THE DEEP ATLANTIC OCEAN
139
(4156 individuals m
−2 , for the >420 mm fraction) in the
Rockall Trough (Dinet et al., 1985, table 5). More data
are available for the metazoan meiofauna, particularly
the nematodes, from the six BIOGAS stations. Dinet
and Vivier (1977) found that total meiofaunal densities
declined with increasing bathymetric depth, as did
the proportion of the fauna present in the upper
1 cm of sediment. Densities also showed considerable
spatial variability at both bathyal and abyssal sites,
and particularly at stations 1, 2 and 4 to the north
and station 5 to the south. Dinet and Vivier (1977)
attributed this variability to the physico-chemical
heterogeneity of the sediments. Interestingly, they could
find no seasonal variation in total meiofaunal densities,
or any correlation between densities and the organiccarbon content of the sediments. In a second study,
Dinet and Vivier (1979) investigated the nematode
fauna at the species level. The populations were very
diverse, most species being represented by only one
or a few individuals. The minimum and maximum
diversity values occurred at the two southern sites
(from about 50 species at station 5 to about 128 species
at station 6) with the four stations of the northern
transect yielding approximately 82–115 species (Dinet
and Vivier, 1979, fig. 5 therein). Analysis of faunal
similarity indicates an approximate separation between
stations on the continental slope and those on the
continental rise. However, there were no consistent
differences between northern (1–4) and southern (5–
6) sites; stations 1 and 6 showed a fairly close
affinity, while station 5 showed little similarity to
other sites. According to Soetaert and Heip’s (1995)
analysis of these and other literature data, the nematode
faunas of the continental rise and the abyssal plain
in the Bay of Biscay closely resemble those from
the Hatteras Abyssal Plain, the HEBBLE site and the
Puerto Rico Trench at the generic level. In terms
of trophic groups, they are dominated by selective
and non-selective deposit feeders and epistrate feeders;
predatory/scavenging forms are uncommon (<10%).
Iberian margin: Gage et al. (1995) and Lamont et al.
(1995) described macrofaunal sediment communities
at two contrasting localities off Portugal, a presumed
high-energy site in the Setubal Canyon (3356–3776 m),
which is believed to experience periods of elevated
current activity, and a tranquil site on the adjacent
Tagus Abyssal Plain (5038 m). In both cases, the
samples were dominated by agglutinated Foraminifera,
and the metazoan fauna comprised nematodes, polychaetes, peracarid crustaceans, molluscs and echinoderms in descending order of numerical importance.
Mean macrofaunal densities (300 mm individuals m
−2 )
were an order of magnitude higher in the canyon
(mean 459.2, s.d. 208.6) than on the Tagus Abyssal
Plain (mean 10.55, s.d. 22.24). On the other hand,
species diversity for bivalves, tanaids and agglutinated
Foraminifera was higher on the abyssal plain. There is
some evidence that the fauna is more aggregated on the
scale of tens of meters (between cores) in the canyon
than on the abyssal plain, although the evidence for
this is not conclusive. In general, the results of these
studies suggest that hydrodynamic disturbance tends
to diminish species diversity and increase the spatial
heterogeneity of assemblages.
Northwest African margin
Studies carried out during the 1970s by the Institut
f¨ ur Hydrobiologie und Fischereiwissenschaft (IHF),
Hamburg, and the Institute of Oceanographic Sciences (IOS), Wormley, have contributed greatly to
knowledge of the benthos under the upwelling region and in other areas of the Northwest African
margin. Downslope transects were worked by IHF
off Cap Blanc (21ºN), near the center of the main
upwelling area where primary production reaches
200 g C m
−2 yr
−1 , as well as off Morocco (Cape Mazagan, 35ºN) and Western Sahara (Cabo Pe˜ na Grande,
24–26ºN) to the north and off the Mauretanian
coast (17ºN) to the south. General results have been reviewed by Thiel (1978, 1979, 1982, 1983); Pfannkuche
et al. (1983) presented data from the Moroccan transect
where upwelling intensity is low.
