374
RomeroetaL
were reached in early spring and early summer (of
the order of20 * 10 5 valves m- 2 dol), and a strong
minimum was seen in September (5 * 10 5 valves
m- 2 dol) (Fig_ 3)_ Although diatoms were the dominant contributor to the opal fraction at all times, the
moderate fall and winter increase in the biogenic
opal flux may be associated with flux increases of
radiolarians and silicoflagellates rather than diatoms
(Boltovskoy et aL 1996; Lange et aL 1998)_
The constant presence ofthe upwelling-related
species Thalassionema nitzschioides var_
nitzschioides and Chaetoceros resting spores indicates the offshore spreading of the coastal
upwelling signaL The mixed flora, composed of
neritic taxa and species representatives of oceanic,
tropical to subtropical conditions (e_g_ Nitzschia
bicapitata, N. interruptestriata, Thalassionema
nitzschioides var. parva, Plate I), observed during the fall and winter may reflect periods of wind
relaxation and intermingling of warm open ocean
water with cool shelf water (Gabric et aL 1993;
Lange et aL 1998)_
Cape Verde
Further south near Cape Verde Islands (12° N and
20° W), the North Equatorial Under Current and
the North Equatorial Counter Current help build
another upwelling cell, the Guinea Dome, injecting
colder water into the westward flowing North
Equatorial Current (Voituriez 1981; Hagen and
Schemainda 1984)_ In this area, upwelling is mainly
present during November through February
(Mittelstaedt 1991)_ This is clearly reflected in the
fluxes measured at our Cape Verde site, with highest values in fall and winter (Fig- 3)_ Total, opal and
diatom fluxes covaried; correlation factors r = 0_93
for total-opal, 0_82 for total-diatoms, and 0_85 for
opal-diatoms_ Diatom fluxes ranged between 2_8
* 10 5 valves m- 2 dol (min_ in summer) and 50 * 10 5
valves m- 2 dol (max_ in winter)_
In contrast to Cape Blanc (Lange et aL 1998),
the Cape Verde site is an open ocean site with
moderate coastal influence (neritic diatoms were
mainly restricted to the late spring), characterized
by small oceanic diatoms such as N. bicapitata,
N. interruptestriata and T. nitzschioides var.
parva (Plate 1)_ Here, the indicator species for
oceanic equatorial upwelling (Lange et aL 1994;
Treppke et aL 1996a), N. bicapitata, contributed
with> 50% to the winter diatom flux peak_
N-S Transect in the Eastern Equatorial
Atlantic, Guinea Basin
There are two seasons in the eastern equatorial
Atlantic: a warm season from October to June, and
a cold one from July to September with most intense surface cooling in the Gulf of Guinea around
100W (Voiturez and Herbland 1977)_ During the
warm season a thermal ridge developes at 2-3°S;
during the cold season upwelling takes place at the
equator bringing cold and nutrient-rich water to the
surface (Voiturez and Herbland 1977)_
Our results show two major seasonal increases
in the particle fluxes, one in the spring and one in
the summer_ Increased spring sedimentation was
present at all sites, with highly variable magnitudes
and timing (Fig_ 4)_ Calculated total, opal and diatom fluxes for the warm spring season showed
highest values north of the equator, a minimum
close to the it, and again increasing values south of
the equator (Fig_ 4)_ For the northern sites, this could
be (a) a response to enhanced production due to
NE trades' supply of nutrients (Schlitz 1980) which
precipitate within the ITCZ (Wefer and Fischer
1993); or (b) a consequence of additional input from
the coast (Lange et aL 1994); or a combination of
both_ It is mainly in winter and spring, when the NE
trades predominate, that neritic (e_g_ Chaetoceros
radicans) and freshwater diatoms (Plate J) reach
highest relative abundances (~1 0%) in the NGuinea Basin as an indication of southward extension of coastal and eolian influences (Lange et aL
1994, 1998)_ Increased spring sedimentation at the
two southern sites may be attributed to a thermal
ridge centered about 2-3°S which occurs as a permanent feature during the warm season leading to
a deep chlorophyll maximum and elevated primary
production (Voituriez and Herbland 1977, 1981)_
Increased summer sedimentation was only documented north of the equator and is related to the
equatorial upwelling which takes place at this time
of the year (Fig.4; Fischer and Wefer 1995, 1996)_
Diatoms peaked in July-August reaching 45 * 10 5
valves m- 2 d-'_ We are unable to provide a mecha-
