Atlantic Westerly Winds Biome
161
of autotrophic cells remained relatively low. Moreover, it was apparent that the smallscale patchiness observed in the distribution of zooplankton resulted from top-down
control through interaction between small and large size classes inducing predator-prey
oscillations that originate, at least in part, in diel vertical migration patterns.
In fact, an unusually large range of zooplankton depth strategies obtains here because
the NADR copepod fauna includes both northern and southern species (Williams and
Conway, 1988), although both C. finmarchicus and C. helgolandicus perform seasonal
ontogenetic migrations, wintering at 400–900 m as C5 and rising to the mixed layer in
spring and summer, although the former remains a little deeper than the latter. Neocalanus
gracilis remains within the upper 200 m throughout the year as does C. tenuicornis, which
consistently avoids the near-surface layer and occurs at 20–200 m in all months. These
two, and the other southern species, reproduce year-round.
The abundance of boreo-arctic C. finmarchicus is significantly lower here than further
poleward and, indeed, NADR is the southern limit of this species, is the central range of
C. helgolandicus, and is at the northern limit of the ranges of C. tenuicornis, Neocalanus
gracilis, Nannocalanus minor, and Calanoides carinatus. Beaugrand et al. (2002a) are
able to specify, on the basis of CPR data, those copepods that are characteristic of the
NADR province, enabling it to be thus distinguished from ARCT and SARC, although
unfortunately, these data do not permit of such distinction southward. Any statement
about the distribution of zooplankton species must be tempered by recognition that
these distributions are highly variable between years and between decades. It is clear
from the CPR data (e.g., Colebrook, 1986; Planque et al., 1997) that both long-term
trends and year-to-year variability are responses to changes in physical conditions, even if
the relationship is not always clearly understood. The North Atlantic Oscillation (NAO)
has proved to be an excellent predictor in the NADR of Calanus abundance over a
38-year time series of CPR data, reflecting the response of copepod populations to
ocean physics either directly or through food chain effects (Planque and Reid, 1998).
As Beaugrand et al. (2002b) have shown, the CPR data for 1960–2000 strongly suggest
a long-term shift in the distribution of key copepod species: biota with warm water
affinities shifting further north on the eastern side of the North Atlantic, and cold water
species shifting south in the west. What is the ultimate cause of the shift in the physical
forcing that is thus expressed remains unclear, although the state of the NAO is certainly
implicated.
The vertical ecology of mesozooplankton in NADR was examined by means of an 18station, meridional LHPR section obtained in August 1975 from 42
N (west of Galicia) to
62
N, near OWS I (Longhurst and Williams, 1979). The profiles to 1000 m are dominated,
day and night and all taxa compounded, by an abundant epiplankton within the mixed
layer, which is separated by a biomass discontinuity from sparser plankton below. The
bottom of the epiplankton layer always occurred just shallower than Z m so that, following
this feature, the depth of the epiplankton deepened northward. Epiplankton biomass
always represented >50% of total biomass to 1000 m that showed a general progression
toward higher values poleward: off western Iberia, biomass was 200–800 mg m
−3 dry
weight, compared with 900–2800 mg m
−3 at 55–60
N. The depth of maximum abundance
was usually clearly observable, and shallower than the deep chlorophyll maximum that
itself usually lay within the upper pycnocline. This represents a looser correspondence
between herbivores and the phytoplankton profiles than occurs in tropical, permanently
stratified water columns (see PNEC, for example).
Almost 95% of all biomass to 1000 m comprised only 12 sorted categories: the herbivores C. finmarchicus, Pseudocalanus elongatus, and Limacina retroversa, the omnivores
Metridia lucens, Pleuromamma robusta, and Acartia clausii, and predators represented by
total coelenterates, annelids and hyperiids, together with Euchaeta norvegica. Of these,
Euphausia, Pleuromamma, and Metridia were interzonal migrants, with daytime depth
161
of autotrophic cells remained relatively low. Moreover, it was apparent that the smallscale patchiness observed in the distribution of zooplankton resulted from top-down
control through interaction between small and large size classes inducing predator-prey
oscillations that originate, at least in part, in diel vertical migration patterns.
In fact, an unusually large range of zooplankton depth strategies obtains here because
the NADR copepod fauna includes both northern and southern species (Williams and
Conway, 1988), although both C. finmarchicus and C. helgolandicus perform seasonal
ontogenetic migrations, wintering at 400–900 m as C5 and rising to the mixed layer in
spring and summer, although the former remains a little deeper than the latter. Neocalanus
gracilis remains within the upper 200 m throughout the year as does C. tenuicornis, which
consistently avoids the near-surface layer and occurs at 20–200 m in all months. These
two, and the other southern species, reproduce year-round.
The abundance of boreo-arctic C. finmarchicus is significantly lower here than further
poleward and, indeed, NADR is the southern limit of this species, is the central range of
C. helgolandicus, and is at the northern limit of the ranges of C. tenuicornis, Neocalanus
gracilis, Nannocalanus minor, and Calanoides carinatus. Beaugrand et al. (2002a) are
able to specify, on the basis of CPR data, those copepods that are characteristic of the
NADR province, enabling it to be thus distinguished from ARCT and SARC, although
unfortunately, these data do not permit of such distinction southward. Any statement
about the distribution of zooplankton species must be tempered by recognition that
these distributions are highly variable between years and between decades. It is clear
from the CPR data (e.g., Colebrook, 1986; Planque et al., 1997) that both long-term
trends and year-to-year variability are responses to changes in physical conditions, even if
the relationship is not always clearly understood. The North Atlantic Oscillation (NAO)
has proved to be an excellent predictor in the NADR of Calanus abundance over a
38-year time series of CPR data, reflecting the response of copepod populations to
ocean physics either directly or through food chain effects (Planque and Reid, 1998).
As Beaugrand et al. (2002b) have shown, the CPR data for 1960–2000 strongly suggest
a long-term shift in the distribution of key copepod species: biota with warm water
affinities shifting further north on the eastern side of the North Atlantic, and cold water
species shifting south in the west. What is the ultimate cause of the shift in the physical
forcing that is thus expressed remains unclear, although the state of the NAO is certainly
implicated.
The vertical ecology of mesozooplankton in NADR was examined by means of an 18station, meridional LHPR section obtained in August 1975 from 42
N (west of Galicia) to
62
N, near OWS I (Longhurst and Williams, 1979). The profiles to 1000 m are dominated,
day and night and all taxa compounded, by an abundant epiplankton within the mixed
layer, which is separated by a biomass discontinuity from sparser plankton below. The
bottom of the epiplankton layer always occurred just shallower than Z m so that, following
this feature, the depth of the epiplankton deepened northward. Epiplankton biomass
always represented >50% of total biomass to 1000 m that showed a general progression
toward higher values poleward: off western Iberia, biomass was 200–800 mg m
−3 dry
weight, compared with 900–2800 mg m
−3 at 55–60
N. The depth of maximum abundance
was usually clearly observable, and shallower than the deep chlorophyll maximum that
itself usually lay within the upper pycnocline. This represents a looser correspondence
between herbivores and the phytoplankton profiles than occurs in tropical, permanently
stratified water columns (see PNEC, for example).
Almost 95% of all biomass to 1000 m comprised only 12 sorted categories: the herbivores C. finmarchicus, Pseudocalanus elongatus, and Limacina retroversa, the omnivores
Metridia lucens, Pleuromamma robusta, and Acartia clausii, and predators represented by
total coelenterates, annelids and hyperiids, together with Euchaeta norvegica. Of these,
Euphausia, Pleuromamma, and Metridia were interzonal migrants, with daytime depth