The Distribution of Living Planktic Foraminifera in Relation to Southeast Atlantic Oceanography
105
at the outermost edge of the ABF reflects the influence of the AC in this area.
To review briefly, we have noted that by using
key faunal elements the three main hydrographic
environments studied can be distinguished. The
abundance patterns show that for at least the tropical and subtropical regimes, the total assem blages
are not as informative as the concentrations of
selected species.
We have shown that other factors such as saI inity and oxygen content have to be considered as
well in the distribution of planktic foraminifera. The
data presented indicate that when all biological
parameters used to define a water mass are carefully considered we may reconstruct the local
hydrographic situation for different depth levels.
However, if faunal dynamics are to be understood,
parameters such as temperature, salinity, oxygen,
and nutrient concentrations may not suffice to understand the mechanisms and interrelationships
between assemblage and habitat. A better knowledge of the food web is crucial as the prey availability controls both the blooming of selected
planktic foraminifers and the grazing upon them.
Comparison of Assemblages from Plankton
Tows with those in Surface Sediments
We compare the distribution of individual planktic
foraminiferal species commonly used for
paleoceanographic reconstruction as observed in
the upper 50 m ofthe Southeast Atlantic with their
geographic distribution patterns in Southeast Atlantic surface sediments.
Globigerinoides ruber (pink and white
morphotypes) reaches values up to 35 percent in
the surface waters of the Angola Gyre and south
of the equatorial upwelling area (Fig. 10). The regional distribution of G. ruber in the underlying
sediment shows a similar pattern with frequencies
of 30 - 45 percent in the Angola Basin and south
of the equator toward the Subtropical Gyre. In
addition, the decline of G. ruber in the Guinea Basin
and in the Benguela Current area is documented
in the surface waters as well as in the sediment.
However, G. ruber is most abundant in surface
sediments of the central Subtropical Gyre, where
unfortunately no multinet data are available. The
higher percentages of G. ruber in the Angola
Basin as well as in the Subtropical Gyre are due to
G. ruber's preference for salt-rich gyre water
(salinities between 36.0 and 36.6). Its diminished
frequencies south of Angola (Fig. 10 and Van
Leeuwen, 1989), where the cold low-salinity
Benguela Current flows (salinities: 34.8 - 35.6), and
in the Guinea Basin (salinities: 35 - 35.6) corroborate this conclusion.
Globigerinoides sacculifer occurred in higher
percentages during the plankton sampling period at
the Walvis Ridge (Fig. 11) and in the equatorial
region. We presume that this distribution pattern
was affected by the reproduction cycle of a full
lunar period (Bijma et al. 1990). The only observed
coincidence with the surface-sediment distribution
is in the equatorial region, where G. sacculifer
reaches maximum values of20 percent. Based on
the surface sediment distribution, it would seem that
G. sacculifer is generally a typical warm water
species, occurring with maximum abundance where
the annual mean water temperatures are >24° C.
Tow samples, however, show a more differentiated
pattern which allows us to trace special surface
water masses.
Globigerina bulloides is known to favor nutrient-rich upwelling areas in the tropical ocean
(Thiede 1975; Prell and Curry 1981; Kroon 1990).
We can corroborate this observation, having recorded greater abundances of G. bulloides near
the coastal upwelling area off Namibia and toward
the eastern equatorial upwelling region (Fig. 12)
where phosphate and nitrate concentrations are
higher than in other parts of the tropical and subtropical South Atlantic. A comparison with its distribution in the surface sediment illustrates that the
surface water frequency pattern is reflected in the
bottom sediment, though the equatorial upwelling
maximum is more pronounced in the surface
water samples. The increasing numbers of G.
bulloides found in the surface sediments south of
30
0
S nicely trace its preference for fcrtile transitional and subpolar areas (Be 1977).
Neogloboquadrina dutertrei is most abundant
in the surface water layer of the tropical Southeast
Atlantic, the equatorial upwelling area (Fig. 13).
