366
Romeroetal.
Introduction
Diatoms are unicellular, eukaryotic, photosynthetic
microorganims ranging from ca. 2 /lm to ca. 2 mm
in size. They are distributed in almost all waters;
they are abundant in the phytoplankton and
phytobenthos of marine and fresh waters, at alllatitudes (Round et al 1990; Hasle and Syvertsen
1996). About 1400-1800 species are marine
planktic (Hasle and Syvertsen 1996), and are found
in the upper 200 m of the ocean. Diatoms have a
highly differentiated cell wall, which is impregnated
with opaline silica, so that their growth is subject to
the availability of silicon (orthosilicic acid, Si(OH1)
in the water (Round et al. 1990). The siliceous exoskeleton is variably resistant to decay. Diatom
losses in settling through great depths are very uneven, and in a number of cases tens and hundreds
of times fewer valves reach the surface sediment
layer compared to their amounts in suspension in
surface waters (see Lisitzin 1971, for a review). Of
the siliceous microorganisms, diatoms are of great
importance in modem biogenic silica production
(Treguer et al. 1995), followed by radiolarians, and
by silicoflagellates (Lisitzin 1971; Takahashi et al.
1990).
A substantial part of the ocean's primary productivity is provided by diatoms (Treguer et al.
1995). In general, they are the dominant primary
producers in temperate and cold areas, and are very
abundant in the recently upwelled waters of eastern boundary currents and in diverging surface
currents where nutrients are brought to the surface.
On an annual basis, the relative contribution of diatom to the primary productivity is highly variable.
Nelson et al. (1995) and Treguer et al. (1995) proposed upper limits of35% on the contribution of
diatoms in oligotrophic areas and 75% in coastal
zones and other nutrient-rich systems. Regardless
of the areas, the general trend is that the relative
abundance of diatoms in the phytoplankton increases as primary productivity does (Raymont
1980). As a general statement we may say that
diatoms are the dominant primary producers in a
number of oceanographic settings that offer both
the high nutrient and turbulence conditions required
by diatoms (e.g. coastal upwelling areas, equatorial divergences, ice-edges, river plumes; see
Nelson et al. 1995; Ragueneau eta!. subm). In contrast, small, non-siliceous pico- and nanoplankton
are of great importance to the total productivity in
oligotrophic regions (see Treguer et a!. 1995, and
references therein).
There seems to be no simple latitudinal pattern
for the annual organic carbon to opal flux ratio.
In areas with high biogenic opal flux such as
the Bransfield Strait and in some coastal upwelling systems with moderate opal fluxes (e.g. off
Namibia), a linear relationship between opal and organic carbon fluxes is seen (De Master et al. 1992;
Fischer and Wefer 1996; Ragueneau et al. subm).
An exponential relationship is observed in the highly
productive belt in the Antarctic; while in general,
tropical and subtropical oceans are characterized
by a non-linear relationship between daily organic
and opal fluxes (Ragueneau et al. subm).
The fate of the biogenic silica produced in the
euphotic layer is governed by the competition between recycling in and export from the upper surface waters. Globally, at least 50% of the silica
produced by diatoms in the euphotic zone dissolves
in the upper 100 m (Nelson et a!. 1995). According
to Bishop et al. (1977), in the equatorial Atlantic the
abundance of diatoms decreases by about 86%
from 50 m to 400 m. At a depth of 500 m only 5%
of the original opal remains, and at 1000 m only 1 %
remains (Treguer et al. 1995). Several authors (e.g.
Berger 1976; Takahashi 1986) have reported that
silicate dissolution below a water depth of 1000 m
is minimal. Finally, the portion of silica that escapes
dissolution, either in the surface waters or in the
water column, settles downward, ultimately reaching the sediment (see Fig. I in Treguer et a!. 1995).
In this paper, we intent to analyze the value of
diatoms as a proxy of export production from the
surface waters by using our equatorial and tropical
Atlantic datasets. Polar and subpolar regions are
not included here. We compare mean annual values and temporal flux variations from locations (Fig.
I, Table 1) including the eastern tropical Atlantic,
off Cape Blanc in the Mauretanian upwelling zone,
and south of Cape Verde within the Guinea Dome;
along a N-S transect, which crosses the equatorial
high-surface chlorophyll band into the transition
zone to the oligotrophic subtropical gyre, in the eastem equatorial Atlantic around 1 OOW in the N-Guinea
Romeroetal.
