., 1991a).
The Barents Sea bloom is well described by Skjoldal and Rey (1989), who suggest that it
may be initiated either by diatoms or by P. pouchetti, although small flagellates dominate
postbloom conditions in the classical manner. Winter nitrate in the photic zone is lower
than in other polar seas (12–14 M) and may be reduced to low or even undetectable
levels after the bloom. A deep chlorophyll maximum develops by midsummer within
the upper 50 m, with great between-year variability, forced by changes in stability and
pycnocline formation in the surface layer. In June 1982 there was no deep chlorophyll
maximum and residual nitrate was about 15 M, whereas in 1980 at the same place and
in the same month, nitrate was undetectable and there was a strong DCM at 30 m. In
1981, the situation was intermediate.