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Interannual variability (1988–1991) of siliceous phytoplankton fluxes off northwest Africa O. E. ROMERO1*, C. B. LANGE2,3 AND G. WEFER1 1DEPARTMENT OF GEOSCIENCES, UNIVERSITÄT BREMEN, PO BOX
CILMAN DRIVE, LA JOLLA, CA ‒, USA
, BREMEN, GERMANY AND 2SCRIPPS INSTITUTION OF OCEANOGRAPHY,
3PRESENT ADDRESS: FACULTAD DE CIENCIAS NATURALES Y OCEANOGRÁFICAS, DEPARTAMENTO DE OCEANOGRAFÍA, UNIVERSIDAD DE CONCEPCIÓN,
CASILLA
-C, CONCEPCIÓN , CHILE
*CORRESPONDING AUTHOR:
[email protected]
Four years of observations (1988–1991) of downward fluxes of diatoms and silicoflagellates at a trap site off Cape Blanc (ca. 20°N, 20°W), northwest Africa, are presented. Significant variations in flux and species composition were observed as well as a marked drop in the export of biogenic opal (and diatoms) from 1988 to 1989; fluxes remained low thereafter. We hypothesize that this diminution might be related to decreased coastal upwelling intensity and offshore spreading of the typical chlorophyll filament, and/or a lesser silicate content of upwelling waters off Cape Blanc. In addition, the more seaward positioning of the mooring may have influenced the fluxes. At all times, diatoms were the most prominent contributors to the biogenic opal flux, and diatom fluxes closely paralleled total mass flux fluctuations. Although species composition varied seasonally, no significant qualitative variations were observed from year to year. In general, the dominance of neritic diatoms, such as Thalassionema nitzschioides var. nitzschioides, resting spores of Chaetoceros and Cyclotella litoralis, reflected the continuous offshore influence of coastal upwelling at the Cape Blanc trap site, with stronger intensity in spring/summer. In contrast, the occurrence of pelagic diatoms (e.g. Nitzschia bicapitata, N. interruptestriata, T. nitzschioides var. parva and Fragilariopsis doliolus), and high silicoflagellate fluxes (mainly Dictyocha messanensis) were linked to inshore transport of oceanic waters, generally in winter. With the exception of some fragile, pelagic diatoms, dominant species found in the settled material also occurred in the underlying sediments, suggesting that diatom thanatocoenosis downcore (Organisms preserved from the top to the bottom in sediment core) can be used as a reliable indicator of the intensity and persistence of the offshore spreading of coastal upwelling.
I N T RO D U C T I O N Recent studies of coastal upwelling systems show that these include plumes and filaments extending offshore up to hundreds of kilometres from the adjacent coast off northwest Africa. Prominent filaments are observed off Cape Ghir (31°N), Cape Juby (28°N) and Cape Blanc (21°N) (Barton, 1998). The latter is the site of a ‘giant filament’ which extends up to 450–600 km offshore (Gabric et al., 1993); it is present for most of the year with substantial intra- and interannual variations (Bricaud et al., 1987; Van Camp et al., 1991). The large offshore transport represents a potential mechanism for exporting cool, nutrient-rich water from the coastal region, and is thought
© Oxford University Press 2002
to be partly responsible for both enhanced offshore primary production (Gabric et al., 1996), and the occurrence of coastal phytoplankton found several hundred kilometres offshore (Chavez et al., 1991; Barton, 1998; Lange et al., 1998; Romero et al., 1999a). As a result of the fairly steady trade winds, upwelling off Cape Blanc occurs throughout the year (Schemainda et al., 1975), with periods of stronger intensity in spring, early summer and autumn (Mittelstaedt, 1991; Van Camp et al., 1991; Barton, 1998). The source waters for the upwelling are either salty and relatively nutrient-poor North Atlantic Central Water, north of about 23°N, or less-saline and nutrient-rich South Atlantic Central Water, south of 21°N. Thus, the upwelling waters near Cape Blanc vary
