166
Chapter 9: The Atlantic Ocean
fully developed immigrant Sargasso biota above and the remains of a refugee slope-water
community below.
More generally, mesozooplankton species distributions echo the distribution of water
types in this complex physical system. Using recurrent group and cluster analysis, Ashjian
and Wishner (1993) examined how 22 categories of 18 species of copepods responded to
the offshore downsloping of physical properties—pycnocline, nutricline, and downstream
velocity profile—along a section across the Gulf Stream in May 1988. Four principal
groups of taxa were identified, of which one (Group 3) comprised large, active diel
migrants. Group 1 comprised epiplanktonic Nannocalanus minor, Neocalanus gracilis, and
Lucicutia ovalis (and their growth stages) that layered in the upper 100 m, in water of about
25
C right across the stream. Group 2 (Calanus finmarchicus, Rhincalanus nasutus, and
Metridia lucens) lay deep, in water <10
C, and always below the downsloping pycnocline.
Group 4 (Calanus tenuicornis stages, and Lucicutia gemina) lay horizontally across the
stream below Group 1, at 100–200 m, in water of 20–24
C.
A seasonal series of mesoplankton samples at 36
N reveals the general annual cycle
of vertical migrations in the Gulf Stream system that will modify the foregoing simple
pattern (Allison and Wishner, 1986). In winter, biomass in the upper 200 m is low both
day and night in the slope water and in the adjacent north wall of the stream relative
to biomass that is about one order of magnitude greater in spring and early summer.
In the Gulf Stream itself and the adjacent Sargasso water, there is little seasonality in
near-surface biomass. In May, biomass is concentrated at 0–100 m both by day and night,
progressively deepening offshore, and little diel migration occurs. By September, diel
migration is strongly established from the slope to the Sargasso water, with a subsurface
maximum at night at about 80–90 m in all areas, near the DCM that develops during
the summer months. Such strong diel migration to 400–600 m in a region with such active
horizontal advection must cause very significant horizontal redistribution of biomass.
Synopsis
Case 2—Nutrient-limited spring production peak. Z m undergoes moderate boreal winter
excursion while Z eu remains fairly constant at 35–50 m, so that thermocline is illuminated
from May to October. Rate increase of P begins in February, before a shoal (thermal)
Z m is established in March–April, and reaches a (nutrient-limited?) maximum in April,
subsequently declining progressively to an annual minimal rate in November (Fig. 9.8); a
shoulder on this decline in August–October appears to be a response to the deepening of
the mixed layer as it passes down out of the photic zone. Dynamic chlorophyll biomass
range tracks the variations in P so that maximum accumulation occurs in April. Relations
of biomass and P suggest that consumption and production are balanced, and that
seasonal vertical migration of consumers is insignificant.
North Atlantic Subtropical Gyral Province
(NAST-E, NAST-W)
Extent of the Province
There is no continental shelf in this province save for the island platforms of Bermuda
and the Azores. NAST is bounded to the west and northwest by the eddy field of the Gulf
Stream and to the northeast by the bifurcation of flow between the Azores Current and
the North Atlantic Current; that is, at about 40–42
N. To the south, it is defined by the
Subtropical Convergence (STC) that lies below the convergence between the trade winds
and the westerlies, along about 25–30
N; this frontal zone is the equatorward limit of
Chapter 9: The Atlantic Ocean
fully developed immigrant Sargasso biota above and the remains of a refugee slope-water
community below.
More generally, mesozooplankton species distributions echo the distribution of water
types in this complex physical system. Using recurrent group and cluster analysis, Ashjian
and Wishner (1993) examined how 22 categories of 18 species of copepods responded to
the offshore downsloping of physical properties—pycnocline, nutricline, and downstream
velocity profile—along a section across the Gulf Stream in May 1988. Four principal
groups of taxa were identified, of which one (Group 3) comprised large, active diel
migrants. Group 1 comprised epiplanktonic Nannocalanus minor, Neocalanus gracilis, and
Lucicutia ovalis (and their growth stages) that layered in the upper 100 m, in water of about
25
C right across the stream. Group 2 (Calanus finmarchicus, Rhincalanus nasutus, and
Metridia lucens) lay deep, in water <10
C, and always below the downsloping pycnocline.
Group 4 (Calanus tenuicornis stages, and Lucicutia gemina) lay horizontally across the
stream below Group 1, at 100–200 m, in water of 20–24
C.
A seasonal series of mesoplankton samples at 36
N reveals the general annual cycle
of vertical migrations in the Gulf Stream system that will modify the foregoing simple
pattern (Allison and Wishner, 1986). In winter, biomass in the upper 200 m is low both
day and night in the slope water and in the adjacent north wall of the stream relative
to biomass that is about one order of magnitude greater in spring and early summer.
In the Gulf Stream itself and the adjacent Sargasso water, there is little seasonality in
near-surface biomass. In May, biomass is concentrated at 0–100 m both by day and night,
progressively deepening offshore, and little diel migration occurs. By September, diel
migration is strongly established from the slope to the Sargasso water, with a subsurface
maximum at night at about 80–90 m in all areas, near the DCM that develops during
the summer months. Such strong diel migration to 400–600 m in a region with such active
horizontal advection must cause very significant horizontal redistribution of biomass.
Synopsis
Case 2—Nutrient-limited spring production peak. Z m undergoes moderate boreal winter
excursion while Z eu remains fairly constant at 35–50 m, so that thermocline is illuminated
from May to October. Rate increase of P begins in February, before a shoal (thermal)
Z m is established in March–April, and reaches a (nutrient-limited?) maximum in April,
subsequently declining progressively to an annual minimal rate in November (Fig. 9.8); a
shoulder on this decline in August–October appears to be a response to the deepening of
the mixed layer as it passes down out of the photic zone. Dynamic chlorophyll biomass
range tracks the variations in P so that maximum accumulation occurs in April. Relations
of biomass and P suggest that consumption and production are balanced, and that
seasonal vertical migration of consumers is insignificant.
North Atlantic Subtropical Gyral Province
(NAST-E, NAST-W)
Extent of the Province
There is no continental shelf in this province save for the island platforms of Bermuda
and the Azores. NAST is bounded to the west and northwest by the eddy field of the Gulf
Stream and to the northeast by the bifurcation of flow between the Azores Current and
the North Atlantic Current; that is, at about 40–42
N. To the south, it is defined by the
Subtropical Convergence (STC) that lies below the convergence between the trade winds
and the westerlies, along about 25–30
N; this frontal zone is the equatorward limit of