Downward Particle Fluxes Within Different Productivity

Deep-Sea Research I 48 (2001) 2251–2282

Downward particle fluxes within different productivity regimes offthe Mauritanian upwelling zone (EUMELI program) A. Borya,b,*, C. Jeandelc, N. Leblondd, A. Vangriesheime, A. Khripounoffe, L. Beaufortf, C. Rabouillea, E. Nicolasd, K. Tachikawag, H. Etcheberh, P. Buat-Menard! h a Laboratoire des Sciences du Climat et de l’Environnement, Domaine du CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France b Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, 61 Route 9W, Palisades, NY 10964-8000, USA c Laboratoire d’Etudes en Geophysique! et Oceano! graphie Spatiales, Observatoire Midi-Pyren! ees,! 14 Avenue E. Belin, 31400 Toulouse, France d Laboratoire de Physique et Chimie Marines, B.P. 08, 06238 Villefranche-sur-mer, France e Institut Franç ais de Recherche pour l’Exploitation de la Mer, B.P. 70, 29280 Plouzane,! France f Centre Europeen! de Recherche et d’Enseignement de Geosciences! de l’Environnement, Universite Aix-Marseille III, B.P. 80, Plateau Petit Arbois, 13545 Aix-en-Provence, France g University of Cambridge, Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ, UK h Universite! Bordeaux 1, Departement! de Geolo! gie et Oceano! graphie, Avenue des Facultes,! 33405 Talence, France Received 27 April 2000; received in revised form 20 October 2000; accepted 2 February 2001

Abstract

A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of 71–78% biogenic origin (i.e. consisting of CaCO3, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (188300N, 218000W) to the remote, open-ocean, oligotrophic site (218000N, 318000W). This decrease largely reflects the difference in total primary production between the two sites, from 260 to 110 g organic C mÀ2 yrÀ1. At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward

*Corresponding author. Fax: +1-845-365-8155. E-mail address: [email protected] (A. Bory).

0967-0637/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 637(01)00010-3 2252 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Particulate ﬂux; Sediment traps; Particulate organic carbon; Particle settling; Current meter data; Coastal upwelling; North Atlantic Ocean; Eastern subtropical; OﬀMauritania

1. Introduction

Sediment trap studies have provided over the last decades invaluable clues for our under- standing of particulate matter transport processes and fluxes in the ocean water column on days- to-years time scales (see Ittekkot et al., 1996, for review). Such studies have permitted, in particular, a direct investigation of the ‘biological pump’ (Berger et al., 1989), that is, the transfer of atmospheric CO2 to the deep ocean resulting from biological activity in the surface water and the subsequent downward export of biogenic particulate matter. It has been shown that settling biogenic particles rapidly transfer the primary production signal to depth, yielding a close temporal coupling between surface and deep oceans (e.g., Asper et al., 1992). Quantitatively, the degree of coupling, however, shows large differences throughout the different provinces of the world ocean (e.g., Bishop, 1989; Berger and Wefer, 1990; Longhurst, 1995; Lampitt and Antia, 1997), and no simple relationship is observed between total primary production and export of biogenic particulate matter (organic carbon in particular; Boyd and Newton, 1999). If ocean total primary production, which can be estimated from remote sensing (Morel et al., 1996), is to be successfully interpreted in terms of carbon export within the ocean, then it is important to better constrain the transfer processes of the particulate matter depending on the productivity regime of the surface waters. The EUMELI program (EUtrophic, MEsotrophic and oLIgotrophic), which took place within the framework of the France-JGOFS program (Joint Ocean Global Flux Study), aimed at studying progressive changes in the biogeochemical processes due to the seaward dissipation of the Mauritanian upwelling and the associated decrease in biological activity from coastal regions to the subtropical gyre. Historical background and objectives of EUMELI are detailed in Morel (2000). Multidisciplinary studies have been carried out (see the special section of Deep-Sea Research Part I, Vol. 43 (8); Morel, 1996) at three sites located along a latitudinal transect representing decreasing primary productivity (eutrophic, hereinafter E; mesotrophic, hereinafter M; oligotrophic, hereinafter O). During this study, sediment traps were deployed at several depths in the water column for almost 2 yr (1991–1992) at the M and O sites. Different aspects of the particle fluxes, specifically 210Pb (Legeleux et al., 1996), Mn, rare earth elements and Nd isotopes (Tachikawa et al., 1997), biogenic barium (Legeleux and Reyss, 1996; Bory and Newton, 2000; Jeandel et al., 2000), anthropogenic lead (Hamelin et al., 1997; Alleman et al., 1999), and lithogenic fluxes (Bory and Newton, 2000), as well as deep particle fluxes in the O region (Khripounoffet al., 1998), have already been published. Here we present the mass, particulate organic carbon (hereinafter POC), particulate inorganic carbon (hereinafter PIC), coccolithophore, opal, and lithogenic downward fluxes obtained during the entire sediment-trap deployments at both sites, together with the relevant current meter data. A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2253

These data are discussed at and between the two sites, in order to document the relationship between surface-water productivity regime and: (1) the export of particulate matter, especially organic carbon, from surface waters; and (2) the transfer processes of the material through the water column. Current meter data also provide insights into the role of water-mass dynamics in the particle-ﬂux pattern in this region.

