370
Romeroetal.
general pattern can be seen which correlates well
with the primary production estimates of Berger
(1989, "Dahlem map") (Fig. 1) and with satellite
imagery (Longhurst et al. 1995).
Fluxes were highest at the sites adjacent to or
influenced by coastal upwelling areas (off
Cape Blanc and at the Walvis Ridge; Fig. 1) and
south of Cape Verde where total primary production is high (90-125 gC m- 2 y-I, Berger 1989;
323 gC m- 2 y' in the Benguela Current, and
700 gC m- 2 y' off NW Africa, Longhurst et al.
1995), and high seasonal variability of fluxes is
expected due to the occurrence of large filaments
with high phytoplankton biomass streaming offshore (van Camp et al. 1991; Gabric et al. 1993;
Lutjeharms and Stockton 1987).
The eastern equatorial sites (Guinea Basin) are
located across the high surface chlorophyll band of
the equatorial divergence zone (primary production
60-90 gC m- 2 y', Berger 1989; 157 gC m- 2 y',
Longhurst et al. 1995). Fluxes were highest in the
Guinea Basin north ofthe equator compared to the
sites at and south ofthe equator (Fig. 1). Although
mean flux values differed in magnitude for 19891
90 (sites GBN3, GBZ4) and 1991 (sites EAI-5),
the pattern of increased flux rates (especially diatoms, Fig. 1) occurred north of the equator but not
south of it, as expected from satellite-derived chlorophyll distributions (Longhurst 1993) and primary
productivity measurements (Voituriez and Herbland
1981). Fischer and Wefer (1995,1996) offer several explanations for this discrepancy and suggest
that this is a result of restricted particle sinking close
to the equatorial divergence, and preferential concentration and sinking of particulates further north
ofthe equator in the vicinity of the convergent frontal zone (Yoder et al. 1994). Another explanation
could be that most of the chlorophyll is included in
non-siliceous pico- and nanoplankton (e.g. naked
flagellates, as is the case in the equatorial Pacific;
Chavez and Smith 1995) which may not get trapped
or preserved in the sediment traps.
Tn the less productive western equatorial Atlantic (Voituriez and Herbland 1981), flux values reflect the transition (Fig. 1) from oligotrophic open
ocean conditions (around 7° S, WA3) of the eastern subtropical gyre (where primary production is
assumed to fall within 35-45 gC m- 2 y') to an area
(around 4° S, site WA 7) probably influenced to a
larger degree by pelagic blooms ofthe equatorial
Atlantic (Longhurst 1993) (contour lines 45-60 gC
m- 2 y'; Berger 1989; Fig. 1); here, flux values were
similar to the N-Guinea Basin (Fig. 1).
A characteristic pattern of the domination of
chain-forming and colonial diatoms with cell diameters of> 5 flm in coastal upwelling systems and
that of small « 5 flm) solitary phytoplankton in the
open ocean equatorial regimes emerged in our
data (see Chavez and Smith 1995, for a review).
Throughout the sampling year, small specimens of
Nitzschia bicapitata (Plate 1) accompanied by
other taxa typical of open-ocean conditions dominated the diatom assemblage in the eastern and
western equatorial Atlantic and near Cape Verde.
Off Cape Blanc and at the Walvis Ridge, on the
other hand, a coastal upwelling assemblage characterized by the colonial diatom Thalassionema
nitzschioides var. nitzschioides and members of
the chain-forming genus Chaetoceros (Plate I),
mixed with representatives of oceanic conditions,
was the rule (Treppke et al. 1996b; Lange et al.
1998).
Total mass flux was clearly dominated (> 40%)
by carbonate-bearing primary ( coccolithophorids)
and/or secondary producers (planktic foraminifera),
particularly in the Guinea Basin and in the western
equatorial Atlantic (> 60%; We fer and Fischer
1993; Fischer and We fer 1996). South of Cape
Verde and off Cape Blanc, total mass fluxes were
mainly composed oflithogenics and carbonate components. Biogenic opal contributions to the total
mass flux (Fig. 1) were higher offNW Africa,
at the Walvis Ridge and in the N-Guinea Basin
(6-12%) than they were in the S-Guinea Basin and
the western equatorial Atlantic (4-7%). Regardless of the overall contribution to the total flux,
diatoms dominated (in numbers per m 2 dol)
the biogenic opal fraction at all sites, followed by
silicoflagellates and radiolarians. An example of
two contrasting environments are given in Fig. 2.