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in nutrient concentration depending on their origin, which is most likely the reason for the corresponding regional differences in primary production (Fütterer, 1983). Because of the relative paucity of data for the waters off Cape Blanc, interannual variations are hard to determine (Barton, 1998). Recent observations prove that year-toyear variations are very strong (Nykjær and Van Camp, 1994; Fischer et al., 1996, 1999; Ratmeyer et al., 1999; Bory et al., 2001; Müller and Fischer, 2001; Thomas et al., 2001; Romero et al., 2002). Whether they might be related to global-scale climatic variations or to a natural level of basin-wide atmospheric and/or oceanic variability (Barton, 1998) is an issue in need of further investigation. With the goal of providing information on export production of particulates and fluxes to the sea floor, the area off Cape Blanc has been intensively studied with sediment traps over the last decade (Wefer and Fischer, 1993; Kalberer et al., 1993; Boltovskoy et al., 1996; Fischer and Wefer, 1996; Fischer et al., 1996, 1999, 2000; Jickells et al., 1996; Lange et al., 1998; Ratmeyer et al., 1999; Romero et
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al., 1999a, b, 2002; Bory et al., 2001; Müller and Fischer, 2001). In this paper, we present new information on intraand interannual patterns of siliceous primary producers off Cape Blanc for a 4 year sampling period (1988–1991), as a continuation of observations initiated by Lange and Romero and their co-workers (Lange et al., 1998; Romero et al., 1999a, b, 2002). In addition, the diatom and silicoflagellate assemblages in the traps are compared to those preserved in surface sediments, to aid interpretations of paleoceanographic signals downcore.
METHOD A total of four moorings were deployed off Cape Blanc (ca. 20°N, 20°W), in the Canary current (Figure 1). Details on sampling intervals and trap depths are given in Table I. The temporal resolution of trap samples is ca. 4 weeks between March 1988 and March 1989 (CB1), ca. 3 weeks between March 1989 and April 1991 (CB2 and 3), and 10 days from May to November 1991 (CB4). For CB1 and CB2, the classical cone-shaped traps
Fig. 1. Location of mooring site CB (Cape Blanc; triangle), off Mauretania, northwest Africa, and representation of the main subsurface currents (CC, Canary Current; NEC, North Equatorial Current). The shadowed areas represent offshore spreading of the two most prominent chlorophyll filaments off northwest Africa: Cape Ghir [~31°N, modified after (Van Camp et al., 1991)] and Cape Blanc [~20°N, modified after (Gabric et al., 1993, 1996)].
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Mark V and Mark VI were used. Kiel SMT 230/23 traps were used for CB3 and CB4. Collection cups were poisoned with HgCl2 before deployment, and NaCl was added to reach a final salinity of 40‰. Samples were poisoned again after recovery with HgCl2 and stored at 4°C. The splitting procedure and chemical analyses of the < 1 mm fraction were carried out at Bremen University (Fischer and Wefer, 1991). Biogenic opal was determined using a sequential leaching technique with modifications (DeMaster, 1981; Müller and Schneider, 1993). For this study 1/16 and 1/64 splits of the original samples were used. They were rinsed with distilled water and prepared according to methodology described in Lange et al. (Lange et al., 1994). Diatom analysis was carried out on permanent slides (Mountex mounting medium) of acid-cleaned material. Qualitative and quantitative analyses were performed at 400 and 1000 magnifications using a Zeiss-Photomicroscope III or a Zeiss-Axioscope with phase-contrast illumination. Several traverses across the cover slip were examined, depending on abundance. Each individual was identified to the lowest taxonomic level possible. The resulting counts yielded estimates of daily fluxes of diatom valves per m2 per day (m–2 day–1) calculated according to Sancetta and Calvert (Sancetta and Calvert, 1988), as well as relative abundances of individual taxa. Counting of replicate slides indicated that the analytical error for the flux estimates is ≤ 15%. To compare the trap data in terms of interannual variability (Figure 2), we organized them on an approximately annual basis. The sampling intervals of each mooring are not coincident with the beginning and end of each calendar year (Table I), and thus the year we considered for the interannual comparison is not strictly one of 365 days. For example, year 1988 starts on March 22 (the beginning of the sampling period for CB1) and ends on December 17
(the last sample taken in 1988); in 1991, sampling ended on November 19.