2. Regional setting and methods

Surface currents in the study region, located southeast of the North Atlantic subtropical gyre, are presented in Fig. 1 (see Bory and Newton, 2000, for details). The dominant feature along the

Fig. 1. Mooring locations (solid triangles), bathymetry, schematic near-surface circulation and total primary production levels (g organic C mÀ2 yrÀ1) at the three EUMELI sites. PP levels are those calculated by Morel (2000) using a standard model operated with chlorophyll concentrations extracted from the 12 ‘‘climatological monthly maps’’ derived from CZCS archives covering the 1978–1986 period (see also Morel, 1996,2000; Morel et al., 1996). Dashed line, approximate contour of equal PP (Auffret et al., 1992). Abbreviations are CC, Canary Current; NEC, North Equatorial Current; NECC, North Equatorial Counter Current; NASG, approximate center of the North Atlantic Subtropical Gyre. Arrows: solid line, all year round; dashed line, fall, winter and spring only; dotted line, summer and fall only. Below the surface waters are, successively, the unstable Cape Verde Frontal Zone (CVFZ, between 150 to 600 m), a major boundary where North Atlantic Central Water meets South Atlantic Central Water (Zenk et al., 1991; Pierre et al., 1993; Vangriesheim et al., 2000), another frontal zone (22–248N) between Mediterranean Water and Antarctic Intermediate Water (between 800 and 1000 m), and the North Atlantic Deep Water (1200 to 4000 m), itself overlying Antarctic Bottom Water. Geographical map was obtained from M. Weinelt (WWW page, hhttp:// www.aquarius.geomar.de/omc/i, 1999). 2254 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 northwest African coast is the upwelling of cool, nutrient-rich subsurface waters caused by the trade winds blowing toward the southwest alongshore (Mittelstaedt, 1991). Between 20 and 258N, the upwelling is fairly active all year round (Speth and Detlefsen, 1982). Although its main impact is confined to shelf-slope waters within about 50–60 km of the coast, its influence can be observed further offshore. As a result of the upwelling system, a strong east-to-west-decreasing gradient in the total primary production (hereinafter PP) of the surface waters is observed (see Fig. 1). The study region is situated underneath a major route of atmospheric mineral dust, which is trans- ported from the Saharan and Sahel regions across the tropical Atlantic (see Prospero, 1996a,b, for review). Sediment traps were located on the Cape Verde Terrace (site M: 188300N, 218000W, 3100 m water depth) and on the Cape Verde abyssal plateau (site O: 218000N, 318000W, 4600 m water depth) at distances of 400–500 and 1400 km, respectively, from the African coast, and about 300 and 700 km from the Cape Verde archipelago (Fig. 1). Settling particles were collected with multisample conical sediment-traps (see Bory and Newton, 2000, for details) moored at 250, 1000, and 2500 depths in the water column; at site M, an additional trap was deployed at 3000 m, that is about 200 m above bottom. Time-series samples were obtained between February 1991 and November 1992 (see appendix tables for details). Sampling intervals varied from 8 to 10 d and were synchronized at all depths and also between the M and O moorings. All moorings were equipped with Aanderaa RCM 7 and 8 current meters, which were located 10 m below each of the traps. Detailed mooring configurations are described elsewhere (Bournot et al., 1995). Sediment-trap sampling procedures were consistent with JGOFS (SCORE, 1990) and are fully detailed elsewhere (Leblond et al., 1995; Bory and Newton, 2000). Swimmers were identified and removed from the samples according to the criteria of Knauer and Asper (1989) and Michaels et al. (1990); this step was preceded by sieving through a 1 mm nylon mesh. Mixed and homogenized samples were divided into sub-samples with a peristaltic automatic splitter (Heussner et al., 1990). Mass flux was obtained by the weighing of several desiccated sub-samples. Sub-samples intended for organic carbon and opal analyses were rinsed with deionized water and centrifuged before being lyophilized. Organic carbon content was determined after carbonate removal (with 2 N HCl) by combustion and detection of liberated CO2 with a LECO element analyzer (see Legeleux, 1994, and Khripounoffet al., 1998, for details). Opal content was measured for one time series only (first trap deployment at site M; Ballouey, 1994) by the method of Mortlock and Froelich (1989). PIC content was calculated from total Ca concentration in the samples as CaCO3; Mg-calcite and the Ca contained in the lithogenic fraction were assumed to be negligible (see Newton et al., 1994; Bory and Newton, 2000). The lithogenic fraction was estimated from the Al concentration in the samples, assuming that lithogenic material is 8.4% Al (the value given for decarbonated marine sediment by Turekian and Wedepohl, 1961). Aluminum and calcium were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Two different preparation procedures were used depending on the samples. Procedure 1 is fully described elsewhere (Bory, 1997; Bory and Newton, 2000). Procedure 2: sub-samples were filtered onto pre-weighed polycarbonate membrane filters (0.4 mm porosity, 47 mm diameter), immediately rinsed with isotonic (0.56 N) ammonium formate to remove salt and excess formalin, and then dried. After weighing, samples were dissolved together with the filters by a two overnight step digestion A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2255 procedure (ultra-pure HNO3 followed by HNO3/HF (1/1) mixture) in PFA Teflon bombs left on a hot plate at 1508C. Analyses were performed with a 138 Ultrace Jobin Yvon ICP-AES. In both procedures, blanks were run repeatedly to check for possible contamination during the preparatory and analytical procedures. The accuracy of the complete dissolution and analytical method was tested by periodic analyses of a geological sediment standard (BCSS-1 or MESS-2, both from the National Research Council of Canada). Precision of the entire procedures was established by replicates. The absolute abundance of coccoliths in the traps was estimated with wet sub-samples diluted (> Â1000) in buffered distilled water and homogenized. A known fraction was then filtered on cellulose Micronsep (MSI) membrane (0.45 mm effective pore size, 47 mm diameter). A fraction of the latter was mounted with Canadian balsam between a slide and cover slip, and the absolute abundance of free (i.e., isolated) coccoliths and coccospheres were counted using a Zeiss Axioscop optical microscope at the appropriate magnification (Â1000). View-fields of 0.01 mm2 were examined until at least 300 coccoliths were counted. Intact coccospheres were determined on larger areas of the filters with weaker magnification (Â500) and larger view-field (0.04 mm2).

