262
Chapter 9: The Atlantic Ocean
period of rising river flow in February–March, there are extreme episodes of settlement
whose blanketing effect is sufficiently strong as to take down planktonic organisms from
the water column into the sediments (Aller and Todorov, 1997). Calanoid copepods
and other planktonic biota are rapidly buried, along with the in situ benthic fauna, to
depths of ∼25cm; in the AMASSEDS surveys, extraordinarily high abundances of buried
copepods were observed at all stations, reaching >5000 copepods m
−2 at the river mouth
station, in water ∼20 m deep. The copepods were intact and had fresh phytoplankton in
their guts, so they must have been carried down and been buried very rapidly. During
the period of falling river discharge (August–October) this phenomenon occurred only
close to the river mouth; at this period, the deposits below the inner river plume are at
their most stable.
More generally, the seasonality of benthic communities within the ambit of the Amazon
plume responds directly to discharge rate: this is observed in the abundance and size
frequency distribution of individual species, as well as in their behaviors as expressed
by burrowing and feeding depths, and also in the specific diversity of communities
(Aller and Stupakoff, 1996). During the period of rising river flow, these indices all take
minimal values, associated with very dynamic overturn and resuspension of at least the
top 100 cm of the soft sediments; in the opposite season of low discharge rates and
weak wind stress, the soft sediments become consolidated and are very rapidly invaded
by burrowing polychaetes that reach high density (∼100 ind cm
−2 ), while meiofauna
(nematodes, harpacicoids and foraminifera) and bacterial numbers increase by about two
orders of magnitude (from ∼3 to ∼220 × 10
9 cells cm
−2 ) through the top 10 cm of the
sediments. With increasing depth, both diversity and abundance of meiofaunal groups
increase.
The macrofauna is dominated by polychaetes and crustacea, whereas mollusks are
relatively unimportant. Early colonizing polychaetes (e.g., capitellids, spionids) are especially characteristic everywhere, with a wide range of other taxa, of which Lumbrinereidae
appear to reach the highest densities. Crustacea are mostly burrowing forms: cumaceans
(Eudorella), burrowing carideans (Ogyrides), thalassinid mud shrimps (Callianassa and
Upogebia), and tube-dwelling amphipods (Erichthonus and Ampelisca). Sediment reworking rates of order 100–200 g m
−2 d
−1 were computed for these and related organisms.
Relatively insignificant numbers of brachyuran crustaceans are also present. Mollusks
include both surface (Tellina) and subsurface (Nucula, Yoldia) deposit feeders, together
with a few predatory gastropods. These species associations recall those that inhabit very
soft muddy deposits off estuary mouths in the Gulf of Guinea, where the same genera
of burrowing crustacea are equally important; the resuspension-deposition of the cycle
deposits recalls that of the mobile mud banks (“chakara”) of the Malabar coast of western India where comparable mortality and recolonization of the benthic macrofauna is
induced.
The demersal fish associations of the shelf regions from the Amazon to the Orinoco
were well surveyed in their near-pristine state in 1957–59 (Lowe-McConnell, 1962),
clearly establishing their general resemblance to those of the Gulf of Guinea shelf, with
a few interesting differences. In fact, we do not lack for early survey data of this region
by local and colonial governments, and also by fishery agencies of the United States and
the USSR (see Longhurst and Pauly, 1987). The dominant coastal species association, on
soft coastal mud, the length of these coasts comprises 11 species of sciaenids (Cynoscion,
Macrodon, Nebris, Stellifer) as well as threadfin (Polynemus), spadefish (Chaetodipterus),
flatfish (Paralichthys), sea catfish (Bagrus, Arius), and various elasmobranchs. Further
offshore and on firmer muddy deposits down to at least 60 m, sciaenids (Micropogon,
Macropogon, Cynoscion) dominate a demersal fish fauna that also includes many coastal
genera characteristic of the Gulf of Guinea: Polynemus, Larimus, Vomer, and so on.
