Atlantic Westerly Winds Biome
169
western subprovinces is not arbitrary, but biologically significant. Floating aggregations
of gulf weed (Sargassum spp.) are abundant to the west but sparse to the east of this line,
or approximately over the Mid-Atlantic Ridge. It is thought that this is consequent on the
partially closed nature of circulation within the Sargasso Sea. There are also significant
morphological and ecological differences between Sargassum in this province and in the
NATR to the south of the STC (Niermann, 1986; Butler et al., 1983, and references
therein).
As mentioned earlier, there are >300 individual seamounts (not all associated with
the Mid-Atlantic Ridge) of sufficient topography to sustain Taylor columns, so it may
be expected that some areas of enhanced surface chlorophyll will be topographically
controlled, even in midocean. Saltzman and Wishner (1997a) briefly review the ecological
effects that may be anticipated where seamounts populate a deep ocean region. Uplifting
of isotherms and upwelling of nutrients, interaction with diel vertical migrant zooplankton
and consequent patch formation, and induction of relatively high biomass of pelagic fish
are among the more frequently noted effects.
NAST(E) includes a sector of the offshore Canary Current, clear of the field of eddies
and filaments generated within the coastal boundary zone. Here, mixed-layer depths are
shoaler, and enhanced chlorophyll biomass is indicated by the satellite images, particularly
south of the Canaries, though it will be important to separate the effects of eddies from
the filaments of high chlorophyll induced by tidal mixing (and other coastal processes)
at the islands. Vertical nutrient flux in cyclonic eddies occurs centrally by isopycnal
transport, and therefore it is strongest where the pycnocline dome is shoalest and where
some diapycnal mixing may occur across the shoaled pycnocline. As in many upwelling
situations, chlorophyll enhancement occurs most strongly a little downstream of the
surface nutrient maximum and so, in this case, chlorophyll is highest around the edge of
the cyclonic eddies. Anticyclonic eddies frequently interact with filaments of water having
enhanced chlorophyll, causing the chlorophyll signal to spiral inward toward the center
of the eddy, as also occurs in Gulf Stream rings.
The STC front along the southern flank of NAST has been studied to the south of the
Azores by Fernandez and Pingree (1996), who show that it supports local enhancement
of primary production and biomass accumulation, as is generally the case for oceanic
fronts. Winter productivity is in the range 08–09 g C m
−2 d
−1 or about twice the rate
for those parts of this province not influenced by frontal or mesoscale eddy processes.
These authors suggest that because of the great spatial extent of the frontal signature
in the Azores Current system, primary production within the subtropical front is of
major significance for regional carbon budgets. A DCM occurs everywhere across the
NAST province at about 50–100 m across the province, following the depth of the
nutricline rather than that of the pycnocline in the density gradient. It lies progressively
deeper to the south and east across the province. As usual, the chlorophyll maximum is
somewhat deeper than the depth of maximum photosynthetic rate and of the biomass
maximum for photosynthetic cells. This must follow from the generalization that the
C/chl ratio decreases with increasing depth of the DCM. Li (1995) shows that the DCM
for cyanobacteria and prochlorophytes can be traced oceanwide across NAST (Fig. 9.9).
It is in the west of the province, in the Sargasso Sea, that the most detailed studies
of ecosystem response have been performed, over a long period of years, because of the
proximity of the “Oceanographic” at Woods Hole, and the existence of the Bermuda
Station for Biological Research at St. George’s. Gordon Riley (1957) obtained data for 2
years at OWS E at 35
N 48
W, to the northeast of Bermuda, and laid the groundwork for
much of what has followed. The most comprehensive description of pelagic ecosystem
here derives from the Bermuda Atlantic Time Series Study (BATS) studies at 31
N, just
south of Bermuda over deep water, at a station that was occupied for several days on each
of 111 occasions from 1989 to 1994. This extraordinary data set allows almost weekly
169
western subprovinces is not arbitrary, but biologically significant. Floating aggregations
of gulf weed (Sargassum spp.) are abundant to the west but sparse to the east of this line,
or approximately over the Mid-Atlantic Ridge. It is thought that this is consequent on the
partially closed nature of circulation within the Sargasso Sea. There are also significant
morphological and ecological differences between Sargassum in this province and in the
NATR to the south of the STC (Niermann, 1986; Butler et al., 1983, and references
therein).
As mentioned earlier, there are >300 individual seamounts (not all associated with
the Mid-Atlantic Ridge) of sufficient topography to sustain Taylor columns, so it may
be expected that some areas of enhanced surface chlorophyll will be topographically
controlled, even in midocean. Saltzman and Wishner (1997a) briefly review the ecological
effects that may be anticipated where seamounts populate a deep ocean region. Uplifting
of isotherms and upwelling of nutrients, interaction with diel vertical migrant zooplankton
and consequent patch formation, and induction of relatively high biomass of pelagic fish
are among the more frequently noted effects.
NAST(E) includes a sector of the offshore Canary Current, clear of the field of eddies
and filaments generated within the coastal boundary zone. Here, mixed-layer depths are
shoaler, and enhanced chlorophyll biomass is indicated by the satellite images, particularly
south of the Canaries, though it will be important to separate the effects of eddies from
the filaments of high chlorophyll induced by tidal mixing (and other coastal processes)
at the islands. Vertical nutrient flux in cyclonic eddies occurs centrally by isopycnal
transport, and therefore it is strongest where the pycnocline dome is shoalest and where
some diapycnal mixing may occur across the shoaled pycnocline. As in many upwelling
situations, chlorophyll enhancement occurs most strongly a little downstream of the
surface nutrient maximum and so, in this case, chlorophyll is highest around the edge of
the cyclonic eddies. Anticyclonic eddies frequently interact with filaments of water having
enhanced chlorophyll, causing the chlorophyll signal to spiral inward toward the center
of the eddy, as also occurs in Gulf Stream rings.
The STC front along the southern flank of NAST has been studied to the south of the
Azores by Fernandez and Pingree (1996), who show that it supports local enhancement
of primary production and biomass accumulation, as is generally the case for oceanic
fronts. Winter productivity is in the range 08–09 g C m
−2 d
−1 or about twice the rate
for those parts of this province not influenced by frontal or mesoscale eddy processes.
These authors suggest that because of the great spatial extent of the frontal signature
in the Azores Current system, primary production within the subtropical front is of
major significance for regional carbon budgets. A DCM occurs everywhere across the
NAST province at about 50–100 m across the province, following the depth of the
nutricline rather than that of the pycnocline in the density gradient. It lies progressively
deeper to the south and east across the province. As usual, the chlorophyll maximum is
somewhat deeper than the depth of maximum photosynthetic rate and of the biomass
maximum for photosynthetic cells. This must follow from the generalization that the
C/chl ratio decreases with increasing depth of the DCM. Li (1995) shows that the DCM
for cyanobacteria and prochlorophytes can be traced oceanwide across NAST (Fig. 9.9).
It is in the west of the province, in the Sargasso Sea, that the most detailed studies
of ecosystem response have been performed, over a long period of years, because of the
proximity of the “Oceanographic” at Woods Hole, and the existence of the Bermuda
Station for Biological Research at St. George’s. Gordon Riley (1957) obtained data for 2
years at OWS E at 35
N 48
W, to the northeast of Bermuda, and laid the groundwork for
much of what has followed. The most comprehensive description of pelagic ecosystem
here derives from the Bermuda Atlantic Time Series Study (BATS) studies at 31
N, just
south of Bermuda over deep water, at a station that was occupied for several days on each
of 111 occasions from 1989 to 1994. This extraordinary data set allows almost weekly