3. Results

3.1. Currents at the mooring locations

Current-meter data is compiled in Bournot et al. (1995) and general current description and analyses are presented in Vangriesheim et al. (2000). Here we present data relevant to the sediment trap sample interpretation. Indeed, current measurements in the vicinity of the traps can provide valuable information on: (1) the likelihood of a significant hydrodynamic bias that might compromise sample collection (Knauer and Asper, 1989; Gust et al., 1992, 1994), and (2) the origin of the advected water masses above the traps and, indirectly, of the collected particles (Siegel et al., 1990; Siegel and Deuser, 1997). Statistics have been calculated for each trap deployment times as well as for each 8- or 10-d sampling periods. At 250 m, horizontal speeds (i.e., integrated over a 1 h period; Fig. 2) often exceeded the 15 cm sÀ1 threshold above which currents are known to significantly affect the collection of settling particles (Baker et al., 1988; Knauer and Asper, 1989). Current speeds were 8 and 8–14 cm sÀ1 on average during each mooring deployment times at sites O and M, with maximum values of 30 and 100 cm sÀ1, respectively. Such high current speeds are likely to have caused important bias to the results. Moreover, equipment (traps and current meters) in such highly dynamic environment largely malfunctioned and the time-series at 250 m was significantly reduced. In addition, the number of swimmers and the abundance of >1 mm particles recorded at that depth (see section 3.2 and 3.3) may have further affected the trapping efficiency and the recovery of the smaller fraction. Therefore, the high probability of sampling artifacts dictated rejection of the particle flux time-series recorded at 250 m for interpretation. Consequently, the results of that time-series are not presented here. Most of the current speeds at 1000 m, and all of them at 2500 m, were below 15 cm sÀ1 (Fig. 2). The average current speed for each mooring deployment was always below 6 cm sÀ1 at 1000 m and 4cmsÀ1 at 2500 m depth. During 25 sampling periods (12 out of 48 at site M and 13 out of 71 at 2256 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Fig. 2. Percentage of the sampling time (for each 10- or 8-d sampling interval) for which current speed exceeded the 15 cm sÀ1 threshold at 250 (fine line) and 1000 (bold line) m. Horizontal lines, no data. site O), speeds >15 cm sÀ1 were recorded at 1000 m (see also Vangriesheim, 1994). However, the speed never exceeded 20 cm sÀ1, and the 15 cm sÀ1 threshold was exceeded for 54% of the time for 22 of these 25 sampling periods and for 7–11% of the time for the other three (one at site M and two at site O; Fig. 2). The mean currents show a prevailing westward direction at almost all the depths of the two sites as indicated by their east and north current vector components U and V (Table 1). Mean currents decrease strongly with depth and move from southwest at 250 m to west at 1000 m at site O, and from west at 250 m to southwest at 1000 m at site M. Mean residual currents during each sampling period, calculated after low-pass filtering (Lanczos filter: cut-offperiod of 4 d) to remove tidal and inertial oscillations (Vangriesheim et al., 2000), are shown in Figs. 3b and 4b as vector plots. At both sites, superimposed onto the dominant westward component, which is clearly apparent at 250 m, the general trends of the residual currents display north–south oscillations, particularly marked at 1000 m depth and generally synchronized at the three depths. These oscillations are thought to be due to Rossby waves generated by the CVFZ instability and propagate with a westward phase speed (Vangriesheim et al., 2000).

3.2. Swimmer abundance in the traps

Identiﬁed and removed ‘‘swimmers’’ (Michaels et al., 1990), whose activity (feeding, defecating, dying) within the traps is the other main potential cause of bias, are most abundant at site M and A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2257

Table 1 Mean current speeda Depth (m) U (cm sÀ1) V (cm sÀ1) Oligotrophic site 250 À2.32 À2.67 1000 À0.80 À0.02 2500 À0.61 0.52

Mesotrophic site 250 À4.69 À0.96 1000 À0.90 À0.42 2500 +0.18 À0.54 3000 n.d. n.d. a Mean east (U ) and north (V ) components of the current speed obtained from in situ current measurements. Abbreviation n.d., no data.

Fig. 3. Temporal variability of the mass ﬂux (Panel a, raw data; Panel c, the same mass ﬂux time-series smoothed by a three-sample running mean) and mean residual currents (Panel b) at the oligotrophic site. Sub-sampling errors were estimated as the standard deviation on sub-samples mass determinations (1s). decrease at both sites roughly by an order of magnitude between each trap depth from 250 to 2500 m (no statistics are available for the 3000 m traps, for which only a coarse removal of the swimmers was carried out; see Tables 5 and 6 in the appendix for details). At site M, although very abundant at 250 m (up to several thousand per sampling cup), swimmers are generally a few 2258 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Fig. 4. Temporal variability of the mass ﬂux (Panel a, raw data; Panel c, the same mass ﬂux time-series smoothed by a three-sample running mean) and mean residual currents (Panel b) at the mesotrophic site. Sub-sampling errors are stated in Fig. 3.

hundred and a few tens at 1000 and 2500 m, respectively. At site O, the number of swimmers is similar at 250 m to that at site M, but 2–3 times lower at 1000 and 2500 m than at site M. At both sites, swimmers were present at levels of less than a few percent (by mass) of the particle fluxes reported at 1000 and 2500 m. Temporal variability of the abundance of swimmers in the collection cups is largely associated with that of the particle flux (increasing abundance of swimmers with increasing particle flux), although no proportional relationship is observed. Among the planktonic organisms identified as swimmers in the sampling cups, copepods were by far the most abundant organisms at all depths, representing up to 90% (by number) of the identified species (70–80 and 50–60%, on average, of M and O populations). Other crustaceans (ostracods, amphipods, mysidaceas, euphosiaceas), gastropods, nauplii (crustacean larvae), polychaetes, appendicularians, and some gelatinous species (medusas, salps, siphonophores) were the other main contributors to the population of swimmers.