R E S U LT S Interannual flux variations Although comparison of annual fluxes may be somehow biased by differences in trap depths (Table I), a striking feature emerges: annual fluxes of siliceous phytoplankton and bulk components, except organic carbon, were highest in 1988 and lowest in 1991 (Figures 2 and 3). However, differences in the interannual pattern for each component are obvious. Diatom and biogenic opal fluxes decreased abruptly from 1988 to 1989; fluxes remained rather constant through 1990, and decreased again in 1991. In contrast, the strongest drop in total particle and silicoflagellate fluxes was observed between 1990 and 1991. Organic carbon showed a peculiar pattern, with an abrupt decrease in flux values between 1988 and 1989, resembling that of biogenic opal and diatoms, and an increase in 1990 and especially in 1991 which is not mirrored by the other parameters measured (Fischer et al., 1996).
Seasonal flux variations From March 1988 to November 1991, diatom fluxes were better correlated with total particle and biogenic opal fluxes (correlation factors r = 0.84 and 0.92) than were silicoflagellates (r = 0.67 and 0.56). Independent of season and year, diatoms were the most prominent contributors to the biogenic opal flux. Daily diatom fluxes always remained one order of magnitude higher than those of silicoflagellates (Figure 3). Radiolaria counts are available for 1988 only (Boltovskoy et al., 1996). Daily radiolaria fluxes were lower than those of the other two siliceous groups and were in the order of 104 skeletons m–2 day–1.
Table I: Cape Blanc sediment traps: location, deployment depths, sampling duration and intervals Water
Trap
Sampling
Samples
Mooring
Trap type (opening)
Position
depth (m)
depth (m)
duration
days
CB1
Mark V
20°45.3N
3646
2195
Mar 22 88–
13 27
(0.5 m2)
19°44.5W
Mark VI
21°08.7N
(1.17 m2)
20°41.2W
Kiel SMT 230
21°08.3N
(0.5 m2)
20°40.3W
Kiel SMT 230
21°08.7N
(0.5 m2)
20°41.2W
CB2
CB3
CB4
Mar 08 89 4092
3502
Mar 15 89–
22 17
Mar 24 90 4094
3557
Ap 29 90–
16 21.5
Ap 08 91 4108
3562
May 03 91– Nov 19 91
20 10
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Fig. 2. Annual flux patterns: Mean yearly fluxes of biogenic opal (g m–2 year–1), diatoms (valves 108 m–2 year–1), silicoflagellates (skeletons 107 m–2 year–1), total particles (g m–2 year–1), and organic carbon (g m–2 year–1) at the Cape Blanc site from 1988 through 1991 (see Table I for mooring position). Total particle, biogenic opal and organic carbon fluxes were taken from Fischer et al. (Fischer et al., 1996, 1999).
On a seasonal basis, increases of a moderate and/or strong nature in diatoms and silicoflagellates were recorded in (1) spring and summer 1988 (diatoms), (2) autumn 1988 through winter 1989 (both), (3) spring 1989 (silicoflagellates), (4) winter 1990 (both), (5) winter 1991 (silicoflagellates), and (6) spring–summer 1991 (both) (Figure 3). After the winter of 1990, the diatom flux remained below 106 valves m–2 day–1, and silicoflagellates never exceeded 2 105 skeletons m–2 day–1. Total particle flux ranged between 8.2 and 361 m–2 day–1 [Figure 3; (Fischer et al., 1996)]. The most obvious peaks were observed in early spring, early summer and autumn 1988, in late summer of 1989, and in winter and spring 1990. Biogenic opal flux followed a similar seasonal pattern; highest values were observed in spring and early summer 1988, and the second highest in autumn 1988, winter 1989 and winter 1990. Organic carbon flux, varying between 0.7 and ca. 24 mg m–2 day–1, peaked in almost every season, and showed higher correlation with biogenic opal and diatoms than with silicoflagellates (r = 0.73 and 0.64 versus 0.19, respectively). Two outstanding maxima were seen in the summer of 1988 and 1991.
