218
Chapter 9: The Atlantic Ocean
Kattegat and thence down to 2–3‰ in the Gulfs of Bothnia and Finland. Stratification
is thus imposed progressively by buoyancy as well as by surface heating. Outflow of
low-salinity water released from the Kattegat forms the narrow (10 km wide), baroclinic
Baltic Current that conforms to the Swedish coast and, after modification within the
Skagerrak, subsequently follows the topography of Norway as a wider coastal current.
The Baltic itself is a relatively small mediterranean sea of mean depth 57 m, having a
shallow sill depth (<20 m) at the Kattegat. Water balance (and an estuary-like circulation)
depends on river discharge and episodic ventilation of the deeper water mass by irruptions
of saline water over the sill. Permanent salinity stratification occurs through the entire
Baltic, with a halocline at 30–40 m, the actual depth depending on the sill depths between
the different basins. The inner Baltic has a very low surface salinity (1–3‰), permitting ice
cover to develop over the entire northern region in winter. Surface irradiance in summer
reinforces stratification. The Gottland Deep (about 200 m depth) is anoxic, because of
the shallow sill and weak ventilation of the central Baltic Sea: the extent of the anoxic
area is rather variable, depending on the relative amount of ventilation that occurs each
year through the Kattegat. General surface circulation in the Baltic is cyclonic, weak, and
replete with mesoscale eddy features; the dominant southwesterly winds generate both
coastal jets, and more or less permanent upwelling at some coastal topographic features.
Response of the Pelagic Ecosystems
It should be borne in mind, when thinking about the ecology of this province, that here
the marine environment has been strongly modified by farming ashore and by mining,
oil drilling, and fishing at sea. Massive extraction of sand and gravel in the southern
North Sea has recycled significant amounts of previously buried inorganic nutrients into
the water column. The intensive agriculture of the European fields releases very large
quantities of nutrients, pesticides, and herbicides: phosphate input to the North Sea from
such sources increased by a factor of about 7 from 1950 to 1980. Industrial fishing has
strongly modified not only fish stocks, but also disturbed the surficial geology and the
benthic invertebrates of the sea floor.
In discussing the physical environment, I have emphasized the distinction between
mixed and stratified regions, and the existence of tidal fronts, because these are fundamental elements in the functioning of continental shelf ecosystems. Because of the
displacement of these fronts with the lunar tidal cycle their exact distribution is not
observable with precision in the 7- and 30-day composite satellite images. Nevertheless,
one of the most prominent features in almost every image of this region is the Friesian
front between the shallow region where bottom stress from tidal currents is very strong
and the deeper southern North Sea, where stratification develops in summer. This seemingly “high chlorophyll” feature is, in fact, a shallow region of relatively very turbid
water. Several exemplary TSM images of this region are available, each representing an
individual day during 1999, and can be compared with the SeaWiFS images for the same
7-day period to support this assumption. These images show that not only is tidal bed
stress responsible for resuspension, but so also is the passage of a storm system. Anyone
who has seen the brown seas off the East Anglian coast in windy weather knows what
the satellite is seeing under such conditions.
Nevertheless, the chlorophyll images serve very well to illustrate the difference between
instantaneous reality and the canonical model for the phytoplankton seasonal cycle in
this province; the complex spatial pattern of apparent chlorophyll enhancement changes
strikingly from month to month, and between years. A review of the 7-day composites
for 1999 shall serve to illustrate the whole. The shelf-break front and slope current often
bears a linear band of high chlorophyll, of much higher concentrations than over the
adjacent shelf. This is seen well in early June, when the slope current bloom extended from
Chapter 9: The Atlantic Ocean
Kattegat and thence down to 2–3‰ in the Gulfs of Bothnia and Finland. Stratification
is thus imposed progressively by buoyancy as well as by surface heating. Outflow of
low-salinity water released from the Kattegat forms the narrow (10 km wide), baroclinic
Baltic Current that conforms to the Swedish coast and, after modification within the
Skagerrak, subsequently follows the topography of Norway as a wider coastal current.
The Baltic itself is a relatively small mediterranean sea of mean depth 57 m, having a
shallow sill depth (<20 m) at the Kattegat. Water balance (and an estuary-like circulation)
depends on river discharge and episodic ventilation of the deeper water mass by irruptions
of saline water over the sill. Permanent salinity stratification occurs through the entire
Baltic, with a halocline at 30–40 m, the actual depth depending on the sill depths between
the different basins. The inner Baltic has a very low surface salinity (1–3‰), permitting ice
cover to develop over the entire northern region in winter. Surface irradiance in summer
reinforces stratification. The Gottland Deep (about 200 m depth) is anoxic, because of
the shallow sill and weak ventilation of the central Baltic Sea: the extent of the anoxic
area is rather variable, depending on the relative amount of ventilation that occurs each
year through the Kattegat. General surface circulation in the Baltic is cyclonic, weak, and
replete with mesoscale eddy features; the dominant southwesterly winds generate both
coastal jets, and more or less permanent upwelling at some coastal topographic features.
Response of the Pelagic Ecosystems
It should be borne in mind, when thinking about the ecology of this province, that here
the marine environment has been strongly modified by farming ashore and by mining,
oil drilling, and fishing at sea. Massive extraction of sand and gravel in the southern
North Sea has recycled significant amounts of previously buried inorganic nutrients into
the water column. The intensive agriculture of the European fields releases very large
quantities of nutrients, pesticides, and herbicides: phosphate input to the North Sea from
such sources increased by a factor of about 7 from 1950 to 1980. Industrial fishing has
strongly modified not only fish stocks, but also disturbed the surficial geology and the
benthic invertebrates of the sea floor.
In discussing the physical environment, I have emphasized the distinction between
mixed and stratified regions, and the existence of tidal fronts, because these are fundamental elements in the functioning of continental shelf ecosystems. Because of the
displacement of these fronts with the lunar tidal cycle their exact distribution is not
observable with precision in the 7- and 30-day composite satellite images. Nevertheless,
one of the most prominent features in almost every image of this region is the Friesian
front between the shallow region where bottom stress from tidal currents is very strong
and the deeper southern North Sea, where stratification develops in summer. This seemingly “high chlorophyll” feature is, in fact, a shallow region of relatively very turbid
water. Several exemplary TSM images of this region are available, each representing an
individual day during 1999, and can be compared with the SeaWiFS images for the same
7-day period to support this assumption. These images show that not only is tidal bed
stress responsible for resuspension, but so also is the passage of a storm system. Anyone
who has seen the brown seas off the East Anglian coast in windy weather knows what
the satellite is seeing under such conditions.
Nevertheless, the chlorophyll images serve very well to illustrate the difference between
instantaneous reality and the canonical model for the phytoplankton seasonal cycle in
this province; the complex spatial pattern of apparent chlorophyll enhancement changes
strikingly from month to month, and between years. A review of the 7-day composites
for 1999 shall serve to illustrate the whole. The shelf-break front and slope current often
bears a linear band of high chlorophyll, of much higher concentrations than over the
adjacent shelf. This is seen well in early June, when the slope current bloom extended from