Atlantic Coastal Biome
251
the discharge of the St. Lawrence River, which reaches 30 × 10
−3 m
−3 sec
−1 at times of
peak flow in summer. This flow, together with Labrador Current water that has entered
directly from the north through the Straits of Belle Isle, drives an outward stream at
the surface through the Cabot Strait at all times. Ice cover forms during winter in the
Labrador Current and over much of the Gulf of St. Lawrence, and during the period of
spring breakup, fractured pack ice is transported south out of the Gulf and along the
eastern coast of Nova Scotia. Ice regimes on the northern parts of the province differ
strongly depending on the origin of the flow in which freezing occurs. Unlike the Gulf
of St. Lawrence where pack ice is formed in situ, the winter ice cover off the Labrador
coast, which extends south to the northern Grand Banks, is characterized by the frequent
presence of icebergs drifted from the glacier fronts of the Canadian arctic. These may
survive individually much farther to the southeast than does the seasonal ice cover, as
Titanic discovered to its cost.
Since the entire Gulf of St. Lawrence has an estuarine circulation, so that the discharge
of brackish water at the surface toward the open ocean induces a landward flow of deep
water through the Cabot Straits and, consequently, the upwelling of nutrient-rich water
at the head of the Laurentian Channel north of the Gaspé Peninsula (Dickie and Trites,
1983). The water that passes out onto the Scotia shelf from the Gulf of St. Lawrence
moves southward along the coast with the shelf water mass, passes cyclonically around
the Gulf of Maine, rounds Georges Bank, and so moves on southward past Cape Cod to
form the shelf water of the Middle Atlantic Bight.
The southern end of the province beyond Cape Hatteras is transitional to the CARB
province to the south; here, the northward flow of the Gulf Stream water above the slope
is contrary to the generally equatorward drift of slope water in this province. Shelf water
in the South Atlantic Bight, although it still passes southward along the coast, is therefore
heavily influenced by admixture of warm, tropical water with major consequences for
nutrient flux onto the shelf (Boicourt et al., 1998). The slope of the South Atlantic Bight,
at about 31
N, bears a NE-trending ridge—the “Charleston Bump”—that deflects the
deep flow of the Gulf Stream, inducing meanders that may form cyclonic frontal eddies
that turn back over the shelf and flood it with oceanic water. Because the Gulf Stream at
these latitudes is bistable, lying along one or the other of two persistent tracks, the frontal
eddies take two modes: smaller and larger. This bistability determines the consequences
for shelf ecology because, in the large eddy mode, warm oceanic water passes around a
cold dome and may then propagate right in to the coastline.
The development of a summer stratified regime from the winter conditions of nearuniform properties to 50–100 m depth that obtain everywhere except in the South Atlantic
Bight is anything but simple. On the northern Grand Banks, buoyancy is enhanced in
spring by the presence of superficial water of low density, the result of melting pack ice,
and also by increasing surface irradiance and consequent heat gain. The same occurs in
the Gulf of St. Lawrence, although here meltwater is relatively less important (Doyon
et al., 2000). On the Scotian Shelf, warm slope water episodically floods across the shelf;
because it is of significantly higher salinity than the shelf water, it is denser and so remains
near-bottom. Such fluxes are an important nutrient input to shelf ecosystems, and are
akin to the transport of nutrient-rich slope water (2–5
C) into the head of the Gulf of
St. Lawrence below the cold intermediate water at −1
to 2
C.
At the southern end of the province, in the South Atlantic Bight, the timing of
stratification in spring is dependent on the direction of wind forcing (Flagg et al., 2002);
downwelling-favorable winds maintain the shallow plume of light water from Chesapeake
Bay close to the coast as it passes southward in the general shelf circulation, but upwellingfavorable winds cause it to spread out across the shelf. This provides the extra stability
needed to enable the ambient surface heat flux to stratify the water column so that mixing
occurs again only at the end of summer.