3.3. Particulate matter collected in the traps

3.3.1. Fraction >1 mm The >1 mm fraction consisted of living organism remains (full shells or fragments, fish scales, crustacean molts, etc.) and large filamentous or gelatinous aggregates in which fecal pellets could A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2259 be entrapped. At 1000 and 2500 m, the >1 mm fraction was generally less than a few percent of the total mass flux (appendix tables), whilst it could represent the main fraction of the particle flux at 250 m. During the second mooring deployment at site M, large quantities of fish scales, eggs and flesh were found in the collection cups at 2500 m from January 17, 1992 until the end of the deployment (May 6, 1992). This may have biased the collection of settling particles, so the corresponding samples were removed from the time-series.

3.3.2. Particle flux (fraction 51 mm), oligotrophic site The particle flux considered in this study consists of the 51 mm fraction (Wefer and Fischer, 1993; Fischer et al., 1996); compositional analyses were carried out exclusively on this fraction. Mass flux ranges from a few mg mÀ2 dÀ1 to 90 mg mÀ2 dÀ1 (Fig. 3a; detailed results are given in the tables of the appendix). Average fluxes are 35 and 37 mg mÀ2 dÀ1 at 1000 and 2500 m depth, respectively (Table 2). Temporal variability is characterized by several peaks occurring throughout the 2 yr of sampling and showing no apparent seasonal variation. The fluxes, however, usually increase (and decrease) at 1000 m before they do at 2500 m, showing an apparent time lag between the flux events at the two depths (Fig. 3c); several of the main flux peaks actually occur with a time lag of about one sampling period (i.e., 10 d) at 2500 m after their occurrence at 1000 m (Fig. 3a), suggesting settling rate of about 150 m dÀ1. The composition of the trapped material is relatively constant, and therefore POC, PIC and lithogenic material fluxes closely follow the mass flux pattern at both depths (Figs. 5 and 6). This is also shown in Fig. 7, where both PIC and lithogenic fractions are linearly correlated with POC; only 1000 m values are presented since fluxes at 2500 m show similar trends (see also Bory and Newton, 2000). Carbonate (calculated as 100/12 Â PIC) and, to a lesser extent, lithogenic material, which represent generally 50–70 and 20–40% of the particulate matter, largely dominate the composition (Table 2), the former diluting the latter during some of the main flux events (Figs. 5 and 6). No large changes in the composition are observed between 1000 and 2500 m: average POC content decrease from 5% to 4%, and PIC and lithogenic material show a slight increase between 1000 (6.6% and 28%) and 2500 m (6.8% and 29%; Table 2).

3.3.3. Particle flux (fraction 51 mm), mesotrophic site The duration of the available time series at site M was somewhat less than at site O: during the second deployment, a malfunction of the 1000 m trap prevented collection at that depth, and the presence in the trap of fish remains (see Section 3.3) reduced the time series by half at 2500 m depth. The average mass flux recorded (about 230 mg mÀ2 dÀ1 at both 1000 and 2500 m depth during the common sampling period) is about 6 times higher than at site O (Table 2). The mass flux ranges over much larger absolute and relative scales compared to site O (Fig. 4a). About 50% of the mass flux was collected in less than a quarter of the whole sampling period, during two large peaks (mass fluxes up to 700–800 mg mÀ2 dÀ1 were recorded at 1000 and 2500 m), which occurred in early and late spring 1991. Another mass flux peak, although inferior, was recorded during summer 1992. In between, mass flux remains relatively low (120 mg mÀ2 dÀ1 on average) and steady. Time-series at 1000, 2500, and 3000 m when available, present very similar patterns, especially during the peaks. As at site O, the flux tends to be larger at 1000 m at the beginning of 2260

Table 2 2251–2282 (2001) 48 I Research Deep-Sea / al. et Bory A. Average mass ﬂux and composition of the particle mattera

Trap Mass ﬂux POC PIC Opal Lithogenic depth Site Sampling (m) (mg mÀ2 dÀ1 ) (%) (mg mÀ2 dÀ1 ) (%) (mg mÀ2 dÀ1 ) (%) (mg mÀ2 dÀ1 ) (%) (mg mÀ2 dÀ1 ) period

Oligotrophic Whole sampling period 1000 35 (625) 5.1 (625) 1.8 6.6 (605) 2.4 n.d. n.d. 27.7 (615) 9.9 at each depth 2500 37 (621) 4.1 (621) 1.5 6.8 (565) 2.7 n.d. n.d. 29.2 (565) 11.4

Common sampling period 1000 35 (621) 5.1 (621) 1.8 6.6 (555) 2.4 n.d. n.d. 28.3 (565) 10.4 between 1000 and 2500 m 2500 37 (621) 4.1 (621) 1.5 6.8 (555) 2.7 n.d. n.d. 29.2 (565) 11.4

Mesotrophic Whole sampling period 1000 225 (393) 8.9 (392) 20.2 6.5 (389) 14.8 n.d. n.d. 21.9 (390) 49.6 at each depth 2500 205 (513) 7.9 (513) 16.2 6.4 (511) 13.1 7.0 (210) 18.8 23.4 (511) 48.1 3000 159 (399) 3.3 (399) 5.3 n.d. n.d. n.d. n.d. n.d. n.d.

