The species richness St (total number of species/sample) ranged from 36
species at the shelf station D to 94 species at the slope station B. This index did not show any trend with depth in the studied area due to the
low value recorded at the deeper station within the canyon (43 species at
station E'). At all the stations, the amphipods represented the more diverse group (up to 56 % of the total at the station E 7 ). The Shannon index H’ was significandy correlated with evenness J (r = 0.884) and allowed to distinguish the shelf station D (H’ = 2.71) from the deeper
ones (H’ > 4). The maximum diversity was recorded at the slope station
B (H’ = 5.18). Such higher diversity values on bathyal bottoms were already mentioned in previous studies (Sorbe, 1984; Elizalde, 1994; Elizalde et al., 1993).
As often mentioned in previous studies on suprabenthic assemblages
(Sorbe, 1984; Elizalde et al., 1993; Sorbe & Weber, 1995; Dauvin et al.,
1995; Cunha et al., 1997 ; Sorbe, 1999), the decreasing vertical gradient
of the faunal density from the sediment-water interface was also observed for most of the stations of the Capbreton area. Such a typical
near-bottom v ertical distribution of the suprabenthic fauna was trophically related with a higher concentration of suitable particulate organic
matter at the sea floor. However, the station E’ showed a maximum
density within the upper 45-75 cm water layer sampled by the sled. This
peculiar trend was probably related to its location at approximately
800 m depth within the canyon, where oxygen depletion at the sea bottom could be responsible for an upward migration of the fauna into
better oxygenated near-bottom waters.
The total density values Dt (cumulative data from the four nets in each
haul) of the suprabenthic fauna were not correlated with depth in the
Capbreton area, although the lowest ones were recorded at the deepest stations within the canyon (station F7: 56.6 ind./100 m
2 ) and on the open
slope (station A: 62.7 ind./100 m
2 ). The low value at station E’ was probably due to the peculiar environmental conditions on the bottom of the
canyon (oxygen depletion, sediment charactcristics) whereas, at station A,
it was certainly related to its oceanic location far from the coast. The
shelf density value (station D: 1621.6 ind./100 m
2 ) was comparable to
the annual mean value mentioned at 90 m depth off Arcachon
(1865 ind./100 m
2 ; Sorbe, 1999). The highest density value was surprisingly observed on the northern flank of the canyon (station C’:
3918.1 ind./100 m
2 ) and was seven times higher than the winter value recorded at approximately 400 m depth off Arcachon (537.8 ind./100 m
2 ;
unpublished data). Amphipods were numerically dominant at all the stations, with percentage contributions ranging between 44.5% and 69.6%,
followed by mysids, cumaceans and isopods with highly variable contributions according to stations. The other groups showed contributions
lower than 10 per cent. Previous studies on suprabenthic assemblages
from the SE ' . of the bay of Biscay generally mentioned the numerical
dominance of amphipods on the shelf (Sorbe, 1984) whereas asellote
91
species at the shelf station D to 94 species at the slope station B. This index did not show any trend with depth in the studied area due to the
low value recorded at the deeper station within the canyon (43 species at
station E'). At all the stations, the amphipods represented the more diverse group (up to 56 % of the total at the station E 7 ). The Shannon index H’ was significandy correlated with evenness J (r = 0.884) and allowed to distinguish the shelf station D (H’ = 2.71) from the deeper
ones (H’ > 4). The maximum diversity was recorded at the slope station
B (H’ = 5.18). Such higher diversity values on bathyal bottoms were already mentioned in previous studies (Sorbe, 1984; Elizalde, 1994; Elizalde et al., 1993).
As often mentioned in previous studies on suprabenthic assemblages
(Sorbe, 1984; Elizalde et al., 1993; Sorbe & Weber, 1995; Dauvin et al.,
1995; Cunha et al., 1997 ; Sorbe, 1999), the decreasing vertical gradient
of the faunal density from the sediment-water interface was also observed for most of the stations of the Capbreton area. Such a typical
near-bottom v ertical distribution of the suprabenthic fauna was trophically related with a higher concentration of suitable particulate organic
matter at the sea floor. However, the station E’ showed a maximum
density within the upper 45-75 cm water layer sampled by the sled. This
peculiar trend was probably related to its location at approximately
800 m depth within the canyon, where oxygen depletion at the sea bottom could be responsible for an upward migration of the fauna into
better oxygenated near-bottom waters.
The total density values Dt (cumulative data from the four nets in each
haul) of the suprabenthic fauna were not correlated with depth in the
Capbreton area, although the lowest ones were recorded at the deepest stations within the canyon (station F7: 56.6 ind./100 m
2 ) and on the open
slope (station A: 62.7 ind./100 m
2 ). The low value at station E’ was probably due to the peculiar environmental conditions on the bottom of the
canyon (oxygen depletion, sediment charactcristics) whereas, at station A,
it was certainly related to its oceanic location far from the coast. The
shelf density value (station D: 1621.6 ind./100 m
2 ) was comparable to
the annual mean value mentioned at 90 m depth off Arcachon
(1865 ind./100 m
2 ; Sorbe, 1999). The highest density value was surprisingly observed on the northern flank of the canyon (station C’:
3918.1 ind./100 m
2 ) and was seven times higher than the winter value recorded at approximately 400 m depth off Arcachon (537.8 ind./100 m
2 ;
unpublished data). Amphipods were numerically dominant at all the stations, with percentage contributions ranging between 44.5% and 69.6%,
followed by mysids, cumaceans and isopods with highly variable contributions according to stations. The other groups showed contributions
lower than 10 per cent. Previous studies on suprabenthic assemblages
from the SE ' . of the bay of Biscay generally mentioned the numerical
dominance of amphipods on the shelf (Sorbe, 1984) whereas asellote
91