Variability in Export Production Documented by Downward Fluxes and Species Composition
367
Basin and S-Guinea Basin; in the less productive
western equatorial Atlantic around 25°W; and at
Walvis Ridge within the Benguela upwelling system. Finally, diatom assemblages in the traps
are compared to assemblages in the underlying
sediments, and the implications of seasonal and
preservational biases are discussed.
Methods
We used the classical cone-shaped traps with 0.5
m 2 opening and 13/20 sampling cups (MARK VI
and Kiel SMT 230/234). Collection cups were poisoned with HgCI 2 before deployment, and NaCI
was added to reach a final salinity of 38-400/00.
Samples were poisoned again after recovery with
HgCI2 and stored at 4° C. The splitting procedure
and chemical analyses of the < I mm fraction
(which clearly dominated particle flux) were carried out at Bremen University. All mooring sites
were deployed at comparable depths (between 600
and 1000 m depth, Table 1) except for the Cape
Blanc mooring (CB 1) which was deployed at about
2000 m depth. Here, the original surface signal may
have been distorted, and flux values at depth may
be the combined result of degradation in the water
column (C.,g fluxes decrease with depth) and additional supply (total, opal, lithogenic, and carbonate fluxes increase with depth) by lateral advection
from the shelf and slope and resuspension (Fischer
and Wefer 1996).
Since most trapping experiments extended over
351-381 days (Table I), mean values were extrapolated to 365 days (one year) except for the 1991
N-S transect (EAI-5) where sampling duration was
230 days. For deployment data see Table 1.
Throughout the text the seasons refer to the Northern Hemisphere (boreal seasons) except for the
Walvis Ridge site where austral seasons are used.
For this study 1/64,1125 and 1/125 splits ofthe
original samples were used. They were rinsed with
distilled water and prepared according to methodology of Lange et al. (1994). Diatom analysis was
carried out on permanent slides (Mountex mounting medium) of acid-cleaned material.
Qualitative and quantitative analyses were done
at x 400 and x 1000 magnifications using a ZeissPhotomicroscope III or Zeiss-Axioscope with
phase-contrast illumination. Several traverses
across the cover-slip were examined, depending on
abundances. Each individual was identified to the
lowest taxonomic level possible. The resulting
counts yielded estimates of daily fluxes of diatoms
valves per m· 2 d· 1 calculated according to Sancetta
and Calvert (1988), as well as relative abundances
of individual taxa. Counting of replicate slides indicated thatthe error for the flux estimates is <15%.
Surface sediment samples were collected with
a box corer of 50 x 50 cm 2 surface area, and were
taken from the uppermost 0.5-1 cm of the sediment
surface. Box core samples were prepared after the
methods of Schrader and Gersonde (1978).
Detailed descriptions ofthe investigation sites,
oceanographic and atmospheric settings are presented in Wefer and Fischer (1993); Lange et al.
(1994,1998); Fischer et al. (1996), and Treppke et
al.(1996b).
Results and Discussion
Although our database is constrained by one-year
records (Table 1), one ocean (Atlantic) and one
group of microorganisms (diatoms), tlle data cover
a large range of primary production values, including coastal and open-ocean upwelling areas and
oligotrophic gyres (Fig. 1). We first address these
regional differences in the context of particle fluxes
and diatom species composition; and then we focus on the seasonal pattern derived from each trap
site. To date, interannual differences (1989 vs. 1991)
in diatom fluxes and assemblages can be addressed
for two sites in the eastern equatorial Atlantic only,
but longer-term studies are in progress. Finally, we
extrapolate the trap results to the surface sediments
underlying the traps in order to assess the accuracy
with which the fossil record reproduces the originalliving assemblage.