Common sampling period 1000 225 (393) 8.9 (392) 20.2 6.5 (389) 14.8 n.d. n.d. 21.8 (389) 49.9 between 1000 and 2500 m 2500 232 (393) 8.4 (392) 19.6 6.6 (389) 15.5 n.d. n.d. 22.0 (389) 52.5

Common sampling period 2500 162 (289) 3.2 (289) 5.3 }} }} } } between 2500 and 3000 m 3000 168 (289) 5.2 (289) 5.2 n.d. n.d. n.d. n.d. n.d. n.d.

a Numbers in brackets indicate the corresponding sampling duration in days. Abreviation n.d., no data. A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2261

Fig. 5. Temporal variability of mass (Panel a), POC (Panel b), PIC (Panel c) and lithogenic (Panel d) fluxes and contents at 1000 m at the oligotrophic site. Errors on POC were estimated to be 3%. Errors on PIC and lithogenic fractions include estimated uncertainties on sample weighing, dilution and ICP-AES analysis; the latter, obtained from analyses replication, was always better than 4% (1s). Blanks’ contents were close to detection limits and negligible in all cases. BCSS-1 and MESS-2 measurements conform to an accuracy of Æ 5% of the measured elements. Standard deviations between independent replicates are all within the error bars. the peaks, and larger at 2500 m at the end of them; this is clearly seen during the two main flux events when time-series are smoothed with a running mean (Fig. 4c). However, ‘‘benchmarks’’ (Honjo, 1996) generally occur within the same sampling period at both depths (Fig. 4a). This indicates a much shorter time lag between flux events at 1000 and 2500 m depth, and therefore much faster settling rates, than at the O site, although settling rates cannot be deduced directly. In contrast to site O, the trapped material is much more variable in composition and shows larger POC contents (Table 2). Calcium carbonate (20% to 90%), organic matter (calculated as 2 Â POC; Gordon Jr, 1970; Deuser et al., 1981; Jickells et al., 1996; 55–50%) or the lithogenic fraction (510% to 50%) variably dominate the particle flux (Figs. 8–10). During the two 1991 spring peaks, increases in POC contents, and to a lesser extent in PIC content, are observed. On the other hand, the 1992 summer peak is characterized by a large increase in PIC content only, which makes up most of the flux at both 1000 and 2500 m (PIC was not measured at 2262 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Fig. 6. Temporal variability of mass (Panel a), POC (Panel b), PIC (Panel c) and lithogenic (Panel d) ﬂuxes and contents at 2500 m at the oligotrophic site. Errors are stated in Fig. 5.

Fig. 7. PIC (diamonds) and lithogenic (crosses) ﬂuxes versus POC ﬂux at 1000 m at the oligotrophic site. A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2263

Fig. 8. Temporal variability of mass (Panel a), POC (Panel b), PIC (Panel c) and lithogenic (d) ﬂuxes and contents at 1000 m at the mesotrophic site. Errors are stated in Fig. 5.

3000 m). As a result of these large compositional changes, fluxes of the main components of the particulate matter show no linear correlation (Fig. 11); only 1000 m values are presented since the results from 2500 m show similar trends. The highest lithogenic contents are observed during the low-flux period (Figs. 8 and 9); during the peaks, biogenic components dilute the lithogenic fraction (Figs. 8, 9 and 12; only 2500 m trap data are shown in Fig. 12 as data from 1000 m present similar trends). Opal, measured only in the 2500 m trap during the first deployment, represents 3–12% of the particle flux; the highest opal fluxes occur at the beginning of the major flux events. On average, particulate matter composition varies little with depth, showing only a slight decrease in the POC content (9–8.5% between 1000 and 2500 m during common sampling periods), and a mild increase in PIC (6.5–6.6%) and lithogenic contents (nearly 22% at both depths; Table 2). 2264 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Fig. 9. Temporal variability of mass (Panel a), POC (Panel b), PIC and opal (Panel c) and lithogenic (d) ﬂuxes and contents at 2500 m at the mesotrophic site. Errors on opal analyses were estimated to be of 3%; errors on other analyses are stated in Fig. 5.

3.4. Coccolithophores

Average coccosphere and coccolith fluxes are presented in Table 3. Coccosphere flux is extremely low at site O compared to site M, whilst coccolith fluxes, although larger at site M, are within the same order of magnitude at both sites. The coccoliths/coccospheres ratio is therefore much larger at site O than at site M (Table 3). At site M, the ratio increases with depth as coccosphere flux decreases and coccolith flux increases between 1000 and 2500 m (Table 3), and the highest ratios are observed during low coccolithophore flux periods (Fig. 13). At that site, coccolithophore flux peaks during the two 1991 large spring flux events (Fig. 13), during which the species Emiliania huxleyi represent more than 93% of the coccolithophore community. A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2265

Fig. 10. Temporal variability of mass (Panel a) and POC (Panel b) ﬂuxes and contents at 2500 m at the mesotrophic site. Errors on POC analyses were estimated to be of 5%.

Fig. 11. PIC (diamonds) and lithogenic (crosses) ﬂuxes versus POC ﬂux at 1000 m at the mesotrophic site.

