240
Chapter 9: The Atlantic Ocean
anywhere in the oceans: the Guinean Trawling Survey (GTS) comprised 480 one-hour
standard tows made by two matched French trawlers at eight standard depths on 63
transects, placed 60 km apart along the 4000-km shelf from Guinea to Angola, and it
was performed in both seasons. The data, submitted to recurrent group analysis, revealed
20 species groups with clear affinities for specific deposit and depth habitats (Fager
and Longhurst, 1968). These were objectively assembled into a warm-water fauna above
the thermocline on level bottoms usually of mud or sandy-mud, a fauna on sands
below the thermocline down to 100 m, and shallow (30–40 m) and deep (70–200 m)
hard-ground species groups. There was a remarkable homogeneity in these associations
the entire length of the coast and also in their relative abundance, with the single
exception of the northernmost section immediately south of Cape Roxo, which was
influenced by the Canary Current, and where biomass was significantly higher than
elsewhere. It was also clear that the objective grouping supported almost entirely the
well-known subjective groups of (i) a fauna of brown or silvery Sciaenidae, Drepanidae,
Polynemidae, and Cynoglossidae above the thermocline on muds and muddy sand, and
(ii) a fauna of red or pink Sparidae, Triglidae, Scorpaenidae, and Platycephalidae below the
thermocline on sandy deposits. At and beyond the shelf edge, a fauna of Chlorophthalmus,
Scomber, Bembrops, Peristedion, and Antigonia lay above the deeper-yet Merluccius and
Halosaurus. On hard ground above and below the thermocline, large red and brown
snappers (Lutjanus, Lethrinus) were characteristic of a reef fauna. The objective grouping
identified very few eurybathic species that were not associated with one of the major
species groups.
The distributions of invertebrate benthic communities similarly follow both the nature
of the deposits and also the distribution of water types. The primary divisions are
(i) between the organisms that occur only on soft muddy bottoms or those on sand,
shell sand, and shell gravel, and (ii) between those that occur only in tropical surface
water above the thermocline or those of the cooler, deeper water (Longhurst, 1958,
1963). In the tropical surface water, above the thermocline, a Macoma community, dominated by small bivalves and polychaetes occupies the soft muddy deposits in shallow
water. An Amphioplus community occurs on slightly deeper muddy deposits, comprising many ophiuroids, polychaetes, and burrowing crustacea, while a Venus community occurs on shelly sands, largely of their own making, and includes forms such as
Branchiostoma and echinoids. A Tellina community occurs on mineral sands inshore
and includes many specialized echinoderms and polychaetes. Deep benthic communities below the thermocline on shelly sands and muds differ strikingly from the shallow forms, being dominated by genera such as Hyalonoecia, Leptometra, Ophiothrix,
and Pennatula. At the shelf edge, both of these benthic communities are replaced by
cold-water forms that are largely conspecific with those at similar depths off Western
Europe.
Preliminary budgeting of autotrophic production and benthic and pelagic consumption on the Sierra Leone shelf suggests that benthic turnover, as measured from the
biomass of macroinvertebrates comprising the communities discussed earlier, is sufficiently slow that the autotrophic biomass unconsumed by pelagic herbivores at most
seasons is grossly in excess of the demand of macrobenthic organisms. This supports the
now-familiar concept that in the benthos, as in the pelagic ecosystem, it is the microbial
organisms that perform the bulk of community respiration.
This concept was reinforced by data from a survey performed by “Meteor” in
1988 of meiobenthos biomass distribution on 13 box-corer transects from the midshelf depths to the deep ocean, along the coast from Guinea to Angola (Soltwedel,
1997). This survey demonstrated a clear forcing of meiobenthic biomass by both regional
phytoplankton production and depth: on upwelling coasts, biomass was higher but,
everywhere, decreased with increasing depth. So, foraminiferan, nematode, harpacticoid,
Chapter 9: The Atlantic Ocean
anywhere in the oceans: the Guinean Trawling Survey (GTS) comprised 480 one-hour
standard tows made by two matched French trawlers at eight standard depths on 63
transects, placed 60 km apart along the 4000-km shelf from Guinea to Angola, and it
was performed in both seasons. The data, submitted to recurrent group analysis, revealed
20 species groups with clear affinities for specific deposit and depth habitats (Fager
and Longhurst, 1968). These were objectively assembled into a warm-water fauna above
the thermocline on level bottoms usually of mud or sandy-mud, a fauna on sands
below the thermocline down to 100 m, and shallow (30–40 m) and deep (70–200 m)
hard-ground species groups. There was a remarkable homogeneity in these associations
the entire length of the coast and also in their relative abundance, with the single
exception of the northernmost section immediately south of Cape Roxo, which was
influenced by the Canary Current, and where biomass was significantly higher than
elsewhere. It was also clear that the objective grouping supported almost entirely the
well-known subjective groups of (i) a fauna of brown or silvery Sciaenidae, Drepanidae,
Polynemidae, and Cynoglossidae above the thermocline on muds and muddy sand, and
(ii) a fauna of red or pink Sparidae, Triglidae, Scorpaenidae, and Platycephalidae below the
thermocline on sandy deposits. At and beyond the shelf edge, a fauna of Chlorophthalmus,
Scomber, Bembrops, Peristedion, and Antigonia lay above the deeper-yet Merluccius and
Halosaurus. On hard ground above and below the thermocline, large red and brown
snappers (Lutjanus, Lethrinus) were characteristic of a reef fauna. The objective grouping
identified very few eurybathic species that were not associated with one of the major
species groups.
The distributions of invertebrate benthic communities similarly follow both the nature
of the deposits and also the distribution of water types. The primary divisions are
(i) between the organisms that occur only on soft muddy bottoms or those on sand,
shell sand, and shell gravel, and (ii) between those that occur only in tropical surface
water above the thermocline or those of the cooler, deeper water (Longhurst, 1958,
1963). In the tropical surface water, above the thermocline, a Macoma community, dominated by small bivalves and polychaetes occupies the soft muddy deposits in shallow
water. An Amphioplus community occurs on slightly deeper muddy deposits, comprising many ophiuroids, polychaetes, and burrowing crustacea, while a Venus community occurs on shelly sands, largely of their own making, and includes forms such as
Branchiostoma and echinoids. A Tellina community occurs on mineral sands inshore
and includes many specialized echinoderms and polychaetes. Deep benthic communities below the thermocline on shelly sands and muds differ strikingly from the shallow forms, being dominated by genera such as Hyalonoecia, Leptometra, Ophiothrix,
and Pennatula. At the shelf edge, both of these benthic communities are replaced by
cold-water forms that are largely conspecific with those at similar depths off Western
Europe.
Preliminary budgeting of autotrophic production and benthic and pelagic consumption on the Sierra Leone shelf suggests that benthic turnover, as measured from the
biomass of macroinvertebrates comprising the communities discussed earlier, is sufficiently slow that the autotrophic biomass unconsumed by pelagic herbivores at most
seasons is grossly in excess of the demand of macrobenthic organisms. This supports the
now-familiar concept that in the benthos, as in the pelagic ecosystem, it is the microbial
organisms that perform the bulk of community respiration.
This concept was reinforced by data from a survey performed by “Meteor” in
1988 of meiobenthos biomass distribution on 13 box-corer transects from the midshelf depths to the deep ocean, along the coast from Guinea to Angola (Soltwedel,
1997). This survey demonstrated a clear forcing of meiobenthic biomass by both regional
phytoplankton production and depth: on upwelling coasts, biomass was higher but,
everywhere, decreased with increasing depth. So, foraminiferan, nematode, harpacticoid,