Temporal succession of trapped diatom and silicoflagellate species assemblages Diatoms Diatom flux and species diversity, as expressed by the Shannon–Weaver Index (Shannon and Weaver, 1949) were negatively correlated (r = –0.80; Figure 4a). Low daily fluxes (0.5 105–9 105 valves m–2 day–1) corresponded with highly diversified assemblages, and vice versa. This inverse relationship was especially marked between spring 1988 and spring 1989 (r = –0.91), when flux seasonality was more pronounced. It is also evident that over the 4 years of the study diatom diversity was constantly higher in 1989–1991 than in 1988 (Figure 4a, Table II). A few species dominated the assemblage throughout the 4 year sampling period (Figure 4b). The flux was mainly composed of marine neritic (coastal areas) and pelagic (open ocean water) species (Table II). Main components of the neritic group were Thalassionema nitzschioides var. nitzschioides, resting spores of Chaetoceros spp. and Cyclotella litoralis. The pelagic group included Thalassionema
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Fig. 3. Seasonal flux patterns: Time-series fluxes at the Cape Blanc site from March 1988 to November 1991 (see Table I for mooring position). Diatom flux in valves 105 m–2 day–1 and silicoflagellate flux in skeletons 104 m–2 day–1. Horizontal stippled lines represent average daily flux for each year. Total particle, biogenic opal and organic fluxes in mg m–2 day–1 [from (Fischer et al., 1996, 1999)].
nitzschioides var. parva, Nitzschia interruptestriata, the N. bicapitata group (including N. bicapitata, N. bifurcata and N. braarudii), Fragilariopsis doliolus and Planktoniella sol, all of them representatives of warm, oligotrophic conditions. In addition, several tycopelagic epiphytic and benthic
diatoms, thriving in near-shore areas, were identified (mainly Actinoptychus vulgaris, Delphineis surirella and Biddulphia alternans). Their occurrence probably reflects littoral influence and transport from the very upper shelf to deeper waters by bottom currents and down-slope
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(a)
(b)
Fig. 4. (a) Diatom flux and Shannon–Weaver Diversity Index at the Cape Blanc site from March 1988 to November 1991. (b) Cumulative percentage of the most abundant neritic and pelagic diatom species or group of species, and the neritic : pelagic (N : P) ratio at the Cape Blanc site from March 1988 to November 1991.
movement. Freshwater diatoms and phytoliths, transported by wind from northern Africa, were constantly present in the trapped assemblage, although freshwater diatoms never exceeded 8% of the total assemblage (Lange et al., 1998; Romero et al., 1999a, b, 2002).
Despite strong interannual variations in the diatom flux, the overall specific composition of the assemblage showed little change between 1988 and 1991. Neritic diatoms were generally more abundant than pelagic ones, especially during 1988 (Figure 4b, Table II).
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Table II: Mean annual relative abundance (%) of dominant diatoms in sediment traps for the period 1988–1991 and in the underlying surface sediment [GeoB1121; data from (Lange et al., 1998)], neritic : pelagic (N : P) ratio and diversity Abundance (%)
Sediment traps
Sediment
Dominant diatom species or
1988a
1989
1990
1991
group of species
2195 mb
3502 m
3557 m
3562 m
3681 m
Neritic group
54.59
27.96
29.56
31.74
55.30
Thalassionema nitzschioides
31.05
13.20
13.18
17.71
12.3
var. nitzschioides Cyclotella litoralis
9.30
6.12
11.92
9.16
15.7
RS Chaetoceros spp.