Previous studies have reported that this species
105
at the outermost edge of the ABF reflects the influence of the AC in this area.
To review briefly, we have noted that by using
key faunal elements the three main hydrographic
environments studied can be distinguished. The
abundance patterns show that for at least the tropical and subtropical regimes, the total assem blages
are not as informative as the concentrations of
selected species.
We have shown that other factors such as saI inity and oxygen content have to be considered as
well in the distribution of planktic foraminifera. The
data presented indicate that when all biological
parameters used to define a water mass are carefully considered we may reconstruct the local
hydrographic situation for different depth levels.
However, if faunal dynamics are to be understood,
parameters such as temperature, salinity, oxygen,
and nutrient concentrations may not suffice to understand the mechanisms and interrelationships
between assemblage and habitat. A better knowledge of the food web is crucial as the prey availability controls both the blooming of selected
planktic foraminifers and the grazing upon them.
Comparison of Assemblages from Plankton
Tows with those in Surface Sediments
We compare the distribution of individual planktic
foraminiferal species commonly used for
paleoceanographic reconstruction as observed in
the upper 50 m ofthe Southeast Atlantic with their
geographic distribution patterns in Southeast Atlantic surface sediments.
Globigerinoides ruber (pink and white
morphotypes) reaches values up to 35 percent in
the surface waters of the Angola Gyre and south
of the equatorial upwelling area (Fig. 10). The regional distribution of G. ruber in the underlying
sediment shows a similar pattern with frequencies
of 30 - 45 percent in the Angola Basin and south
of the equator toward the Subtropical Gyre. In
addition, the decline of G. ruber in the Guinea Basin
and in the Benguela Current area is documented
in the surface waters as well as in the sediment.
However, G. ruber is most abundant in surface
sediments of the central Subtropical Gyre, where
unfortunately no multinet data are available. The
higher percentages of G. ruber in the Angola
Basin as well as in the Subtropical Gyre are due to
G. ruber's preference for salt-rich gyre water
(salinities between 36.0 and 36.6). Its diminished
frequencies south of Angola (Fig. 10 and Van
Leeuwen, 1989), where the cold low-salinity
Benguela Current flows (salinities: 34.8 - 35.6), and
in the Guinea Basin (salinities: 35 - 35.6) corroborate this conclusion.
Globigerinoides sacculifer occurred in higher
percentages during the plankton sampling period at
the Walvis Ridge (Fig. 11) and in the equatorial
region. We presume that this distribution pattern
was affected by the reproduction cycle of a full
lunar period (Bijma et al. 1990). The only observed
coincidence with the surface-sediment distribution
is in the equatorial region, where G. sacculifer
reaches maximum values of20 percent. Based on
the surface sediment distribution, it would seem that
G. sacculifer is generally a typical warm water
species, occurring with maximum abundance where
the annual mean water temperatures are >24° C.
Tow samples, however, show a more differentiated
pattern which allows us to trace special surface
water masses.
Globigerina bulloides is known to favor nutrient-rich upwelling areas in the tropical ocean
(Thiede 1975; Prell and Curry 1981; Kroon 1990).
We can corroborate this observation, having recorded greater abundances of G. bulloides near
the coastal upwelling area off Namibia and toward
the eastern equatorial upwelling region (Fig. 12)
where phosphate and nitrate concentrations are
higher than in other parts of the tropical and subtropical South Atlantic. A comparison with its distribution in the surface sediment illustrates that the
surface water frequency pattern is reflected in the
bottom sediment, though the equatorial upwelling
maximum is more pronounced in the surface
water samples. The increasing numbers of G.
bulloides found in the surface sediments south of
30
0
S nicely trace its preference for fcrtile transitional and subpolar areas (Be 1977).
Neogloboquadrina dutertrei is most abundant
in the surface water layer of the tropical Southeast
Atlantic, the equatorial upwelling area (Fig. 13).
Previous studies have reported that this species