150
Chapter 9: The Atlantic Ocean
In the early stages of the bloom, during April and May, chlorophyll concentrations
of <025 mg m
−3 extend well beneath the mixed layer to 150–200 m, suggesting that
ungrazed cells at that time sink quickly through the weak pycnocline. As discussed in
Chapter 5, nitrate is only occasionally exhausted in the euphotic zone and, as Fig. 5.2
shows, minimal values are not reached until mid-July. This is, of course, the consequence
of the geostrophic nitrate ridge that lies across the periphery of the North Atlantic gyre
and of small wind-mixing events. Chlorophyll values remain relatively high throughout
the summer, usually >20 mg chl m
−3 in the core of the near-surface, high-chlorophyll
layer. Late summer blooms of coccoliths, probably Emiliania huxleyi, are consistently
observed as backscattered light in surface color images in the southern part of SARC
(Brown, 1995b).
For the SARC province we have almost unrivalled information about the threedimensional, quantitative evolution of zooplankton communities. In 1948–48, at OWS
M, weekly and throughout the year, depth-stratified Nansen net tows (0–2000 m) were
obtained and complete analysis made of zooplankton taxa on almost all profiles (Østvedt,
1955). At OWS I, the dynamics of the zooplankton were investigated from 1971 to
1974, again almost weekly, from late March to mid-October, using the Longhurst-Hardy
Plankton Recorder (LHPR). The 111 profiles obtained comprise about 4700 individual,
metered plankton samples, each representing depth intervals of approximately 10 m from
the surface to 500 m, and in some cases deeper. One hundred and twenty species or
species groups of copepods were identified, of which only seven comprised about 90%
of the total numbers of individuals: Oithona spp. 29%, Acartia tonsa 19%, Calanus
finmarchicus 16%, Metridia lucens 14%, Euchaeta norvegica 5%, Oncaea spp. 4%, and
Pleuromamma robusta 3% (Williams, 1973). These C. finmarchicus data were revisited
recently by Irigoien et al. (2000).
The profiles at OWS M and OWS I confirm Gran’s earlier suggestions that the
zooplankton species associations in the Norwegian Sea comprise (i) a surface epiplankton,
(ii) a plankton of intermediate depths corresponding to the subsurface layers of Atlantic
water, and (iii) a deeper plankton association. The surface plankton is by far the most
diverse, and not only because some individuals of all seasonal vertical migrants are found
there in all months. The Atlantic water zone is characterized especially by the occurrence
of Paraeuchaeta norvegica and (during daytime) of diel migrants such as Pleuromamma
and Metridia. The deeper, colder zone is characterized by Calanus hyperboreus (which
does not rise seasonally in this province) and the large amphipod Cyclocaris guilelmi.
It is, of course, Calanus finmarchicus and its seasonal life cycle that has attracted most
attention in studies of the North Atlantic plankton, although in this province, as elsewhere,
the smaller epipelagic copepods are numerically dominant, so that the total population
of copepods is always biased surfaceward, numbers being an order of magnitude higher
in the upper 30 m than at all depths below 100 m. This should be remembered when
assessing the food available to—for instance—fish larvae (see later discussion) and is
also reflected in the distribution of zooplankton predators, such as chaetognaths. These,
likewise, have a distribution in the LHPR profiles biased surfaceward at all seasons. A
similar argument can be made for other predators. Coelenterates are dominated at OWS
I by Aglanthe digitale (>80% dry weight biomass) and individual distribution is once
again biased surfaceward with most individuals in the upper 100 m. Only in late July
were some larger individuals found to the bottom of the profiles in 800 m.