9.67
6.87
3.15
3.78
14.4
Coscinodiscus thorii
2.58
0.14
0.11
0.09
0.9
Coscinodiscus asteromphalus
1.12
0.05
0.00
0.00
6.6
Actinocyclus octonarius
0.87
1.58
1.20
1.00
5.4
19.23
30.14
21.15
25.62
12.40
Pelagic group Nitzschia bicapitata group
4.82
7.06
3.71
7.29
0.1
Thalassionema nitzschioides var. parva
2.77
7.24
3.43
3.15
0.6
Nitzschia interruptestriata
2.98
5.99
4.21
3.39
0.5
Proboscia alata
3.10
0.27
0.39
2.81
0.1
Roperia tesselata
2.14
2.02
2.15
2.66
1.8
Fragilariopsis doliolus
1.60
4.39
4.41
3.24
3.9
Planktoniella sol
1.82
3.17
2.85
3.08
5.4
Ratio N : P
2.84
0.93
1.40
1.24
4.46
Littoral group
4.60
2.44
4.20
3.21
3.10
Actinoptychus vulgaris
2.23
1.36
3.08
1.74
0.6
Delphineis surirella
1.62
0.59
0.82
1.26
0.2
Biddulphia alternans
0.75
0.49
0.30
0.21
2.3
Freshwater diatoms
2.67
6.54
7.33
3.48
8.3
Total percentage
81.09
67.08
62.24
64.05
79.1
Average diversity
3.00
3.45
3.45
3.43
ayear; btrap
3.19
depth.
Thalassionema nitzschioides var. nitzschioides was the dominant species of the neritic group and the main contributor to the diatom flux. A slight shift in species composition within the neritic group was observed, with higher relative abundances of Chaetoceros resting spores in 1989 and of Cyclotella litoralis in 1990 (Figure 4b, Table II). Dominance in the pelagic diatom assemblage changed somewhat from year to year. In 1988, the N. bicapitata group and Proboscia alata co-dominated the pelagic association, followed by a co-dominance of T. nitzschioides var. parva and N. bicapitata in 1989. Fragilariopsis doliolus and N. interruptestriata contributed the most in 1990 (Table II). Significant variations occurred in the neritic : pelagic diatom ratio. In general, the neritic : pelagic ratio was
positively correlated with diatom flux (r = 0.73) and inversely correlated with diversity (r = –0.65; compare Figures 4a and b). Higher neritic : pelagic ratios are generally observed during periods of high flux and low diversity, in early summer and late autumn 1988, late summer 1989, winter 1990, summer–autumn 1990 and late summer 1991 (Figure 4b). A strong influence of pelagic diatoms was not observed until early 1989 lasting through mid-summer of the same year, and then again in winter 1991 (Figure 4b). Silicoflagellates Seasonal and year-to-year variations in the composition of the silicoflagellate flora were rather low, and only five species were identified in the traps. The pelagic,
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warm water species Dictyocha messanensis (Takahashi and Blackwelder, 1992) dominated the association at all times, and no intra- or inter-annual significant changes were observed.