RomeroetaL
were reached in early spring and early summer (of
the order of20 * 10 5 valves m- 2 dol), and a strong
minimum was seen in September (5 * 10 5 valves
m- 2 dol) (Fig_ 3)_ Although diatoms were the dominant contributor to the opal fraction at all times, the
moderate fall and winter increase in the biogenic
opal flux may be associated with flux increases of
radiolarians and silicoflagellates rather than diatoms
(Boltovskoy et aL 1996; Lange et aL 1998)_
The constant presence ofthe upwelling-related
species Thalassionema nitzschioides var_
nitzschioides and Chaetoceros resting spores indicates the offshore spreading of the coastal
upwelling signaL The mixed flora, composed of
neritic taxa and species representatives of oceanic,
tropical to subtropical conditions (e_g_ Nitzschia
bicapitata, N. interruptestriata, Thalassionema
nitzschioides var. parva, Plate I), observed during the fall and winter may reflect periods of wind
relaxation and intermingling of warm open ocean
water with cool shelf water (Gabric et aL 1993;
Lange et aL 1998)_
Cape Verde
Further south near Cape Verde Islands (12° N and
20° W), the North Equatorial Under Current and
the North Equatorial Counter Current help build
another upwelling cell, the Guinea Dome, injecting
colder water into the westward flowing North
Equatorial Current (Voituriez 1981; Hagen and
Schemainda 1984)_ In this area, upwelling is mainly
present during November through February
(Mittelstaedt 1991)_ This is clearly reflected in the
fluxes measured at our Cape Verde site, with highest values in fall and winter (Fig- 3)_ Total, opal and
diatom fluxes covaried; correlation factors r = 0_93
for total-opal, 0_82 for total-diatoms, and 0_85 for
opal-diatoms_ Diatom fluxes ranged between 2_8
* 10 5 valves m- 2 dol (min_ in summer) and 50 * 10 5
valves m- 2 dol (max_ in winter)_
In contrast to Cape Blanc (Lange et aL 1998),
the Cape Verde site is an open ocean site with
moderate coastal influence (neritic diatoms were
mainly restricted to the late spring), characterized
by small oceanic diatoms such as N. bicapitata,
N. interruptestriata and T. nitzschioides var.
parva (Plate 1)_ Here, the indicator species for
oceanic equatorial upwelling (Lange et aL 1994;
Treppke et aL 1996a), N. bicapitata, contributed
with> 50% to the winter diatom flux peak_
N-S Transect in the Eastern Equatorial
Atlantic, Guinea Basin
There are two seasons in the eastern equatorial
Atlantic: a warm season from October to June, and
a cold one from July to September with most intense surface cooling in the Gulf of Guinea around
100W (Voiturez and Herbland 1977)_ During the
warm season a thermal ridge developes at 2-3°S;
during the cold season upwelling takes place at the
equator bringing cold and nutrient-rich water to the
surface (Voiturez and Herbland 1977)_
Our results show two major seasonal increases
in the particle fluxes, one in the spring and one in
the summer_ Increased spring sedimentation was
present at all sites, with highly variable magnitudes
and timing (Fig_ 4)_ Calculated total, opal and diatom fluxes for the warm spring season showed
highest values north of the equator, a minimum
close to the it, and again increasing values south of
the equator (Fig_ 4)_ For the northern sites, this could
be (a) a response to enhanced production due to
NE trades' supply of nutrients (Schlitz 1980) which
precipitate within the ITCZ (Wefer and Fischer
1993); or (b) a consequence of additional input from
the coast (Lange et aL 1994); or a combination of
both_ It is mainly in winter and spring, when the NE
trades predominate, that neritic (e_g_ Chaetoceros
radicans) and freshwater diatoms (Plate J) reach
highest relative abundances (~1 0%) in the NGuinea Basin as an indication of southward extension of coastal and eolian influences (Lange et aL
1994, 1998)_ Increased spring sedimentation at the
two southern sites may be attributed to a thermal
ridge centered about 2-3°S which occurs as a permanent feature during the warm season leading to
a deep chlorophyll maximum and elevated primary
production (Voituriez and Herbland 1977, 1981)_
Increased summer sedimentation was only documented north of the equator and is related to the
equatorial upwelling which takes place at this time
of the year (Fig.4; Fischer and Wefer 1995, 1996)_
Diatoms peaked in July-August reaching 45 * 10 5
valves m- 2 d-'_ We are unable to provide a mecha-