251
the discharge of the St. Lawrence River, which reaches 30 × 10
−3 m
−3 sec
−1 at times of
peak flow in summer. This flow, together with Labrador Current water that has entered
directly from the north through the Straits of Belle Isle, drives an outward stream at
the surface through the Cabot Strait at all times. Ice cover forms during winter in the
Labrador Current and over much of the Gulf of St. Lawrence, and during the period of
spring breakup, fractured pack ice is transported south out of the Gulf and along the
eastern coast of Nova Scotia. Ice regimes on the northern parts of the province differ
strongly depending on the origin of the flow in which freezing occurs. Unlike the Gulf
of St. Lawrence where pack ice is formed in situ, the winter ice cover off the Labrador
coast, which extends south to the northern Grand Banks, is characterized by the frequent
presence of icebergs drifted from the glacier fronts of the Canadian arctic. These may
survive individually much farther to the southeast than does the seasonal ice cover, as
Titanic discovered to its cost.
Since the entire Gulf of St. Lawrence has an estuarine circulation, so that the discharge
of brackish water at the surface toward the open ocean induces a landward flow of deep
water through the Cabot Straits and, consequently, the upwelling of nutrient-rich water
at the head of the Laurentian Channel north of the Gaspé Peninsula (Dickie and Trites,
1983). The water that passes out onto the Scotia shelf from the Gulf of St. Lawrence
moves southward along the coast with the shelf water mass, passes cyclonically around
the Gulf of Maine, rounds Georges Bank, and so moves on southward past Cape Cod to
form the shelf water of the Middle Atlantic Bight.
The southern end of the province beyond Cape Hatteras is transitional to the CARB
province to the south; here, the northward flow of the Gulf Stream water above the slope
is contrary to the generally equatorward drift of slope water in this province. Shelf water
in the South Atlantic Bight, although it still passes southward along the coast, is therefore
heavily influenced by admixture of warm, tropical water with major consequences for
nutrient flux onto the shelf (Boicourt et al., 1998). The slope of the South Atlantic Bight,
at about 31
N, bears a NE-trending ridge—the “Charleston Bump”—that deflects the
deep flow of the Gulf Stream, inducing meanders that may form cyclonic frontal eddies
that turn back over the shelf and flood it with oceanic water. Because the Gulf Stream at
these latitudes is bistable, lying along one or the other of two persistent tracks, the frontal
eddies take two modes: smaller and larger. This bistability determines the consequences
for shelf ecology because, in the large eddy mode, warm oceanic water passes around a
cold dome and may then propagate right in to the coastline.
The development of a summer stratified regime from the winter conditions of nearuniform properties to 50–100 m depth that obtain everywhere except in the South Atlantic
Bight is anything but simple. On the northern Grand Banks, buoyancy is enhanced in
spring by the presence of superficial water of low density, the result of melting pack ice,
and also by increasing surface irradiance and consequent heat gain. The same occurs in
the Gulf of St. Lawrence, although here meltwater is relatively less important (Doyon
et al., 2000). On the Scotian Shelf, warm slope water episodically floods across the shelf;
because it is of significantly higher salinity than the shelf water, it is denser and so remains
near-bottom. Such fluxes are an important nutrient input to shelf ecosystems, and are
akin to the transport of nutrient-rich slope water (2–5
C) into the head of the Gulf of
St. Lawrence below the cold intermediate water at −1
to 2
C.
At the southern end of the province, in the South Atlantic Bight, the timing of
stratification in spring is dependent on the direction of wind forcing (Flagg et al., 2002);
downwelling-favorable winds maintain the shallow plume of light water from Chesapeake
Bay close to the coast as it passes southward in the general shelf circulation, but upwellingfavorable winds cause it to spread out across the shelf. This provides the extra stability
needed to enable the ambient surface heat flux to stratify the water column so that mixing
occurs again only at the end of summer.