DISCUSSION Temporal variations of siliceous phytoplankton fluxes As a result of the strong along-shore component of the trade winds, coastal upwelling off Cape Blanc occurs almost year-round (Schemainda et al., 1975). Short-term intensifications and relaxations of the wind stress on a scale of days to weeks modify the low seasonality of the upwelling intensity (Nykjær and Van Camp, 1994). The long, extended boundary of the chlorophyll filament, separating coastal upwellings from oceanic waters, represents a great opportunity for mixing between the two water masses (Barton, 1998). Seasonal and year-to-year variability is reflected in the flux of diatoms and silicoflagellates, and in the diatom species composition. Strong contributions of neritic diatoms in the offshore realm are linked to seaward transport of coastal waters and biota mainly in spring–summer. Higher fluxes of silicoflagellates and the dominance of pelagic diatoms, mainly in winter, reflect the intermingling of warm open-ocean, more nutrient-poor waters (Lange et al., 1998; Romero et al., 1999a), in coincidence with periods of wind relaxation (Gabric et al., 1993). Well-known interannual fluctuations in the upwelling intensity off Cape Blanc are also reflected in the productivity of the surface waters (Van Camp et al., 1991; Barton, 1998). An example of this variability is shown by strong interannual variations of the diatom flux and qualitative composition of the diatom flora collected at the Cape Blanc site. We hypothesize that the decrease in both diatom flux and the neritic : pelagic ratio after 1988 may be a consequence of three non-mutually exclusive hypotheses: first, a reduction in both coastal upwelling intensity and offshore spreading of the chlorophyll filament; second, a change in the quality of water reaching the study site, with lowered silicate contents which may have reduced diatom production off Cape Blanc, especially during 1991; and/or third, a shift in the deployment location and trap depth to more offshore, less productive and deeper waters. The first hypothesis is supported by the lessened diatom flux and contribution of neritic diatoms from 1989 to 1991. According to satellite observations, the Cape Blanc mooring occasionally lies within the chlorophyll filament of cool, pigment-rich waters spreading at least 450 km seaward ca. 24° and 20°N off nortwest Africa (Lange et al., 1998; Romero et al., 1999a). Although colour satellite
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measurements for the period in study are not available, earlier observations show that seaward extension and the pigment concentration of the filament strongly varies on both seasonal and interannual bases (Gabric et al., 1993; Thomas et al., 2001). In contrast to the first hypothesis is the assumption presented by Fischer et al., who suggested an increase in coastal upwelling during 1991 (Fischer et al., 1999). These authors based their conclusion on temperature estimates derived from the pteropod Limacina inflata and the foraminiferan Globigerinoides ruber for the same time interval studied in the present work, which yielded lowest annual mean estimates for 1991. The small temperature reduction can also be seen in satellite Sea Surface Temperature (SST) as well as in alkenone unsaturation maxima gained at the Cape Blanc site in 1991 (Müller and Fischer, 2001). However, this temperature decrease caused no increase either in the siliceous phytoplankton flux or in the total flux of particulates (Figure 2). Instead, the change in SST may have caused variations in the composition of the phytoplankton community (see discussion below). At the mesotrophic EUMELI trap site, located south of our Cape Blanc deployment, Bory et al. attributed interannual differences in the magnitude of the particle peaks between 1991 and 1992 to the pre-eminence of the calcium carbonate fraction (Bory et al., 2001). As for the second hypothesis, it has been demonstrated that diatoms play a decisive role in the phytoplankton community of Eastern Boundary Current Systems, such as that off Cape Blanc (Blasco et al., 1981). Hence, lowered silicate content in surface and subsurface waters could limit the total production and the export of siliceous phytoplankton to the deep sea (Dugdale et al., 1995; Berger and Lange, 1998). Though silicate measurements at or close to the Cape Blanc trap site for the time period presented are not available, the sharp increase in the flux of organic carbon in 1991, and the enhancement in the relative contribution of calcium carbonate to the total mass flux from 44% in 1988 to 52% in 1991 (Fischer et al., 1996) indicate a change in the composition of the primary producer community off Cape Blanc. Based on unusually high alkenone fluxes at the Cape Blanc site, Müller and Fischer also speculated about a possible enhanced contribution of coccolithophorids in 1991 (Müller and Fischer, 2001). In addition, analysis of the raw, untreated trap samples revealed higher relative contributions of cyanobacteria during 1991 than in earlier years; these organisms may have been responsible in part for the observed organic carbon peak. Additionally, the observed decrease in diatom flux and increase in the relative contribution of oceanic diatoms after 1988 may be related to a change in deployment position and trap depth (the third hypothesis; Table I). Moorings CB2–4 were located further north and ca. 60