4. Discussion

4.1. Assessment of the trapping eﬃciency

Data on the hydro-dynamical conditions in the vicinity of the traps (inferred from the com- prehensive current meter record), and the swimmer abundance in the sampling cups, permit an assessment of the performance of the traps in this study. The magnitude of the currents recorded at 1000, 2500, and 3000 m depth is generally low enough to preclude major hydrodynamic bias to 2266 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Fig. 12. Lithogenic content versus POC (Panel a) and PIC (Panel b) contents at 2500 m at the mesotrophic site; ‘‘peaks’’ correspond to >200 mg mÀ2 dÀ1 mass ﬂux events.

Table 3 Mean coccolith and coccosphere fluxesa Trap Sampling period Coccolith flux Coccosphere flux Coccoliths/ depth (m) (106 mÀ2 dÀ1) (103 mÀ2 dÀ1) coccospheres Oligotrophic site 2500 15/3/91–23/7/91 623 35b 17976b 2500 15/3/91–24/11/92 457 n.d. n.d. Mesotrophic site 250 23/2/91–4/4/91 2290 20721 111 1000 23/2/91–4/4/91 4174 10320 405 2500 23/2/91–4/4/91 5770 7689 750 2500 13/2/91–24/11/92 1186 1346 881 a Flux in number of individuals per square meter per day. Abbreviation n.d., no data. b Semi-quantitative values due to low number of coccospheres counted. A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2267

Fig. 13. Top: POC, opal, and lithogenic ﬂuxes (left-hand axes) plotted together with the number of swimmers in the collection cup (right-hand axes) at 2500 m at the mesotrophic site. Bottom: PIC ﬂux (left-hand axes) plotted together with the number of coccospheres, coccoliths and the coccoliths/spheres ratio (right-hand axes) for the same time series.

bottom-moored trap collection of fast-settling particles (Baker et al., 1988; Honjo and Doherty, 1988; Knauer and Asper, 1989; Bacon, 1996; Yu et al., 2001). Only one of the three periods for which the 15 cm sÀ1 threshold was exceeded for a significant fraction of the sampling time at 1000 m (see Section 3.1), which is also when the highest maximum speed is recorded at that depth at site M (15–25 March, 1991; Fig. 2, top Panel, and Fig. 4b), coincides with an unusually low mass flux at 1000 m compared to 2500 m. Such relative decrease is in agreement with the tendency towards undertrapping above 1200 m reported from radionuclide measurements by Bacon (1996) and Yu et al. (2001). No other apparent correlation is evident between current speed and downward particle flux, giving some confidence in the quality of the flux data. The validity of the latter is supported for all the moorings by the consistency between time-series recorded at different depths (see Section 3.3) despite changing hydrodynamic conditions with depth. These results support the 15 cm sÀ1 current speed as being a critical threshold for trapping efficiency (Baker et al., 1988; Knauer and Asper, 1989). Similarly, considering the low level of swimmers at 1000 and 2500 m (see Section 3.2), related artifacts, further minimized by the use of formaldehyde in the sampling cups (Bory and Newton, 2000), are therefore likely to be negligible. Thus, except at 2500 m at site M during the second part of the second deployment (presence of fish remains; see Section 3.3), sampling does not seem to have suffered serious problems.

4.2. Magnitude of the downward particle ﬂuxes

The magnitude of the mass flux shows an important decrease between the M and the O regions, in agreement with both the decrease of the PP and the increasing distance from the aeolian particle source. Biogenic fluxes in the M region are the highest recorded in the North Atlantic Ocean (Jickells et al., 1996). On the other hand, the magnitude of particle fluxes at site O is typical of open ocean records, including those in the Sargasso Sea (Deuser, 1986). 2268 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Carbonates, which dominate the composition of the settling material at both sites, represent a slightly larger fraction of the mass flux at site O in agreement with the lower nutrient condition (Eppley, 1989; Berger and Wefer, 1990) in this area (Morel, 2000). Averaged coccolithophore fluxes at site M are of the same order of magnitude as at Station NABE-34 (348N218W; Broerse et al., 2001). In the M region, where the higher nutrient supply is more favorable to the develop- ment of new primary production species like diatoms (e.g., Goldman, 1993), opal (not measured at site O) is recorded, although it is a minor component of the sinking material. The flux of POC is, on average, about one order of magnitude higher at site M. Such relative difference is much larger than the one in the mean total primary production, which decreases by only a factor of 2.3 between the M and the O regions (Fig. 1). The apparent higher E ratio (i.e., percentage of total primary production exported to depth) in the M region compared to the O region is consistent with the difference in productivity between the two provinces (Eppley, 1989; Wollast, 1998); this point is discussed in further detail in Section 4.5. Lithogenic material also contributes importantly to the trapped material, demonstrating the significance of the atmospheric inputs in this area (Bory and Newton, 2000).

