224
Chapter 9: The Atlantic Ocean
Where organic material becomes an important component of the deposits, and mud
(particles <60 m) comprises >25% of the whole sediment, we shall expect some version
of an Amphiura association of sediment-feeding organisms dominated by ophiuroids
feeding on the sediment surface, and polychaetes feeding within it. Once again, increasing
mud content is associated with changing composition of the fauna. In softer sediments
(30–70% fines) we shall find Maldane and Clymene, and in very soft mud (>80% fines)
there will also be the polychaetes Pectinaria, Sternaspis and Maldane, and the bivalve
mollusks Abra nitride. These Amphiura associations seem to have a higher turnover rate
than the Venus community, in one case the annual P/B ratio being reported as very close
to unity.
These are dynamic species associations, and proportional representation of component species is neither stable, nor uniform from place to place. It is this that renders the
common-sense classification of species associations so vulnerable to statistical criticism.
A typical Amphiura community (sens. Thorson) was studied by Buchanan and Warwick
(1974) for many years in the North Sea. In 1958, at the start of this period, the nominate
species, the ophiuroid Amphiura filiformis, was very abundant (>12–15 m
−2 ) and dominated the contents of a grab-haul. But by 1971, there were only 2 ind m
−2 , because no
recruitment had occurred for many years. Nevertheless, the authors of the study found no
contradiction later on in using the shorthand of an “Amphiura community” to describe
their object of study.
Synopsis
Case 2—Nutrient-limited spring production but modified by shallow-water processes. The
mixed layer undergoes boreal winter excursion from 10–20 m (May–August) to 50–60 m
(December–March), while Z eu is consistently at 20–25 m. The thermocline is thus illuminated only during boreal summer. The winter mixing carries the pycnocline to the
bottom from midshelf shoreward and to moderate depths seaward of this line. P rises
rapidly in boreal spring to a May maximum, the subsequent decline showing only weakly
the habitual autumn change in rate. Significant accumulation of chlorophyll in boreal
winter October–March is perhaps largely an effect of river discharge and winter resuspension on satellite imagery, and seasonal maximum chlorophyll accumulation is usually
30 days prior to the peak P rate (Fig. 9.22).
Canary Current Coastal Province (CNRY)
Extent of the Province
The Canary Current Coastal Province comprises the southerly coastal flow of the eastern
boundary current of the North Atlantic from Cape Finisterre in northern Portugal to
Cape Verde in Senegal, its seaward boundary being the convergent front at the outer limit
of the zone of anticyclonic curl of wind stress approximately 200–400 km offshore. The
province includes the field of eddies associated with this convergence. It also includes the
Canary Islands that lie about 100 km offshore from Cap Juby (28
N).
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
Although the continental shelf here has nowhere the same width as is typical of NECS,
especially along the western coast of Iberia, it is somewhat wider along the northwestern
coast of Africa than in any other eastern boundary current province; here, there are
exceptionally wide areas in the bights between Capes Ghir and Jubi and between Capes
Bojador and Blanc where the 200-m contour lies 100–120 km offshore, whereas off the
capes themselves it narrows to 20–30 km. The Arguin Bank just south of Cape Blanc
Chapter 9: The Atlantic Ocean
Where organic material becomes an important component of the deposits, and mud
(particles <60 m) comprises >25% of the whole sediment, we shall expect some version
of an Amphiura association of sediment-feeding organisms dominated by ophiuroids
feeding on the sediment surface, and polychaetes feeding within it. Once again, increasing
mud content is associated with changing composition of the fauna. In softer sediments
(30–70% fines) we shall find Maldane and Clymene, and in very soft mud (>80% fines)
there will also be the polychaetes Pectinaria, Sternaspis and Maldane, and the bivalve
mollusks Abra nitride. These Amphiura associations seem to have a higher turnover rate
than the Venus community, in one case the annual P/B ratio being reported as very close
to unity.
These are dynamic species associations, and proportional representation of component species is neither stable, nor uniform from place to place. It is this that renders the
common-sense classification of species associations so vulnerable to statistical criticism.
A typical Amphiura community (sens. Thorson) was studied by Buchanan and Warwick
(1974) for many years in the North Sea. In 1958, at the start of this period, the nominate
species, the ophiuroid Amphiura filiformis, was very abundant (>12–15 m
−2 ) and dominated the contents of a grab-haul. But by 1971, there were only 2 ind m
−2 , because no
recruitment had occurred for many years. Nevertheless, the authors of the study found no
contradiction later on in using the shorthand of an “Amphiura community” to describe
their object of study.
Synopsis
Case 2—Nutrient-limited spring production but modified by shallow-water processes. The
mixed layer undergoes boreal winter excursion from 10–20 m (May–August) to 50–60 m
(December–March), while Z eu is consistently at 20–25 m. The thermocline is thus illuminated only during boreal summer. The winter mixing carries the pycnocline to the
bottom from midshelf shoreward and to moderate depths seaward of this line. P rises
rapidly in boreal spring to a May maximum, the subsequent decline showing only weakly
the habitual autumn change in rate. Significant accumulation of chlorophyll in boreal
winter October–March is perhaps largely an effect of river discharge and winter resuspension on satellite imagery, and seasonal maximum chlorophyll accumulation is usually
30 days prior to the peak P rate (Fig. 9.22).
Canary Current Coastal Province (CNRY)
Extent of the Province
The Canary Current Coastal Province comprises the southerly coastal flow of the eastern
boundary current of the North Atlantic from Cape Finisterre in northern Portugal to
Cape Verde in Senegal, its seaward boundary being the convergent front at the outer limit
of the zone of anticyclonic curl of wind stress approximately 200–400 km offshore. The
province includes the field of eddies associated with this convergence. It also includes the
Canary Islands that lie about 100 km offshore from Cap Juby (28
N).
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
Although the continental shelf here has nowhere the same width as is typical of NECS,
especially along the western coast of Iberia, it is somewhat wider along the northwestern
coast of Africa than in any other eastern boundary current province; here, there are
exceptionally wide areas in the bights between Capes Ghir and Jubi and between Capes
Bojador and Blanc where the 200-m contour lies 100–120 km offshore, whereas off the
capes themselves it narrows to 20–30 km. The Arguin Bank just south of Cape Blanc