But to return to C. finmarchicus itself, which does nevertheless maintain the largest
species-specific copepod biomass at OWS I. Four distinct phases can be identified in the
seasonal cycle in the profiles there from 1971 to 1974:
1. Rise of the overwintered population: From the middle of March until the end of April,
the overwintered population of C5s and adults dominates the biomass between 0 and
800 m. During this period, and prior to the spring bloom in some years, but coincident
Chapter 9: The Atlantic Ocean
In the early stages of the bloom, during April and May, chlorophyll concentrations
of <025 mg m
−3 extend well beneath the mixed layer to 150–200 m, suggesting that
ungrazed cells at that time sink quickly through the weak pycnocline. As discussed in
Chapter 5, nitrate is only occasionally exhausted in the euphotic zone and, as Fig. 5.2
shows, minimal values are not reached until mid-July. This is, of course, the consequence
of the geostrophic nitrate ridge that lies across the periphery of the North Atlantic gyre
and of small wind-mixing events. Chlorophyll values remain relatively high throughout
the summer, usually >20 mg chl m
−3 in the core of the near-surface, high-chlorophyll
layer. Late summer blooms of coccoliths, probably Emiliania huxleyi, are consistently
observed as backscattered light in surface color images in the southern part of SARC
(Brown, 1995b).
For the SARC province we have almost unrivalled information about the threedimensional, quantitative evolution of zooplankton communities. In 1948–48, at OWS
M, weekly and throughout the year, depth-stratified Nansen net tows (0–2000 m) were
obtained and complete analysis made of zooplankton taxa on almost all profiles (Østvedt,
1955). At OWS I, the dynamics of the zooplankton were investigated from 1971 to
1974, again almost weekly, from late March to mid-October, using the Longhurst-Hardy
Plankton Recorder (LHPR). The 111 profiles obtained comprise about 4700 individual,
metered plankton samples, each representing depth intervals of approximately 10 m from
the surface to 500 m, and in some cases deeper. One hundred and twenty species or
species groups of copepods were identified, of which only seven comprised about 90%
of the total numbers of individuals: Oithona spp. 29%, Acartia tonsa 19%, Calanus
finmarchicus 16%, Metridia lucens 14%, Euchaeta norvegica 5%, Oncaea spp. 4%, and
Pleuromamma robusta 3% (Williams, 1973). These C. finmarchicus data were revisited
recently by Irigoien et al. (2000).
The profiles at OWS M and OWS I confirm Gran’s earlier suggestions that the
zooplankton species associations in the Norwegian Sea comprise (i) a surface epiplankton,
(ii) a plankton of intermediate depths corresponding to the subsurface layers of Atlantic
water, and (iii) a deeper plankton association. The surface plankton is by far the most
diverse, and not only because some individuals of all seasonal vertical migrants are found
there in all months. The Atlantic water zone is characterized especially by the occurrence
of Paraeuchaeta norvegica and (during daytime) of diel migrants such as Pleuromamma
and Metridia. The deeper, colder zone is characterized by Calanus hyperboreus (which
does not rise seasonally in this province) and the large amphipod Cyclocaris guilelmi.
It is, of course, Calanus finmarchicus and its seasonal life cycle that has attracted most
attention in studies of the North Atlantic plankton, although in this province, as elsewhere,
the smaller epipelagic copepods are numerically dominant, so that the total population
of copepods is always biased surfaceward, numbers being an order of magnitude higher
in the upper 30 m than at all depths below 100 m. This should be remembered when
assessing the food available to—for instance—fish larvae (see later discussion) and is
also reflected in the distribution of zooplankton predators, such as chaetognaths. These,
likewise, have a distribution in the LHPR profiles biased surfaceward at all seasons. A
similar argument can be made for other predators. Coelenterates are dominated at OWS
I by Aglanthe digitale (>80% dry weight biomass) and individual distribution is once
again biased surfaceward with most individuals in the upper 100 m. Only in late July
were some larger individuals found to the bottom of the profiles in 800 m.
But to return to C. finmarchicus itself, which does nevertheless maintain the largest
species-specific copepod biomass at OWS I. Four distinct phases can be identified in the
seasonal cycle in the profiles there from 1971 to 1974:
1. Rise of the overwintered population: From the middle of March until the end of April,
the overwintered population of C5s and adults dominates the biomass between 0 and
800 m. During this period, and prior to the spring bloom in some years, but coincident