Beyond 60 m, although the sandy deposits resemble those of the outer shelf of the Gulf of
Chapter 9: The Atlantic Ocean
period of rising river flow in February–March, there are extreme episodes of settlement
whose blanketing effect is sufficiently strong as to take down planktonic organisms from
the water column into the sediments (Aller and Todorov, 1997). Calanoid copepods
and other planktonic biota are rapidly buried, along with the in situ benthic fauna, to
depths of ∼25cm; in the AMASSEDS surveys, extraordinarily high abundances of buried
copepods were observed at all stations, reaching >5000 copepods m
−2 at the river mouth
station, in water ∼20 m deep. The copepods were intact and had fresh phytoplankton in
their guts, so they must have been carried down and been buried very rapidly. During
the period of falling river discharge (August–October) this phenomenon occurred only
close to the river mouth; at this period, the deposits below the inner river plume are at
their most stable.
More generally, the seasonality of benthic communities within the ambit of the Amazon
plume responds directly to discharge rate: this is observed in the abundance and size
frequency distribution of individual species, as well as in their behaviors as expressed
by burrowing and feeding depths, and also in the specific diversity of communities
(Aller and Stupakoff, 1996). During the period of rising river flow, these indices all take
minimal values, associated with very dynamic overturn and resuspension of at least the
top 100 cm of the soft sediments; in the opposite season of low discharge rates and
weak wind stress, the soft sediments become consolidated and are very rapidly invaded
by burrowing polychaetes that reach high density (∼100 ind cm
−2 ), while meiofauna
(nematodes, harpacicoids and foraminifera) and bacterial numbers increase by about two
orders of magnitude (from ∼3 to ∼220 × 10
9 cells cm
−2 ) through the top 10 cm of the
sediments. With increasing depth, both diversity and abundance of meiofaunal groups
increase.
The macrofauna is dominated by polychaetes and crustacea, whereas mollusks are
relatively unimportant. Early colonizing polychaetes (e.g., capitellids, spionids) are especially characteristic everywhere, with a wide range of other taxa, of which Lumbrinereidae
appear to reach the highest densities. Crustacea are mostly burrowing forms: cumaceans
(Eudorella), burrowing carideans (Ogyrides), thalassinid mud shrimps (Callianassa and
Upogebia), and tube-dwelling amphipods (Erichthonus and Ampelisca). Sediment reworking rates of order 100–200 g m
−2 d
−1 were computed for these and related organisms.
Relatively insignificant numbers of brachyuran crustaceans are also present. Mollusks
include both surface (Tellina) and subsurface (Nucula, Yoldia) deposit feeders, together
with a few predatory gastropods. These species associations recall those that inhabit very
soft muddy deposits off estuary mouths in the Gulf of Guinea, where the same genera
of burrowing crustacea are equally important; the resuspension-deposition of the cycle
deposits recalls that of the mobile mud banks (“chakara”) of the Malabar coast of western India where comparable mortality and recolonization of the benthic macrofauna is
induced.
The demersal fish associations of the shelf regions from the Amazon to the Orinoco
were well surveyed in their near-pristine state in 1957–59 (Lowe-McConnell, 1962),
clearly establishing their general resemblance to those of the Gulf of Guinea shelf, with
a few interesting differences. In fact, we do not lack for early survey data of this region
by local and colonial governments, and also by fishery agencies of the United States and
the USSR (see Longhurst and Pauly, 1987). The dominant coastal species association, on
soft coastal mud, the length of these coasts comprises 11 species of sciaenids (Cynoscion,
Macrodon, Nebris, Stellifer) as well as threadfin (Polynemus), spadefish (Chaetodipterus),
flatfish (Paralichthys), sea catfish (Bagrus, Arius), and various elasmobranchs. Further
offshore and on firmer muddy deposits down to at least 60 m, sciaenids (Micropogon,
Macropogon, Cynoscion) dominate a demersal fish fauna that also includes many coastal
genera characteristic of the Gulf of Guinea: Polynemus, Larimus, Vomer, and so on.
Beyond 60 m, although the sandy deposits resemble those of the outer shelf of the Gulf of