4.3. Temporal variability of the downward particle ﬂuxes

At site O, the absence of a marked seasonal variability in the downward particle flux time-series is consistent with the lack of seasonal variation in PP (Morel, 1996; Morel et al., 1996) due to the non-occurrence of a deep winter mixed layer at this latitude (Levitus, 1982) and to the steadiness of external forcing for the biological activity (the intertropical convergence zone does not reach 208N in summer and regular trade winds prevail all year long). Similarly, the 1991 and 1992 mean fluxes show little difference (32 Æ 2 and 39 Æ 2mgmÀ2 dÀ1 at 1000 m, respectively) despite the different sampling periods (February–December and January–November, respectively). The rather regular and fast oscillations of the downward particle flux (the two fastest being of a month and 50-d periods as estimated by power-density spectrum analysis at 4400 m depth; Khripounoff et al., 1998) could be due (1) to the periodic westward currents associated with the Cape Verde Frontal Zone Oscillations advecting pigment-rich water masses from regions nearer the coastal upwelling (which is permanent at this latitude) as suggested by Berthon (1992) and Khripounoff et al. (1998), or (2) to the North Equatorial Current, which is known to induce westward progres- sing mesoscale eddies that could yield patches of higher PP by upwelling nutrients to surface waters as suggested by Dadou et al. (1996). In any case, a majority of high fluxes at 1000 and 2500 m depth is associated with a marked westward component of the current speed at 250 m (Fig. 14a), the closest available data set of surface currents (which is still true if we consider a 10–20 d delay between the occurrences of currents at 250 m and particle fluxes at depth). This supports the hypothesis of a role of westward surface currents in the downward particle flux variability at site O. At site M as well, no seasonal pattern is apparent during the 2-yr sampling period despite a marked temporal variability of the downward particle flux. This is in agreement with our observations as part of the BOFS program (Biogeochemical Ocean Flux Study, UK-JGOFS): the deployment of a mooring in the same area in 1990/1991 showed little evidence of a seasonal particle flux signal (Bory and Newton, 2000). The largest flux events, that is the two huge 1991 spring peaks and the more moderate 1992 summer peak, are likely to reflect mainly the periodic A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2269

Fig. 14. Mass ﬂuxes at 1000 (circles) and 2500 (diamonds) m versus the east component of the mean residual currents (U) at 250 m at the oligotrophic (Panel a) and the mesotrophic (Panel b) sites. ‘‘Peaks’’ (closed circles and closed diamonds) are deﬁned in Fig. 12.

influence of the upwelling in this area. Indeed, offCape Blanc, meandering structures (giant filaments) of pigment-rich waters several tens of kilometers in width and extending beyond the shelf break seaward to the M region are clearly observed on Coastal Zone Color Scanner (hereinafter CZCS) images (Berthon, 1992). These complex features vary in time and space according to the intensity of the horizontal advection and wind stress but present no apparent periodicity (Morel et al., 1996; Morel, 2000). The main flux events could therefore correspond to particle export from these filaments. Unfortunately, CZCS ended in 1986 and no images were available during the mooring deployments. However, as observed at site O, the marked westward currents recorded at 250 m depth during the main flux events (still true with a 10–20 d delay) is in agreement with this hypothesis (Fig. 14b). Furthermore, an impact of the upwelling in the M region is supported by studies of the phytoplankton community structure. Claustre (1994) reports for September–October 1991 and May–June 1992 that the latter was characteristic of a declining bloom that had evolved from an autotrophic community initially produced in nutrient-rich eutrophic conditions and that would have been advected to this area. 2270 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2271

Still, even if no major difference is apparent in the current conditions recorded at site M during the 1991 and 1992 flux peaks (Fig. 4b), the 1991 spring events differ strongly in magnitude and composition from the 1992 summer peak. The classic sequence (Honjo, 1996) of the peaks of the major flux components was observed during spring 1991 (second event in particular; see Fig. 13): (1) large POC peak, likely to result from aggregated phytoplanktonic bloom, with (2) the presence of opal (i.e., diatoms, usually developing early in blooms when nitrate is still available; Michaels and Silver, 1988; Goldman, 1993), and coccolithophores, followed by (3) PIC (zooplanktonic foraminifers, or debris of coccolithophores and foraminifers) and lithogenic peaks, resulting from grazing, packaging and export by zooplankton (whose presence could also be reflected at that time by the abundance of swimmers in the traps), suggest the occurrence of at least one (possibly two) complete bloom at that time at site M as proposed by Bory and Newton (2000). This implies a rapid transport of a nutrient-rich water mass filament from the upwelling region to the M site at that time. On the other hand, the lower magnitude of the 1992 summer peak and the pre-eminence of the calcium carbonate fraction (Eppley, 1989; Berger and Wefer, 1990) suggest the advection of a filament of older biomass from shoreward regions. In between these events, the rather low and steady fluxes reflect the open ocean oligotrophic productivity regime of this region when it is not (or only weakly) influenced by the upwelling. Overall, the high variability of the downward particle flux recorded at site M throughout 1991–1992 is in agreement with the fluctuating character of the productivity regimes in this area, previously seen from satellite and field PP measurements (Morel et al., 1996; Morel, 2000). The strong difference between 1991 and 1992 flux time-series at site M, however, might also be partly the result of the patchiness of the particle source area and the subsequent mesoscale variability of the downward particle flux, rather than solely a significant interannual difference in the