Introduction
Diatoms are unicellular, eukaryotic, photosynthetic
microorganims ranging from ca. 2 /lm to ca. 2 mm
in size. They are distributed in almost all waters;
they are abundant in the phytoplankton and
phytobenthos of marine and fresh waters, at alllatitudes (Round et al 1990; Hasle and Syvertsen
1996). About 1400-1800 species are marine
planktic (Hasle and Syvertsen 1996), and are found
in the upper 200 m of the ocean. Diatoms have a
highly differentiated cell wall, which is impregnated
with opaline silica, so that their growth is subject to
the availability of silicon (orthosilicic acid, Si(OH1)
in the water (Round et al. 1990). The siliceous exoskeleton is variably resistant to decay. Diatom
losses in settling through great depths are very uneven, and in a number of cases tens and hundreds
of times fewer valves reach the surface sediment
layer compared to their amounts in suspension in
surface waters (see Lisitzin 1971, for a review). Of
the siliceous microorganisms, diatoms are of great
importance in modem biogenic silica production
(Treguer et al. 1995), followed by radiolarians, and
by silicoflagellates (Lisitzin 1971; Takahashi et al.
1990).
A substantial part of the ocean's primary productivity is provided by diatoms (Treguer et al.
1995). In general, they are the dominant primary
producers in temperate and cold areas, and are very
abundant in the recently upwelled waters of eastern boundary currents and in diverging surface
currents where nutrients are brought to the surface.
On an annual basis, the relative contribution of diatom to the primary productivity is highly variable.
Nelson et al. (1995) and Treguer et al. (1995) proposed upper limits of35% on the contribution of
diatoms in oligotrophic areas and 75% in coastal
zones and other nutrient-rich systems. Regardless
of the areas, the general trend is that the relative
abundance of diatoms in the phytoplankton increases as primary productivity does (Raymont
1980). As a general statement we may say that
diatoms are the dominant primary producers in a
number of oceanographic settings that offer both
the high nutrient and turbulence conditions required
by diatoms (e.g. coastal upwelling areas, equatorial divergences, ice-edges, river plumes; see
Nelson et al. 1995; Ragueneau eta!. subm). In contrast, small, non-siliceous pico- and nanoplankton
are of great importance to the total productivity in
oligotrophic regions (see Treguer et a!. 1995, and
references therein).
There seems to be no simple latitudinal pattern
for the annual organic carbon to opal flux ratio.
In areas with high biogenic opal flux such as
the Bransfield Strait and in some coastal upwelling systems with moderate opal fluxes (e.g. off
Namibia), a linear relationship between opal and organic carbon fluxes is seen (De Master et al. 1992;
Fischer and Wefer 1996; Ragueneau et al. subm).
An exponential relationship is observed in the highly
productive belt in the Antarctic; while in general,
tropical and subtropical oceans are characterized
by a non-linear relationship between daily organic
and opal fluxes (Ragueneau et al. subm).
The fate of the biogenic silica produced in the
euphotic layer is governed by the competition between recycling in and export from the upper surface waters. Globally, at least 50% of the silica
produced by diatoms in the euphotic zone dissolves
in the upper 100 m (Nelson et a!. 1995). According
to Bishop et al. (1977), in the equatorial Atlantic the
abundance of diatoms decreases by about 86%
from 50 m to 400 m. At a depth of 500 m only 5%
of the original opal remains, and at 1000 m only 1 %
remains (Treguer et al. 1995). Several authors (e.g.
Berger 1976; Takahashi 1986) have reported that
silicate dissolution below a water depth of 1000 m
is minimal. Finally, the portion of silica that escapes
dissolution, either in the surface waters or in the
water column, settles downward, ultimately reaching the sediment (see Fig. I in Treguer et a!. 1995).
In this paper, we intent to analyze the value of
diatoms as a proxy of export production from the
surface waters by using our equatorial and tropical
Atlantic datasets. Polar and subpolar regions are
not included here. We compare mean annual values and temporal flux variations from locations (Fig.
I, Table 1) including the eastern tropical Atlantic,
off Cape Blanc in the Mauretanian upwelling zone,
and south of Cape Verde within the Guinea Dome;
along a N-S transect, which crosses the equatorial
high-surface chlorophyll band into the transition
zone to the oligotrophic subtropical gyre, in the eastem equatorial Atlantic around 1 OOW in the N-Guinea