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nautical miles offshore than mooring CB1, hence closer to more oligotrophic and less chlorophyll filament-influenced waters (Figure 1). Higher productivity and diatom levels close to the coast and lower values in offshore waters off Cape Blanc (Berger, 1989; Antoine et al., 1996; Longhurst et al., 1995) are probably linked to higher concentrations of silicate in the source waters of the coastal undercurrent compared to the reduced silicate concentration of the more shallow offshore thermocline (Dugdale et al., 1995). Below the surface waters between ~ 150 to ~ 600 m south of the Cape Blanc site, the unstable Cape Verde Front occurs. This front builds where the North Atlantic Central Water meets the less saline, nutrient-richer South Atlantic Central Water (Barton and Hughes, 1982). The boundary between both water masses is convoluted, variable in position and characterized by intense mixing and interleaving processes [(Barton, 1998) and references therein]. Year-to-year perturbations of the location of the Cape Verde Front might influence the nutrient availability off Cape Blanc. Thus, a stronger influence of the lower silicate-containing North Atlantic Central Water on the Cape Blanc trap site might have resulted in a lowered diatom flux. Loss of sinking material because of dissolution may have been enhanced by a change in trap depth to deeper waters for CB2–4 (Table I). At least 50% of the biogenic silica produced by the siliceous phytoplankton in the euphotic zone dissolves in the upper 100 m (Nelson et al., 1995). Results gained at a trap site in an Eastern Boundary Current System in the southeast Pacific support our assumption. Strong differences in the diatom fluxes between two traps deployed at ca. 1500 and 2600 m depth off northern Chile, without any significant change in the diatom assemblage composition, were attributed to dissolution of valves in the deep-water column (Romero et al., 2001). A comparison of our study with that of Jickells et al. ( Jickells et al., 1996) for a trap site just south (ca. 19°N, 20°W) of our Cape Blanc mooring shows (1) a similar per cent contribution of biogenic components to the mass
flux, and (2) a striking difference in the average daily mass flux for the overlapping period October 1990 to June 1991 (Table III). The latter is most noteworthy because the reverse would be expected. The latitudinal migration of the chlorophyll filament would affect both sites in spring and summer, but only our site during autumn and winter [see Figure 1 in (Lange et al., 1998)], and therefore higher fluxes would be expected at 21°N than at 19°N. The specific composition of the diatom assemblage reported by Jickells and co-workers is puzzling. Their data show a dominance of the warm-water species Thalassiosira lineata (cited as Coscinodiscus lineatus), and unidentified pennate diatoms, while T. nitzschioides var. nitzschioides, the dominant diatom species throughout the 4 year sampling period at the Cape Blanc site, is not even mentioned at 19°N [see Figure 2 in ( Jickells et al., 1996)]. Their composition would point rather to open-ocean conditions and subdued influence of the chlorophyll filament.
Traps versus sediments Lange et al. compared the diatom composition of trap CB1 (1988) with that of the underlying surface sediment (GeoB1121) (Lange et al., 1998). They concluded that the preserved assemblage resembles the flora trapped in spring and early autumn, dominated by robust to moderately silicified neritic species, mostly associated with periods of high diatom flux. In contrast, the abundance in the sediment of several pelagic diatom species, such as the N. bicapitata group, scarcely exceeded 1%, and this trap–sediment discrepancy was attributed to the delicate structure of their frustules. For other pelagic species with moderately silicified valves, such as F. doliolus and P. sol, abundances in the traps were similar to those found in the surface sediments (Table II), and their occurrence in the preserved record may be used as an indication of inshore incursions of open-ocean waters into the coastal area of Cape Blanc. When comparing the diatom composition of GeoB1121 surface sediments with that of the 4 year trap experiment, we observed that the sediment assemblage resembled much more closely that of 1988 than that of
