Atlantic Polar Biome
151
with it in others, C5 individuals and adults rise from overwintering depths that are
in excess of 500 m. By mid-April, essentially the whole population lies shallower than
100 m, and from about April 10, the first copepodites of the new generation begin to
appear.
2. Production of first generation: Throughout May and during the first 10 days of June,
population biomass increases very fast with the growth of copepodites, and in the
second half of this period adults of the first new generation appear. The population
remains almost entirely within the upper 150 m, with many profiles showing crowding
into the upper 50 m. Diel migration, if it occurs, is shallow.
3. Multigeneration period, some individuals descending: From mid-June until midSeptember, population biomass in the upper 500 m declines progressively, even as
the late copepodites and adults of later generations appear. Although there must be
continual loss of biomass to predation, the decline during the late summer is mainly
caused by the progressive migration of cohorts to depths >500 m. After mid-June,
most profiles show bimodal populations with layers of high abundance in the upper
50–100 m and also layers deeper than 250–350 m. The upper population contains early
and late copepodites, whereas the deep populations are almost entirely C4 and C5.
4. Main population at overwintering depths: Between mid-September and mid-October,
there are few early copepodites (and no C1 at all). The bimodal vertical distribution
is progressively replaced by profiles in which most of the biomass is layered deep,
usually below 300–350 m, and in such a way as to suggest that only the upper parts
of the deep layers are being sampled.
At OWS M, the same seasonality occurs, Calanus finmarchicus biomass being concentrated in midwinter at 600–1000 m. In May, the surviving population aggregates shallower
than 100 m. The population of C. finmarchicus in the southern part of the Norwegian
coastal current starts its reproductive period earlier in the year than the oceanic population and may attain three generations in a single summer period, south of the Lofoten
islands, beyond which only a single generation is achieved; there is, of course, a continual
advection of individuals coastwise into the Barents Sea (Pedersen et al., 2001). In the East
Icelandic Current, spring warming of surface layers starts in May, and peaks in August;
below 75 m, temperatures remain subzero during the summer (Astthorsson and Gislayson, 2003). Here, C. finmarchicus has a 1-year life cycle while the C. hyperboreus cycle
is of at least 2 years. The former species peaks in abundance in July (c 16000 ind m
−2 ),
whereas C. hyperboreus and C. glacialis numbers peak a little earlier in June (c. 370 and
c7700 ind m
−2 , respectively), apparently more coordinated with the timing of the spring
bloom (<1 mg chl m
−3 in late May–early June).
Østvedt observed that the numbers of C. finmarchicus declined progressively at OWS
M during winter so that only about 15% survived until spring, and the same was observed
at OWS I, where 70–80% of the initial overwintering biomass fails to return to the surface
in spring (Longhurst and Williams, 1992). The same progressive decline in numbers of an
overwintering population can be observed in the data for Pseudocalanus minutus, another
seasonal vertical migrant.
Pareuchaeta norvegica, the second largest component of copepod biomass at OWS I,
also rises from overwintering depths below 500 m soon after mid-March. Reproduction
occurs at depth in late winter so the rising population is structured quite differently
from that of C. finmarchicus. Early (C1–C3) copepodites already numerically dominate
the population. During the summer the biomass of P. norvegica is maintained at a
more consistent level than that of C. finmarchicus, and there is less evidence of the
progressive establishment of deep layers and little indication that the population descends
to overwintering depths before mid-October. Thus, P. norvegica spends a shorter period
at depth during the winter than C. finmarchicus. The next ranking species in terms of
absolute biomass, M. lucens and P. robusta, appear not to undertake seasonal migrations
151
with it in others, C5 individuals and adults rise from overwintering depths that are
in excess of 500 m. By mid-April, essentially the whole population lies shallower than
100 m, and from about April 10, the first copepodites of the new generation begin to
appear.
2. Production of first generation: Throughout May and during the first 10 days of June,
population biomass increases very fast with the growth of copepodites, and in the
second half of this period adults of the first new generation appear. The population
remains almost entirely within the upper 150 m, with many profiles showing crowding
into the upper 50 m. Diel migration, if it occurs, is shallow.
3. Multigeneration period, some individuals descending: From mid-June until midSeptember, population biomass in the upper 500 m declines progressively, even as
the late copepodites and adults of later generations appear. Although there must be
continual loss of biomass to predation, the decline during the late summer is mainly
caused by the progressive migration of cohorts to depths >500 m. After mid-June,
most profiles show bimodal populations with layers of high abundance in the upper
50–100 m and also layers deeper than 250–350 m. The upper population contains early
and late copepodites, whereas the deep populations are almost entirely C4 and C5.
4. Main population at overwintering depths: Between mid-September and mid-October,
there are few early copepodites (and no C1 at all). The bimodal vertical distribution
is progressively replaced by profiles in which most of the biomass is layered deep,
usually below 300–350 m, and in such a way as to suggest that only the upper parts
of the deep layers are being sampled.
At OWS M, the same seasonality occurs, Calanus finmarchicus biomass being concentrated in midwinter at 600–1000 m. In May, the surviving population aggregates shallower
than 100 m. The population of C. finmarchicus in the southern part of the Norwegian
coastal current starts its reproductive period earlier in the year than the oceanic population and may attain three generations in a single summer period, south of the Lofoten
islands, beyond which only a single generation is achieved; there is, of course, a continual
advection of individuals coastwise into the Barents Sea (Pedersen et al., 2001). In the East
Icelandic Current, spring warming of surface layers starts in May, and peaks in August;
below 75 m, temperatures remain subzero during the summer (Astthorsson and Gislayson, 2003). Here, C. finmarchicus has a 1-year life cycle while the C. hyperboreus cycle
is of at least 2 years. The former species peaks in abundance in July (c 16000 ind m
−2 ),
whereas C. hyperboreus and C. glacialis numbers peak a little earlier in June (c. 370 and
c7700 ind m
−2 , respectively), apparently more coordinated with the timing of the spring
bloom (<1 mg chl m
−3 in late May–early June).
Østvedt observed that the numbers of C. finmarchicus declined progressively at OWS
M during winter so that only about 15% survived until spring, and the same was observed
at OWS I, where 70–80% of the initial overwintering biomass fails to return to the surface
in spring (Longhurst and Williams, 1992). The same progressive decline in numbers of an
overwintering population can be observed in the data for Pseudocalanus minutus, another
seasonal vertical migrant.
Pareuchaeta norvegica, the second largest component of copepod biomass at OWS I,
also rises from overwintering depths below 500 m soon after mid-March. Reproduction
occurs at depth in late winter so the rising population is structured quite differently
from that of C. finmarchicus. Early (C1–C3) copepodites already numerically dominate
the population. During the summer the biomass of P. norvegica is maintained at a
more consistent level than that of C. finmarchicus, and there is less evidence of the
progressive establishment of deep layers and little indication that the population descends
to overwintering depths before mid-October. Thus, P. norvegica spends a shorter period
at depth during the winter than C. finmarchicus. The next ranking species in terms of
absolute biomass, M. lucens and P. robusta, appear not to undertake seasonal migrations