Atlantic Coastal Biome
217
regions, here and elsewhere, I shall assume that the “chlorophyll” signal in SeaWiFS
images largely represents the relative distribution of suspended fines and CDOM rather
than phytoplankton biomass. This is, of course, an approximation: the mapping of total
suspended material (TSM) for coastal water quality control is a delicate matter and
techniques are not yet resolved. Consult the Dutch POWERS-II Report if this problem
is of concern to you.
In addition to tidal fronts, a variety of other shallow-water, coastal, and estuarine
fronts must be considered here; estuarine fronts, characterized by turbidity and salinity
gradients, occur in many places along all coastlines, although not many have been studied
in detail. Because the hydrography of the Baltic Sea is dominated by large freshwater
inputs (see later discussion) and has a narrow entrance to the North Sea through the
Kattegat, a very strong salinity gradient occurs, having the characteristics of a frontal
zone, within this channel to the east of Denmark.
The spring-summer shelf-edge front appears as a meandering belt of relatively cool
surface water (1–2
C anomaly) just beyond the shelf edge and overlying the upper slope.
Satellite AVHRR shows this to be a quasipermanent feature, at least 800 km in length
and wider at the end of summer than in spring. For this retrograde shelf edge front,
explanations have evolved more slowly than for tidal fronts although, following Pingree
et al. (1986), it is now understood that an internal tide is generated at the 200-m isobath
and propagates from there both offshore and onto the shelf. As Joint et al. (2001) suggest,
the resultant progressive waves force the vertical transport of cool, nutrient-rich water
into the euphotic zone. An understanding of these processes were among the objective of
the OMEX I studies (1993–1995) of the ocean margin of Western Europe: both appear
to contribute to the linear zone of enhanced productivity above the shelf break of the
Celtic Sea and Bay of Biscay.
The continental shelf of Western Europe lies beside a region of weak poleward drift so,
as noted earlier, transport over the shelf is dominated by local winds: strong geostrophic
flow along the continental slope is lacking. The outer shelf is largely occupied by homohaline water that is thermally stratified in summer, except from northern Denmark to
the west of Scandinavia, where low-salinity Baltic outflow induces haline stratification.
Influx of Atlantic water occurs principally as southerly flow in the Norwegian Trench,
and on either side of the Shetlands; flow through the Straits of Dover is trivial. Interannual variability in the supply of Atlantic water is thought to be a major factor in forcing
regime shifts in the North Sea ecosystem.
On the middle shelf regions to the west of Brittany, where values of the stratification parameter are extremely high (so that stratification occurs very early in the year),
nutrient dynamics in summer are strongly influenced by several zones of remnant “winter water.” This is colder and denser than the surrounding water and remains trapped
in the areas where summer stratification is most strongly developed; these “bourrelets
froids” (Le Fèvre, 1986) are dome-shaped and have the potential to act both as a nutrient
reserve and as an accumulator of nutrients during the summer months. Underlying the
Armorican bourrelet lies the region of the “Grand Vasière”, or big mudhole, noted earlier.
Especially in the Bay of Biscay, and along the Friesian coast, the effluents from the
Gironde, Loire, and Rhine form buoyant plumes that extend both seaward and coastwise
from the river mouths: the Bay of Biscay has a very generalized near-coastal region
of lowered salinity, whereas the Loire plume extends northwest into the mouth of the
Channel under conditions of exceptional discharge. As will be discussed later, these
buoyant plumes have important consequences for the development of planktonic biota.
In the northeastern part of the province the circulation pattern is buoyancy-dominated,
although even here, tidal energy dissipation may far exceed the energy input by surface
wind stress (Rodhe, 1998). Beyond the region of tidal fronts in the southwestern North
Sea, climatological salinity is progressively reduced from 34.5‰ to 10–15‰ in the
217
regions, here and elsewhere, I shall assume that the “chlorophyll” signal in SeaWiFS
images largely represents the relative distribution of suspended fines and CDOM rather
than phytoplankton biomass. This is, of course, an approximation: the mapping of total
suspended material (TSM) for coastal water quality control is a delicate matter and
techniques are not yet resolved. Consult the Dutch POWERS-II Report if this problem
is of concern to you.
In addition to tidal fronts, a variety of other shallow-water, coastal, and estuarine
fronts must be considered here; estuarine fronts, characterized by turbidity and salinity
gradients, occur in many places along all coastlines, although not many have been studied
in detail. Because the hydrography of the Baltic Sea is dominated by large freshwater
inputs (see later discussion) and has a narrow entrance to the North Sea through the
Kattegat, a very strong salinity gradient occurs, having the characteristics of a frontal
zone, within this channel to the east of Denmark.
The spring-summer shelf-edge front appears as a meandering belt of relatively cool
surface water (1–2
C anomaly) just beyond the shelf edge and overlying the upper slope.
Satellite AVHRR shows this to be a quasipermanent feature, at least 800 km in length
and wider at the end of summer than in spring. For this retrograde shelf edge front,
explanations have evolved more slowly than for tidal fronts although, following Pingree
et al. (1986), it is now understood that an internal tide is generated at the 200-m isobath
and propagates from there both offshore and onto the shelf. As Joint et al. (2001) suggest,
the resultant progressive waves force the vertical transport of cool, nutrient-rich water
into the euphotic zone. An understanding of these processes were among the objective of
the OMEX I studies (1993–1995) of the ocean margin of Western Europe: both appear
to contribute to the linear zone of enhanced productivity above the shelf break of the
Celtic Sea and Bay of Biscay.
The continental shelf of Western Europe lies beside a region of weak poleward drift so,
as noted earlier, transport over the shelf is dominated by local winds: strong geostrophic
flow along the continental slope is lacking. The outer shelf is largely occupied by homohaline water that is thermally stratified in summer, except from northern Denmark to
the west of Scandinavia, where low-salinity Baltic outflow induces haline stratification.
Influx of Atlantic water occurs principally as southerly flow in the Norwegian Trench,
and on either side of the Shetlands; flow through the Straits of Dover is trivial. Interannual variability in the supply of Atlantic water is thought to be a major factor in forcing
regime shifts in the North Sea ecosystem.
On the middle shelf regions to the west of Brittany, where values of the stratification parameter are extremely high (so that stratification occurs very early in the year),
nutrient dynamics in summer are strongly influenced by several zones of remnant “winter water.” This is colder and denser than the surrounding water and remains trapped
in the areas where summer stratification is most strongly developed; these “bourrelets
froids” (Le Fèvre, 1986) are dome-shaped and have the potential to act both as a nutrient
reserve and as an accumulator of nutrients during the summer months. Underlying the
Armorican bourrelet lies the region of the “Grand Vasière”, or big mudhole, noted earlier.
Especially in the Bay of Biscay, and along the Friesian coast, the effluents from the
Gironde, Loire, and Rhine form buoyant plumes that extend both seaward and coastwise
from the river mouths: the Bay of Biscay has a very generalized near-coastal region
of lowered salinity, whereas the Loire plume extends northwest into the mouth of the
Channel under conditions of exceptional discharge. As will be discussed later, these
buoyant plumes have important consequences for the development of planktonic biota.
In the northeastern part of the province the circulation pattern is buoyancy-dominated,
although even here, tidal energy dissipation may far exceed the energy input by surface
wind stress (Rodhe, 1998). Beyond the region of tidal fronts in the southwestern North
Sea, climatological salinity is progressively reduced from 34.5‰ to 10–15‰ in the