146
Chapter 9: The Atlantic Ocean
by the “mean community body size” for copepods computed by Hays (1996). A line
from Iceland to the Flemish Cap, approximately coinciding with the province boundary
proposed here, divides small copepods (adults, mostly about 0.5 mg wet weight) to the
south in NADR from large copepods to the north in ARCT (mostly about 2.0 mg wet
weight). The same line divides two characteristic patterns of diel vertical migration
(DVM). Mean community percentage biomass translocation by DVM was computed by
Hays from about 130,000 Hardy Continuous Plankton Recorder samples archived for this
region, and these show clearly that in ARCT this index takes values of about 80%, whereas
in NADR values of 10–50% are more usual. DVM is strongest in summer, weakest in
winter in both provinces, and not only intensity but also timing differs on either side
of the line defined earlier: in ARCT, maximum values for DVM occur at midsummer,
whereas in NADR there are spring and autumn peaks.
Seasonal ontogenetic vertical migration follows the same pattern as in BPLR although
C. hyperboreus descends to depths of as much as 1000–1500 m in the Greenland Sea
(Hirsche, 1991). Though some populations of this copepod may require only a single
year to complete their life cycle, the probable duration for most populations in ARCT
is 2 years. When deep convection events occur, they disrupt the arrangement of the
deeper water masses within which overwintering populations of copepods reside. Richter
(1994) proposed that this process causes the sporadic but massive recruitment failures of
copepods that are known to occur here.
Although the ARCT province is a meeting place for polar, subpolar, and temperate
faunas, and although their points of entry to the province are evident, the Russian
studies here emphasized that recirculation and mesoscale activity ensures that the various
faunistic elements remain partitioned only in the most general way. There is some
fidelity of polar forms such as Calanus hyperboreus, Metridia longa, Themisto libellula,
and Limacina helicina to water entrained from the Labrador coastal current and the
East Greenland Current, but individuals are more widely distributed than that. Similarly,
Calanus finmarchicus, Oithona similis, Limacina retroversa, and Physophora hydrostatica are
characteristically more southerly species and are preferentially distributed where Irminger
water, carrying some Atlantic signature, dominates—that is, around the northern limb
of the subpolar gyre in the Labrador and Irminger basins. Thus C. hyperboreus is a
dominant copepod in the extreme NE parts of the Norwegian Basin gyre and in the
western parts of the Labrador Sea gyre, but not elsewhere. Although the fidelity of the
copepod faunas to Atlantic and Arctic water masses seems very clear, the effect of depth
cannot be excluded. In spring 2000, the Labrador water that normally occupies the shelf
spread out over the slope region, but this did not displace the slope fauna that, with a
strong Atlantic biogeographic element, remained in situ (Head et al., 2003). The CPR
analysis confirms that the characterizing species in the two subpolar gyres are different:
Calanus hyperboreus, C. glacialis, and Metridia longa occupies the Labrador Sea gyre and
C. finmarchicus, Heterorhabdus norvegica, Euchaeta norvegica, and Scolecithricella spp. that
of the Norwegian Sea.
Overwintering strategies differ between species (Hirsche, 1991), partly by depth selection and partly by differential timing of their reproductive periods. C. hyperboreus overwinters as C3s to adults and C. finmarchicus as C4s to adults but mostly C5s, whereas
C. glacialis overwinters mostly as CIVs. Metridia longa overwinters in a more advanced
stage, >50% of all individuals in fall already being adult. C. hyperboreus overwinters
deeper (1000–1500 m) than the other species. Experiments in the Labrador Sea suggest
that egg production in spring by C. finmarchicus is fueled importantly by stored lipid
reserves conserved during winter (Cabal et al., 1997).