Romeroetal.
general pattern can be seen which correlates well
with the primary production estimates of Berger
(1989, "Dahlem map") (Fig. 1) and with satellite
imagery (Longhurst et al. 1995).
Fluxes were highest at the sites adjacent to or
influenced by coastal upwelling areas (off
Cape Blanc and at the Walvis Ridge; Fig. 1) and
south of Cape Verde where total primary production is high (90-125 gC m- 2 y-I, Berger 1989;
323 gC m- 2 y' in the Benguela Current, and
700 gC m- 2 y' off NW Africa, Longhurst et al.
1995), and high seasonal variability of fluxes is
expected due to the occurrence of large filaments
with high phytoplankton biomass streaming offshore (van Camp et al. 1991; Gabric et al. 1993;
Lutjeharms and Stockton 1987).
The eastern equatorial sites (Guinea Basin) are
located across the high surface chlorophyll band of
the equatorial divergence zone (primary production
60-90 gC m- 2 y', Berger 1989; 157 gC m- 2 y',
Longhurst et al. 1995). Fluxes were highest in the
Guinea Basin north ofthe equator compared to the
sites at and south ofthe equator (Fig. 1). Although
mean flux values differed in magnitude for 19891
90 (sites GBN3, GBZ4) and 1991 (sites EAI-5),
the pattern of increased flux rates (especially diatoms, Fig. 1) occurred north of the equator but not
south of it, as expected from satellite-derived chlorophyll distributions (Longhurst 1993) and primary
productivity measurements (Voituriez and Herbland
1981). Fischer and Wefer (1995,1996) offer several explanations for this discrepancy and suggest
that this is a result of restricted particle sinking close
to the equatorial divergence, and preferential concentration and sinking of particulates further north
ofthe equator in the vicinity of the convergent frontal zone (Yoder et al. 1994). Another explanation
could be that most of the chlorophyll is included in
non-siliceous pico- and nanoplankton (e.g. naked
flagellates, as is the case in the equatorial Pacific;
Chavez and Smith 1995) which may not get trapped
or preserved in the sediment traps.
Tn the less productive western equatorial Atlantic (Voituriez and Herbland 1981), flux values reflect the transition (Fig. 1) from oligotrophic open
ocean conditions (around 7° S, WA3) of the eastern subtropical gyre (where primary production is
assumed to fall within 35-45 gC m- 2 y') to an area
(around 4° S, site WA 7) probably influenced to a
larger degree by pelagic blooms ofthe equatorial
Atlantic (Longhurst 1993) (contour lines 45-60 gC
m- 2 y'; Berger 1989; Fig. 1); here, flux values were
similar to the N-Guinea Basin (Fig. 1).
A characteristic pattern of the domination of
chain-forming and colonial diatoms with cell diameters of> 5 flm in coastal upwelling systems and
that of small « 5 flm) solitary phytoplankton in the
open ocean equatorial regimes emerged in our
data (see Chavez and Smith 1995, for a review).
Throughout the sampling year, small specimens of
Nitzschia bicapitata (Plate 1) accompanied by
other taxa typical of open-ocean conditions dominated the diatom assemblage in the eastern and
western equatorial Atlantic and near Cape Verde.
Off Cape Blanc and at the Walvis Ridge, on the
other hand, a coastal upwelling assemblage characterized by the colonial diatom Thalassionema
nitzschioides var. nitzschioides and members of
the chain-forming genus Chaetoceros (Plate I),
mixed with representatives of oceanic conditions,
was the rule (Treppke et al. 1996b; Lange et al.
1998).
Total mass flux was clearly dominated (> 40%)
by carbonate-bearing primary ( coccolithophorids)
and/or secondary producers (planktic foraminifera),
particularly in the Guinea Basin and in the western
equatorial Atlantic (> 60%; We fer and Fischer
1993; Fischer and We fer 1996). South of Cape
Verde and off Cape Blanc, total mass fluxes were
mainly composed oflithogenics and carbonate components. Biogenic opal contributions to the total
mass flux (Fig. 1) were higher offNW Africa,
at the Walvis Ridge and in the N-Guinea Basin
(6-12%) than they were in the S-Guinea Basin and
the western equatorial Atlantic (4-7%). Regardless of the overall contribution to the total flux,
diatoms dominated (in numbers per m 2 dol)
the biogenic opal fraction at all sites, followed by
silicoflagellates and radiolarians. An example of
two contrasting environments are given in Fig. 2.