Atlantic Polar Biome
149
1986). It is joined by Atlantic water that forms a rather variable slope current north of
the Shetlands, and large offshore eddies are persistently generated as this flow enters the
Shetland-Faeroe Channel (Sherwin et al., 1999).
Low-salinity water from the Baltic and eastern North Sea passes around the southwestern coast of Norway into the Norwegian Coastal Current. This is bounded by a
salinity-dominated coastal front, with which is associated a highly unstable eddy field
(Johannessen, 1986), modifying both its Atlantic and coastal components as they pass
northward. Off North Cape, most water from the combined flow passes eastward into
the southern Barents Sea, while some continues poleward above the continental slope
west of Spitzbergen and so on to Fram Strait as the West Spitzbergen Current. The water
passing into the southern Barents Sea feeds a gyral circulation, which returns westward
to the south of Spitzbergen. A meandering convergence extends toward Bear Island from
the central Barents Sea, and this front is both the southern limit of winter ice cover and
the boundary between SARC and the adjacent BPLR region.
In the oceanic parts of this province, winter mixing is deep, especially along the OPF
and to the south of Iceland, although not so deep as in the southernmost parts of the
ARCT province. There has been a significant change of physical conditions in these seas
during the past 30 years, several time series suggesting a progressive decrease in both
salinity and temperature of the surface waters of the Norwegian Sea. Decreasing overflow
of deep cold water across the Faeroe-Iceland Ridge since 1950 is associated with decreasing
flow of warm Atlantic water into the Nordic seas (Hansen et al., 2001). Increased windinduced eastward advection of Arctic water in recent decades has resulted in an extension
of an intermediate layer of water of Arctic origin over the whole Norwegian Sea, and to
a freshening of the Atlantic water. The OPF has moved eastward in response by up to
300 km, and there is a close correlation between the location of this front and the winter
index of the North Atlantic Oscillation (Blindheim et al., 2000).
Response of the Pelagic Ecosystems
We have good information on the cycles of pelagic production and consumption for
several regions in the SARC province. It was, of course, from his studies of data from
Ocean Weather Station (OWS) M at 64
N in the Norwegian Sea that Sverdrup (1953)
formulated his classical model of a spring bloom, establishing firmly the relations between
mixed-layer depth and critical depth. His results were closely matched by the longer series
of observations by Bob Williams at OWS I at 59
N just south of Iceland, also in this
province. Here, serial observations were made from April to October in each year from
1971 to 1975, including weekly multidepth profiles of density, nutrients, chlorophyll, and
zooplankton (e.g., Williams and Robinson, 1973; Williams and Hopkins, 1976). Further
useful work near the same position was undertaken in the summer of 1980 during the
JGOFS North Atlantic Bloom Experiment (NABE). The weather ships observations have
been confirmed by daily data obtained more recently from a moored optical profiling
array (Dickey et al., 1994) near OWS I.
Because of the presence of low-salinity surface water of Arctic origin, the spring bloom
at OWS I occurs earlier than would be predicted by some models of mixed-layer evolution
and initiation of the algal bloom for the North Atlantic (Wolf and Woods, 1988: Strass
and Woods, 1988). Chlorophyll and
14 C data for 1972–1975 show that phytoplankton
growth rapidly follows the reestablishment of a shallow mixed layer (e.g., Williams and
Robinson, 1973). Ephemeral blooms associated with temporary near-surface pycnoclines
after periods of calm weather occur even earlier in the year, and continuous spring
bloom conditions are usually established by mid-April, with chlorophyll biomass reaching
3–4 mg m
−3 by mid-May. But there is much between-year variability: in 1972, nitrate was
reduced from >100 to <20 M in only 7 days in April, whereas in 1973 the equivalent
uptake took the entire months of June.
149
1986). It is joined by Atlantic water that forms a rather variable slope current north of
the Shetlands, and large offshore eddies are persistently generated as this flow enters the
Shetland-Faeroe Channel (Sherwin et al., 1999).
Low-salinity water from the Baltic and eastern North Sea passes around the southwestern coast of Norway into the Norwegian Coastal Current. This is bounded by a
salinity-dominated coastal front, with which is associated a highly unstable eddy field
(Johannessen, 1986), modifying both its Atlantic and coastal components as they pass
northward. Off North Cape, most water from the combined flow passes eastward into
the southern Barents Sea, while some continues poleward above the continental slope
west of Spitzbergen and so on to Fram Strait as the West Spitzbergen Current. The water
passing into the southern Barents Sea feeds a gyral circulation, which returns westward
to the south of Spitzbergen. A meandering convergence extends toward Bear Island from
the central Barents Sea, and this front is both the southern limit of winter ice cover and
the boundary between SARC and the adjacent BPLR region.
In the oceanic parts of this province, winter mixing is deep, especially along the OPF
and to the south of Iceland, although not so deep as in the southernmost parts of the
ARCT province. There has been a significant change of physical conditions in these seas
during the past 30 years, several time series suggesting a progressive decrease in both
salinity and temperature of the surface waters of the Norwegian Sea. Decreasing overflow
of deep cold water across the Faeroe-Iceland Ridge since 1950 is associated with decreasing
flow of warm Atlantic water into the Nordic seas (Hansen et al., 2001). Increased windinduced eastward advection of Arctic water in recent decades has resulted in an extension
of an intermediate layer of water of Arctic origin over the whole Norwegian Sea, and to
a freshening of the Atlantic water. The OPF has moved eastward in response by up to
300 km, and there is a close correlation between the location of this front and the winter
index of the North Atlantic Oscillation (Blindheim et al., 2000).
Response of the Pelagic Ecosystems
We have good information on the cycles of pelagic production and consumption for
several regions in the SARC province. It was, of course, from his studies of data from
Ocean Weather Station (OWS) M at 64
N in the Norwegian Sea that Sverdrup (1953)
formulated his classical model of a spring bloom, establishing firmly the relations between
mixed-layer depth and critical depth. His results were closely matched by the longer series
of observations by Bob Williams at OWS I at 59
N just south of Iceland, also in this
province. Here, serial observations were made from April to October in each year from
1971 to 1975, including weekly multidepth profiles of density, nutrients, chlorophyll, and
zooplankton (e.g., Williams and Robinson, 1973; Williams and Hopkins, 1976). Further
useful work near the same position was undertaken in the summer of 1980 during the
JGOFS North Atlantic Bloom Experiment (NABE). The weather ships observations have
been confirmed by daily data obtained more recently from a moored optical profiling
array (Dickey et al., 1994) near OWS I.
Because of the presence of low-salinity surface water of Arctic origin, the spring bloom
at OWS I occurs earlier than would be predicted by some models of mixed-layer evolution
and initiation of the algal bloom for the North Atlantic (Wolf and Woods, 1988: Strass
and Woods, 1988). Chlorophyll and
14 C data for 1972–1975 show that phytoplankton
growth rapidly follows the reestablishment of a shallow mixed layer (e.g., Williams and
Robinson, 1973). Ephemeral blooms associated with temporary near-surface pycnoclines
after periods of calm weather occur even earlier in the year, and continuous spring
bloom conditions are usually established by mid-April, with chlorophyll biomass reaching
3–4 mg m
−3 by mid-May. But there is much between-year variability: in 1972, nitrate was
reduced from >100 to <20 M in only 7 days in April, whereas in 1973 the equivalent
uptake took the entire months of June.