Table III: Comparison of average daily mass fluxes and percentage composition at our Cape Blanc site with data of Jickells et al. (Jickells et al., 1996) at ca. 19°00N, 20°11W for the period October 1990 to June 1991
Our study Jickells et al. (1996)
Total particle
Biogenic
Calcium
Organic
flux
opal
carbonate
carbon
Lithogenic
(mg m–2 day–1)
(%)
(%)
(%)
(%)
99.4
5.5
41.0
6.5
26.5
245.0
6.9
45.0
8.7
30.7
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1989–1991 (Table II). Apparently, an overwhelming neritic signal has been the rule throughout the late Quaternary. Abrantes’ study of core 16030–1 (21°14N, 18°03W) reveals that the sediments younger than 80 000 years are dominated by Chaetoceros spores, accompanied by T. nitzschioides var. nitzschioides, representing high coastal upwelling productivity, especially during glacial times (Abrantes, 1991). Diatom concentration in the surface sediments underlying the trap site [8.84 105 valves g–1 sediment; (Lange et al., 1998, Romero et al., 1999a)] is similar to that given by Stabell for young sediments of ODP Site 657 [2.0 105 valves g–1 sediment; 21°20N, 20°57W (Stabell, 1989)], but lower than concentrations at ODP site 658 [ca. 18 105 valves g–1 sediment; 20°45N, 18°35W (Stabell, 1989)]. This difference is related to the shallower depth of site 658, which is located on the shelf underlying more productive coastal waters (Berger, 1989; Longhurst et al., 1995; Antoine et al., 1996), and thus reflects higher diatom production than under the more pelagic conditions of our site Cape Blanc and ODP 657. It is noticeable that the relative loss of silicoflagellate skeletons is markedly higher than that of diatom valves (Table IV). While the diatom accumulation rate in the surface sediment remains in the same order of magnitude as the average yearly fluxes calculated from the traps, the accumulation rate of silicoflagellates is reduced by one order of magnitude. This difference in the preservation of diatoms and silicoflagellates off Cape Blanc may be related to a preferential dissolution effect on the silicoflagellate skeletons (Hurd, 1983). However, Dictyocha messanensis is the main contributor to both the trapped and the
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sediment assemblage, a species with rather robust silica rods. Although we are unable to explain this difference, this degree of loss has also been observed in other areas of the world (Romero et al., 1999a, 2001).
Outlook Our 4 year observations confirm that seasonal and interannual variations in export production of siliceous organisms are strong in the area off Cape Blanc. They suggest that these fluctuations are a direct result of shortand long-term variability in the interplay between nutrient-rich, coastal waters and oligotrophic, offshore conditions. Sinking particles are still being collected with sediment traps in the area off Cape Blanc and research on the flux of particles is proceeding. In addition, satellite observations (SeaWIFS data) of surface chlorophyll and sea surface temperatures have re-started since September 1997 (no data exist between August 1986 and August 1997). Taken jointly, these observations will help to link oceanographic conditions with biological response at seasonal and interannual time scales, and also add valuable information for the understanding and interpretation of the paleoceanographic signal preserved in the sediments.
AC K N O W L E D G E M E N T S
Diatoms
Silicoflagellates
The authors would like to thank G. Fischer for fruitful discussions and suggestions which helped to improve the final draft of this manuscript, and F. Reid for correcting the final draft. The final version greatly benefited from reviews by two anonymous referees and Dr Jenkinson ( JPR Editor). The competent assistance of the officers and crew of the research vessel F/S ‘Meteor’ in the recovery of the moorings and the retrieval of sediment traps is acknowledged. Financial support for this work was provided by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 261 at the Bremen University, Contribution no. 348). Minor funds were also provided by the Marine Life Research Group of Scripps Institution of Oceanography.
1988a
4.8 108
4.9 107
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1989
2.0
4.9
1990
2.1 108
5.2 107
1991
1.1 108
1.6 107
1.4 108
1.0 106
Table IV: Comparison between yearly fluxes of diatoms and silicoflagellates from sediment traps and accumulation rates in the surface sediment [GeoB 1121; data from (Lange et al., 1998)]
Yearly fluxes – Traps 108
107
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Received on May 29, 2000; accepted on June 5, 2002
