Atlantic Westerly Winds Biome
177
much of this undercurrent water is entrained back into the fast outflowing surface current
(Caspers, 1957; Sorokin, 1983). On entering the Black Sea, the remainder spills over the
shelf break into deeper water.
The basin-scale circulation is cyclonic and appears as a coastal current that is the
analogue of the coastal flow of the Mediterranean. It is more continuous in the Black
Sea, however, because of the less complex coastal topography there. From the mouth
of the Bosphorus a strong current runs eastward along the Paphlagonian coast, forming
the strongest flow of the Rim Current (40–80 km wide) that circles the whole Black Sea
at the 200-m depth contour. The Rim Current is associated with two principal cyclonic
gyres that occupy the eastern and western basins (divided to the south of the Crimea).
These are constrained by the shelf edge that runs zonally across the basin at the latitude
of southern Crimea, so that flow on the northern shallow shelves themselves is more
variable. Smaller anticyclonic gyres lie between the main gyres and the coast, trapped
by topographic features. During winter, the two-gyre circulation may break down, to be
replaced with a single, more elongated cyclonic gyre, and, by the end of winter, very cold
shelf water has been formed on the wide shelf regions to the northeast and to the west of
the Crimea. This water is progressively advected around the western side of the Black Sea
and its effect may be traced even along the southern coast. Especially along the southern
and eastern coasts, there is strong mesoscale vorticity in the meandering and filamentous
flow, whose features propagate eastward at 10–15 km a day. This field of vorticity widens
at Cape Baba.
This circulation pattern explains the topography of the halocline and the oxic/anoxic
interface that lies at about 150 m near the centers of circulation of the two main cyclonic
gyres and deepens to >200 m around the coastal margins. Only below a small permanent
anticyclonic gyre in the southeastern part of the sea does the oxic/anoxic interface deepen
away from the coast. Because the chemistry of the oxic/anoxic interface is so intimately
connected with biological processes, we shall defer discussion of it to the following section.
Response of the Pelagic Ecosystems
The ecological characteristics of the two seas are sufficiently different that to place them
in a single province is largely a matter of convenience. Both, however, were significantly
modified during the 20th century, not only from land-based sources of contamination
but also by reduced runoff from the major rivers entering the basins. Nitrate values in
the mixed layer of the Black Sea have increased significantly in the past 25 years. Also
very significant has been the loss of the annual Nile flood, held in recent decades behind
the Aswan High Dam, resulting in a very significant modification of the ecology of the
eastern Mediterranean. The artificial Lessepsian connection between the Mediterranean
basin and the Red Sea is of great significance for taxonomic biogeography because of
immigration of Indo-Pacific species through the Suez Canal. This transport, and the
introduction of exotic species in ballast water of tankers, was discussed in Chapter 2.
The ecological response of the two seas to seasonal changes environmental forcing is,
for all these reasons, quite different as is clearly demonstrated by the seasonal chlorophyll
images from the SeaWiFS and MODIS sensors since 1997. The Mediterranean shows a
clear seasonal winter–spring bloom, stronger in the western than the eastern basin, and
from June until October almost the whole sea is deeply oligotrophic. The Black Sea, on
the other hand, appears now to have an almost uniformly high level of algal biomass
over deep water—a green field that is relatively invariant seasonally. Even higher biomass
is consistently indicated over the northern and western shelf areas, with permanent “hot
spots” in the Azov Sea and at the margin of the Danube-Dniester deltaic region.
Mediterranean Sea The seasonal cycle of primary production and consumption
resembles that of the subtropical Atlantic. Winter mixing causes nitrate to become
177
much of this undercurrent water is entrained back into the fast outflowing surface current
(Caspers, 1957; Sorokin, 1983). On entering the Black Sea, the remainder spills over the
shelf break into deeper water.
The basin-scale circulation is cyclonic and appears as a coastal current that is the
analogue of the coastal flow of the Mediterranean. It is more continuous in the Black
Sea, however, because of the less complex coastal topography there. From the mouth
of the Bosphorus a strong current runs eastward along the Paphlagonian coast, forming
the strongest flow of the Rim Current (40–80 km wide) that circles the whole Black Sea
at the 200-m depth contour. The Rim Current is associated with two principal cyclonic
gyres that occupy the eastern and western basins (divided to the south of the Crimea).
These are constrained by the shelf edge that runs zonally across the basin at the latitude
of southern Crimea, so that flow on the northern shallow shelves themselves is more
variable. Smaller anticyclonic gyres lie between the main gyres and the coast, trapped
by topographic features. During winter, the two-gyre circulation may break down, to be
replaced with a single, more elongated cyclonic gyre, and, by the end of winter, very cold
shelf water has been formed on the wide shelf regions to the northeast and to the west of
the Crimea. This water is progressively advected around the western side of the Black Sea
and its effect may be traced even along the southern coast. Especially along the southern
and eastern coasts, there is strong mesoscale vorticity in the meandering and filamentous
flow, whose features propagate eastward at 10–15 km a day. This field of vorticity widens
at Cape Baba.
This circulation pattern explains the topography of the halocline and the oxic/anoxic
interface that lies at about 150 m near the centers of circulation of the two main cyclonic
gyres and deepens to >200 m around the coastal margins. Only below a small permanent
anticyclonic gyre in the southeastern part of the sea does the oxic/anoxic interface deepen
away from the coast. Because the chemistry of the oxic/anoxic interface is so intimately
connected with biological processes, we shall defer discussion of it to the following section.
Response of the Pelagic Ecosystems
The ecological characteristics of the two seas are sufficiently different that to place them
in a single province is largely a matter of convenience. Both, however, were significantly
modified during the 20th century, not only from land-based sources of contamination
but also by reduced runoff from the major rivers entering the basins. Nitrate values in
the mixed layer of the Black Sea have increased significantly in the past 25 years. Also
very significant has been the loss of the annual Nile flood, held in recent decades behind
the Aswan High Dam, resulting in a very significant modification of the ecology of the
eastern Mediterranean. The artificial Lessepsian connection between the Mediterranean
basin and the Red Sea is of great significance for taxonomic biogeography because of
immigration of Indo-Pacific species through the Suez Canal. This transport, and the
introduction of exotic species in ballast water of tankers, was discussed in Chapter 2.
The ecological response of the two seas to seasonal changes environmental forcing is,
for all these reasons, quite different as is clearly demonstrated by the seasonal chlorophyll
images from the SeaWiFS and MODIS sensors since 1997. The Mediterranean shows a
clear seasonal winter–spring bloom, stronger in the western than the eastern basin, and
from June until October almost the whole sea is deeply oligotrophic. The Black Sea, on
the other hand, appears now to have an almost uniformly high level of algal biomass
over deep water—a green field that is relatively invariant seasonally. Even higher biomass
is consistently indicated over the northern and western shelf areas, with permanent “hot
spots” in the Azov Sea and at the margin of the Danube-Dniester deltaic region.
Mediterranean Sea The seasonal cycle of primary production and consumption
resembles that of the subtropical Atlantic. Winter mixing causes nitrate to become