Regional Differences
To address regional variability we first compared
the average annual total, biogenic opal and diatom
flux values for each mooring site (Fig. I). Although
the mean annual values for the different regions
were based on different sampling periods (Table 1),
and may therefore not be directly comparable, a
367
Basin and S-Guinea Basin; in the less productive
western equatorial Atlantic around 25°W; and at
Walvis Ridge within the Benguela upwelling system. Finally, diatom assemblages in the traps
are compared to assemblages in the underlying
sediments, and the implications of seasonal and
preservational biases are discussed.
Methods
We used the classical cone-shaped traps with 0.5
m 2 opening and 13/20 sampling cups (MARK VI
and Kiel SMT 230/234). Collection cups were poisoned with HgCI 2 before deployment, and NaCI
was added to reach a final salinity of 38-400/00.
Samples were poisoned again after recovery with
HgCI2 and stored at 4° C. The splitting procedure
and chemical analyses of the < I mm fraction
(which clearly dominated particle flux) were carried out at Bremen University. All mooring sites
were deployed at comparable depths (between 600
and 1000 m depth, Table 1) except for the Cape
Blanc mooring (CB 1) which was deployed at about
2000 m depth. Here, the original surface signal may
have been distorted, and flux values at depth may
be the combined result of degradation in the water
column (C.,g fluxes decrease with depth) and additional supply (total, opal, lithogenic, and carbonate fluxes increase with depth) by lateral advection
from the shelf and slope and resuspension (Fischer
and Wefer 1996).
Since most trapping experiments extended over
351-381 days (Table I), mean values were extrapolated to 365 days (one year) except for the 1991
N-S transect (EAI-5) where sampling duration was
230 days. For deployment data see Table 1.
Throughout the text the seasons refer to the Northern Hemisphere (boreal seasons) except for the
Walvis Ridge site where austral seasons are used.
For this study 1/64,1125 and 1/125 splits ofthe
original samples were used. They were rinsed with
distilled water and prepared according to methodology of Lange et al. (1994). Diatom analysis was
carried out on permanent slides (Mountex mounting medium) of acid-cleaned material.
Qualitative and quantitative analyses were done
at x 400 and x 1000 magnifications using a ZeissPhotomicroscope III or Zeiss-Axioscope with
phase-contrast illumination. Several traverses
across the cover-slip were examined, depending on
abundances. Each individual was identified to the
lowest taxonomic level possible. The resulting
counts yielded estimates of daily fluxes of diatoms
valves per m· 2 d· 1 calculated according to Sancetta
and Calvert (1988), as well as relative abundances
of individual taxa. Counting of replicate slides indicated thatthe error for the flux estimates is <15%.
Surface sediment samples were collected with
a box corer of 50 x 50 cm 2 surface area, and were
taken from the uppermost 0.5-1 cm of the sediment
surface. Box core samples were prepared after the
methods of Schrader and Gersonde (1978).
Detailed descriptions ofthe investigation sites,
oceanographic and atmospheric settings are presented in Wefer and Fischer (1993); Lange et al.
(1994,1998); Fischer et al. (1996), and Treppke et
al.(1996b).
Results and Discussion
Although our database is constrained by one-year
records (Table 1), one ocean (Atlantic) and one
group of microorganisms (diatoms), tlle data cover
a large range of primary production values, including coastal and open-ocean upwelling areas and
oligotrophic gyres (Fig. 1). We first address these
regional differences in the context of particle fluxes
and diatom species composition; and then we focus on the seasonal pattern derived from each trap
site. To date, interannual differences (1989 vs. 1991)
in diatom fluxes and assemblages can be addressed
for two sites in the eastern equatorial Atlantic only,
but longer-term studies are in progress. Finally, we
extrapolate the trap results to the surface sediments
underlying the traps in order to assess the accuracy
with which the fossil record reproduces the originalliving assemblage.
Regional Differences
To address regional variability we first compared
the average annual total, biogenic opal and diatom
flux values for each mooring site (Fig. I). Although
the mean annual values for the different regions
were based on different sampling periods (Table 1),
and may therefore not be directly comparable, a