In general, the benthic system is strongly influenced
by upwelling intensity, and hence surface primary
production, and by hydrographic conditions (Thiel,
1978; Pfannkuche et al., 1983). Concentrations of
chloroplastic pigments (chlorophyll breakdown products), which indicate organic-matter inputs, are two
orders of magnitude higher off Cap Blanc than
off Morocco; the Western Saharan transect yielded
intermediate values (Thiel, 1978, fig. 4 therein). Along
the Cap Blanc transect, pigment concentrations are
high on the shelf, low on the upper slope (200 m) and
then increase to much higher values between 400 and
1500 m before diminishing gradually with increasing
depth. The low values at 200 m are believed to reflect
the influence of a high-energy benthic boundary layer
on the sedimentation of organic matter. Off Morocco,
139
(4156 individuals m
−2 , for the >420 mm fraction) in the
Rockall Trough (Dinet et al., 1985, table 5). More data
are available for the metazoan meiofauna, particularly
the nematodes, from the six BIOGAS stations. Dinet
and Vivier (1977) found that total meiofaunal densities
declined with increasing bathymetric depth, as did
the proportion of the fauna present in the upper
1 cm of sediment. Densities also showed considerable
spatial variability at both bathyal and abyssal sites,
and particularly at stations 1, 2 and 4 to the north
and station 5 to the south. Dinet and Vivier (1977)
attributed this variability to the physico-chemical
heterogeneity of the sediments. Interestingly, they could
find no seasonal variation in total meiofaunal densities,
or any correlation between densities and the organiccarbon content of the sediments. In a second study,
Dinet and Vivier (1979) investigated the nematode
fauna at the species level. The populations were very
diverse, most species being represented by only one
or a few individuals. The minimum and maximum
diversity values occurred at the two southern sites
(from about 50 species at station 5 to about 128 species
at station 6) with the four stations of the northern
transect yielding approximately 82–115 species (Dinet
and Vivier, 1979, fig. 5 therein). Analysis of faunal
similarity indicates an approximate separation between
stations on the continental slope and those on the
continental rise. However, there were no consistent
differences between northern (1–4) and southern (5–
6) sites; stations 1 and 6 showed a fairly close
affinity, while station 5 showed little similarity to
other sites. According to Soetaert and Heip’s (1995)
analysis of these and other literature data, the nematode
faunas of the continental rise and the abyssal plain
in the Bay of Biscay closely resemble those from
the Hatteras Abyssal Plain, the HEBBLE site and the
Puerto Rico Trench at the generic level. In terms
of trophic groups, they are dominated by selective
and non-selective deposit feeders and epistrate feeders;
predatory/scavenging forms are uncommon (<10%).
Iberian margin: Gage et al. (1995) and Lamont et al.
(1995) described macrofaunal sediment communities
at two contrasting localities off Portugal, a presumed
high-energy site in the Setubal Canyon (3356–3776 m),
which is believed to experience periods of elevated
current activity, and a tranquil site on the adjacent
Tagus Abyssal Plain (5038 m). In both cases, the
samples were dominated by agglutinated Foraminifera,
and the metazoan fauna comprised nematodes, polychaetes, peracarid crustaceans, molluscs and echinoderms in descending order of numerical importance.
Mean macrofaunal densities (300 mm individuals m
−2 )
were an order of magnitude higher in the canyon
(mean 459.2, s.d. 208.6) than on the Tagus Abyssal
Plain (mean 10.55, s.d. 22.24). On the other hand,
species diversity for bivalves, tanaids and agglutinated
Foraminifera was higher on the abyssal plain. There is
some evidence that the fauna is more aggregated on the
scale of tens of meters (between cores) in the canyon
than on the abyssal plain, although the evidence for
this is not conclusive. In general, the results of these
studies suggest that hydrodynamic disturbance tends
to diminish species diversity and increase the spatial
heterogeneity of assemblages.
Northwest African margin
Studies carried out during the 1970s by the Institut
f¨ ur Hydrobiologie und Fischereiwissenschaft (IHF),
Hamburg, and the Institute of Oceanographic Sciences (IOS), Wormley, have contributed greatly to
knowledge of the benthos under the upwelling region and in other areas of the Northwest African
margin. Downslope transects were worked by IHF
off Cap Blanc (21ºN), near the center of the main
upwelling area where primary production reaches
200 g C m
−2 yr
−1 , as well as off Morocco (Cape Mazagan, 35ºN) and Western Sahara (Cabo Pe˜ na Grande,
24–26ºN) to the north and off the Mauretanian
coast (17ºN) to the south. General results have been reviewed by Thiel (1978, 1979, 1982, 1983); Pfannkuche
et al. (1983) presented data from the Moroccan transect
where upwelling intensity is low.
In general, the benthic system is strongly influenced
by upwelling intensity, and hence surface primary
production, and by hydrographic conditions (Thiel,
1978; Pfannkuche et al., 1983). Concentrations of
chloroplastic pigments (chlorophyll breakdown products), which indicate organic-matter inputs, are two
orders of magnitude higher off Cap Blanc than
off Morocco; the Western Saharan transect yielded
intermediate values (Thiel, 1978, fig. 4 therein). Along
the Cap Blanc transect, pigment concentrations are
high on the shelf, low on the upper slope (200 m) and
then increase to much higher values between 400 and
1500 m before diminishing gradually with increasing
depth. The low values at 200 m are believed to reflect
the influence of a high-energy benthic boundary layer
on the sedimentation of organic matter. Off Morocco,