The life cycle of Calanus finmarchicus will be reviewed in the next section, devoted
to the SARC province, so here it is sufficient only to note some characteristics of its
behavior in the ARCT province. In the Labrador Sea, C. finmarchicus produces a single
Chapter 9: The Atlantic Ocean
by the “mean community body size” for copepods computed by Hays (1996). A line
from Iceland to the Flemish Cap, approximately coinciding with the province boundary
proposed here, divides small copepods (adults, mostly about 0.5 mg wet weight) to the
south in NADR from large copepods to the north in ARCT (mostly about 2.0 mg wet
weight). The same line divides two characteristic patterns of diel vertical migration
(DVM). Mean community percentage biomass translocation by DVM was computed by
Hays from about 130,000 Hardy Continuous Plankton Recorder samples archived for this
region, and these show clearly that in ARCT this index takes values of about 80%, whereas
in NADR values of 10–50% are more usual. DVM is strongest in summer, weakest in
winter in both provinces, and not only intensity but also timing differs on either side
of the line defined earlier: in ARCT, maximum values for DVM occur at midsummer,
whereas in NADR there are spring and autumn peaks.
Seasonal ontogenetic vertical migration follows the same pattern as in BPLR although
C. hyperboreus descends to depths of as much as 1000–1500 m in the Greenland Sea
(Hirsche, 1991). Though some populations of this copepod may require only a single
year to complete their life cycle, the probable duration for most populations in ARCT
is 2 years. When deep convection events occur, they disrupt the arrangement of the
deeper water masses within which overwintering populations of copepods reside. Richter
(1994) proposed that this process causes the sporadic but massive recruitment failures of
copepods that are known to occur here.
Although the ARCT province is a meeting place for polar, subpolar, and temperate
faunas, and although their points of entry to the province are evident, the Russian
studies here emphasized that recirculation and mesoscale activity ensures that the various
faunistic elements remain partitioned only in the most general way. There is some
fidelity of polar forms such as Calanus hyperboreus, Metridia longa, Themisto libellula,
and Limacina helicina to water entrained from the Labrador coastal current and the
East Greenland Current, but individuals are more widely distributed than that. Similarly,
Calanus finmarchicus, Oithona similis, Limacina retroversa, and Physophora hydrostatica are
characteristically more southerly species and are preferentially distributed where Irminger
water, carrying some Atlantic signature, dominates—that is, around the northern limb
of the subpolar gyre in the Labrador and Irminger basins. Thus C. hyperboreus is a
dominant copepod in the extreme NE parts of the Norwegian Basin gyre and in the
western parts of the Labrador Sea gyre, but not elsewhere. Although the fidelity of the
copepod faunas to Atlantic and Arctic water masses seems very clear, the effect of depth
cannot be excluded. In spring 2000, the Labrador water that normally occupies the shelf
spread out over the slope region, but this did not displace the slope fauna that, with a
strong Atlantic biogeographic element, remained in situ (Head et al., 2003). The CPR
analysis confirms that the characterizing species in the two subpolar gyres are different:
Calanus hyperboreus, C. glacialis, and Metridia longa occupies the Labrador Sea gyre and
C. finmarchicus, Heterorhabdus norvegica, Euchaeta norvegica, and Scolecithricella spp. that
of the Norwegian Sea.
Overwintering strategies differ between species (Hirsche, 1991), partly by depth selection and partly by differential timing of their reproductive periods. C. hyperboreus overwinters as C3s to adults and C. finmarchicus as C4s to adults but mostly C5s, whereas
C. glacialis overwinters mostly as CIVs. Metridia longa overwinters in a more advanced
stage, >50% of all individuals in fall already being adult. C. hyperboreus overwinters
deeper (1000–1500 m) than the other species. Experiments in the Labrador Sea suggest
that egg production in spring by C. finmarchicus is fueled importantly by stored lipid
reserves conserved during winter (Cabal et al., 1997).
The life cycle of Calanus finmarchicus will be reviewed in the next section, devoted
to the SARC province, so here it is sufficient only to note some characteristics of its
behavior in the ARCT province. In the Labrador Sea, C. finmarchicus produces a single