Atlantic Coastal Biome
265
(1993). The latitude of the confluence, which determines where separation of the Brazil
and Falklands Currents from the continent occurs, is farther to the north during austral
winter and spring; this may be related to the general seasonal shift of wind systems and
to a seasonal meridional shift of the subtropical gyre (Peterson and Stramma, 1991).
Imposed on this seasonality, there is also some short-term variability in the southward
extent of the Brazil Current. When a loop that has pushed unusually far south retreats
to the north, it may be shed as a series of warm-core eddies that pass into the Antarctic
Circumpolar Current (Partos and Piccolo, 1988).
Although intrusions of SACW, and also shelf-edge upwelling, occur off southeastern
Brazil and on the shelf in the vicinity of the Abrolhos and Royal Charlotte Banks,
these processes have been most intensively studied at Cabo Frio (23
S), in relation
to the offshore meander that occurs there and the upwelling that regularly occurs to
the northeast of the cape (Peterson and Stramma, 1991). This is a complex process,
combining wind stress and the topography of a major canyon that cuts across the shelf
(Mascarenas et al., 1971; Moreira da Silva, 1971). Northeast winds force water to upwell
across the plateau that lies east of the cape and to surface either at Cabo Frio itself, or
somewhat to the north. To the south of Cape Frio, a coastal cyclonic eddy forms an
eastward compensation current. Upwelling may be inhibited during periods when the
Brazil Current remains for a period topographically locked to the coastline, so that the
Cabo Frio meander does not form.
Upwelling-downwelling episodes occur very quickly in response to changes in wind
direction, which themselves are very variable and episodic, responding to the cyclical
development and collapse of the atmospheric anticyclone east of southern Brazil. A pulse
of northeast winds will very rapidly draw SACW, of 18
C and 10 M NO 3 , to the surface
from about 300 m. Such episodes occur more frequently in austral summer than in
winter, during which the frequent passage of cold fronts and southwest winds maintains
an almost continuous downwelling, oligotrophic situation along the outer shelf.
In the extreme south of this province, the single major source of freshwater is the
Parana-Plate system at 35
S that discharges sediment at a rate of ∼90 × 10
6 t y
−1 ; this
is a significant flux of solids and has major consequences for the ecology of the inner
shelf of the southern part of the BRAZ province, because it transported equatorward
and alongshore. The seasonal variability of the equatorward extension of the low-salinity
plume is forced by changes in wind stress rather than by changes in the discharge rate of
the River Plate and from the Patos Lagoon.
Response of the Pelagic Ecosystems
Although the upwelling at Cape Frio has occupied much of the attention of regional
studies here, the satellite chlorophyll data for this coast reveal that it is not a major
feature and is only with some difficulty identified in the routine 8-day images; high
chlorophyll values appears to occur only very close to the coast and offshore filaments
are relatively very weak compared with those from many other upwelling sites. The
austral spring-summer bloom that occurs over the shelf dominates seasonal changes
observed in chlorophyll images that represent the entire BRAZ province; this bloom
involves the entire region that lies poleward of the ITCZ, and takes higher values of
chlorophyll biomass over the shelf than offshore. Consequently, during austral spring
and summer, the entire shelf area is progressively occupied by water of relatively high
chlorophyll biomass that is separable from offshore water that remains relatively oligotrophic.
An unusually prominent high-chlorophyll feature occurs on the inshore zone of the
area of wide shelf south of 28
S, and this probably represents both production on the shelf
and also the discharge from the River Plate; the plume appears to be constrained by a
265
(1993). The latitude of the confluence, which determines where separation of the Brazil
and Falklands Currents from the continent occurs, is farther to the north during austral
winter and spring; this may be related to the general seasonal shift of wind systems and
to a seasonal meridional shift of the subtropical gyre (Peterson and Stramma, 1991).
Imposed on this seasonality, there is also some short-term variability in the southward
extent of the Brazil Current. When a loop that has pushed unusually far south retreats
to the north, it may be shed as a series of warm-core eddies that pass into the Antarctic
Circumpolar Current (Partos and Piccolo, 1988).
Although intrusions of SACW, and also shelf-edge upwelling, occur off southeastern
Brazil and on the shelf in the vicinity of the Abrolhos and Royal Charlotte Banks,
these processes have been most intensively studied at Cabo Frio (23
S), in relation
to the offshore meander that occurs there and the upwelling that regularly occurs to
the northeast of the cape (Peterson and Stramma, 1991). This is a complex process,
combining wind stress and the topography of a major canyon that cuts across the shelf
(Mascarenas et al., 1971; Moreira da Silva, 1971). Northeast winds force water to upwell
across the plateau that lies east of the cape and to surface either at Cabo Frio itself, or
somewhat to the north. To the south of Cape Frio, a coastal cyclonic eddy forms an
eastward compensation current. Upwelling may be inhibited during periods when the
Brazil Current remains for a period topographically locked to the coastline, so that the
Cabo Frio meander does not form.
Upwelling-downwelling episodes occur very quickly in response to changes in wind
direction, which themselves are very variable and episodic, responding to the cyclical
development and collapse of the atmospheric anticyclone east of southern Brazil. A pulse
of northeast winds will very rapidly draw SACW, of 18
C and 10 M NO 3 , to the surface
from about 300 m. Such episodes occur more frequently in austral summer than in
winter, during which the frequent passage of cold fronts and southwest winds maintains
an almost continuous downwelling, oligotrophic situation along the outer shelf.
In the extreme south of this province, the single major source of freshwater is the
Parana-Plate system at 35
S that discharges sediment at a rate of ∼90 × 10
6 t y
−1 ; this
is a significant flux of solids and has major consequences for the ecology of the inner
shelf of the southern part of the BRAZ province, because it transported equatorward
and alongshore. The seasonal variability of the equatorward extension of the low-salinity
plume is forced by changes in wind stress rather than by changes in the discharge rate of
the River Plate and from the Patos Lagoon.
Response of the Pelagic Ecosystems
Although the upwelling at Cape Frio has occupied much of the attention of regional
studies here, the satellite chlorophyll data for this coast reveal that it is not a major
feature and is only with some difficulty identified in the routine 8-day images; high
chlorophyll values appears to occur only very close to the coast and offshore filaments
are relatively very weak compared with those from many other upwelling sites. The
austral spring-summer bloom that occurs over the shelf dominates seasonal changes
observed in chlorophyll images that represent the entire BRAZ province; this bloom
involves the entire region that lies poleward of the ITCZ, and takes higher values of
chlorophyll biomass over the shelf than offshore. Consequently, during austral spring
and summer, the entire shelf area is progressively occupied by water of relatively high
chlorophyll biomass that is separable from offshore water that remains relatively oligotrophic.
An unusually prominent high-chlorophyll feature occurs on the inshore zone of the
area of wide shelf south of 28
S, and this probably represents both production on the shelf
and also the discharge from the River Plate; the plume appears to be constrained by a