3————————————————————————————————————————————————— Fig. 15. Modelization of the mass flux at the mesotrophic site during the major 1991 spring peaks. For each of the two peaks (Panel a), a ‘‘continuous’’ flux signal was retrieved from the 1000 m data using the sum of 3 Gauss functions (Panel b). The continuous signal was obtained by tuning the G function parameters and, using an iterative method, by comparing its integration on a 10-d interval (simulating the sediment trap resolution sampling) and the 1000 m trap data. This was carried out until the former satisfactorily matched the latter, that is when difference between each modeled and real data point was less than 3% (Panel c). Particle flux during the fourth sampling period on Panel a was increased in the model, as the former is thought to have been undersampled at that time (see Section 4.1); the increase was so that 1000 and 2500 m flux sums over the total peak period are equal. It was then possible to study the changing pattern of the sampled flux due to the delay (Panel d) in the arrival of the flux events in the lower trap. To better represent the changing pattern of the sampled flux with depth, the ‘‘pulse spreading’’ between 1000 and 2500 m depth due to non-homogenous settling rates among a given flux event was also taken into account by flattening the continuous flux signal (Panel e). This can easily be done on G functions without modifying their sum; the same flattening factor was applied to all functions. The results of a two- (Panel e) or 3-d delay give the best fits to the 2500 m time-series (Panel f ); this is true even if no pulse spreading (Panel d) is taken into account. When the latter is taken into account, modeled (Panel g) and field (Panel a) are in very good agreement. Such results allow the estimation of a 5005settling rate5750 m dÀ1 at that time. The same method was applied to the moderate and last flux peak of the mesotrophic time series to give a 2145settling rate5300 m dÀ1 at that time (i.e., 5- to 7-d time lag). At the oligotrophic site, this approach was applied to the third peak of the first deployment and to the last one of the second deployment. Best fits were obtained for a 9- to 10-d time lag and a 11- to 12-d time lag, respectively, giving estimates of 150–167 and of 125– 136 m dÀ1. 2272 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 influence of upwelling. Indeed, during a given sampling interval period, sediment traps sample only a limited surface of the ocean (a ‘‘streak’’ of a few hundred meters in width at most; Siegel and Deuser, 1997) within the statistical collection area, the order of magnitude of the latter being about hundred kilometers in diameter for a trap located at 1000 m in this area as estimated in Bory and Newton (2000). Whether the traps sample particles exported from the biomass-rich filaments or not would then explain the year-to-year changes in the downward particle flux pattern. However, an assessment of the mesoscale variability of the particle flux in the mesotrophic region was obtained between February and June 1991 by comparing fluxes measured 100 km apart (Bory and Newton, 2000). The significant but still moderate difference (about 30% on average) recorded between mass fluxes at the two sites suggests that spatial heterogeneity is unlikely to be able to explain on its own the apparent drastic changes in the particle flux pattern between 1991 and 1992. In any case, the apparent year-to-year variability at site M underscores the interest of long term (Deuser, 1996) and mesoscales (Newton et al., 1994; Bory and Newton, 2000) studies in mesotrophic regions for budget (carbon in particular) export estimates.

4.4. Particle transport with depth and settling rates

The simultaneous sampling of the settling particles at several depths permits the quantification of some characteristics of particle transport in the water column, such as sedimentation rates and lateral advection. Averaged mass fluxes as well as those of the major components show a very good conservation with depth at both sites O and M (see Bory and Newton, 2000, for details), showing no major lateral advection of resuspended material from the shelf in the two regions below 1000 m depth, even in the deep water column as shown by the record from 200 m above bottom at site M. This is in agreement with nephelometric profiles carried out at both sites, which have not revealed any nepheloid layer in the intermediate or deep water column (Vangriesheim et al., 1993). Furthermore, temporal variations of the downward particle flux obtained at different depths in the water column of the two sites are largely mutually coherent; similar results have been obtained in the same region by Ratmeyer et al. (1999). Thus, particle flux time-series show overall a good ‘‘vertical’’ transfer of the flux signal that clearly indicates a dominant surface origin for the particulate matter collected in all the traps. Moreover, this indicates that, at the two sites, particle source patches are generally large enough so that traps located at different depths (therefore sampling different areas of the ocean surface; e.g., Siegel and Deuser, 1997) sample particles from the same patches, despite the prevailing horizontal currents in the water column. At site M, considering the very good agreement of the fluxes at 1000 and 2500 m depth during the two 1991 spring flux events, a simple model was developed to determine the settling rates (Fig. 15). The aim was to reproduce the changing pattern of the sampled flux (at 1000 and 2500 m) due to delay in the arrival of the real ‘‘continuous’’ flux signal between the two depths. This approach gives an estimated settling rate of a 500–750 m dÀ1 at that time. Such fast-settling particles at site M, associated with large mass fluxes (up to 800 mg mÀ2 dÀ1), could result from high particle concentrations in surface waters that would favor aggregation (e.g., Alldredge and Gotschalk, 1988). However, considering the smaller flux events, settling rates at site M are likely to be lower, as indicated for instance by the last peak of the time-series, which corresponds to a A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2273 settling rate of 210–300 m dÀ1 between 1000 and 2500 m depth. Applying the same method to the O site gives settling rates of 130–160 m dÀ1 between 1000 and 2500 m (see Fig. 15 legend for details), which are in the typical range for particles in the open ocean (Fowler and Knauer, 1986; Newton et al., 1994; Honjo, 1996). These settling rates are similar to the one observed between 2500 and 4400 m at the same site (Khripounoffet al., 1998), thus showing little change in settling rate between the intermediate and deep water column. A rough estimate of the distance between the areas sampled at a given time by the 1000 and 2500 m traps can be derived from the settling rates (assuming no major changes throughout the water column) and the mean current velocity profile at that time (with the approximation of a homogenous current velocity field). This distance (which corresponds to the distance between the centers of the respective statistical collection areas; see Siegel and Deuser, 1997) has been estimated to be less than 7 km during the major flux peaks at site M, and therefore much less than the width of the biomass-rich filaments (see Section 4.3). This is consistent with the fact that traps at 1000 and 2500 m depth appear to sample the same particle source patches at that time.

4.5. POC export

Average POC fluxes over the entire 1991–1992 sampling period represent a much higher fraction of the mean primary production at site M (Fig. 1), 2.8% at 1000 m, compared to site O, where it represents only 0.6% at the same depth. Although comparing datasets obtained over different time periods is somewhat problematic, the fact that POC and PP are both averaged over long time periods should reduce the uncertainties associated with this time mismatch. Moreover, the few field PP measurements obtained in 1991 and 1992 are in very good agreement with mean PP estimates (Morel, 2000). These results are in agreement with the fact that in mesotrophic conditions, organic carbon is transferred more readily and efficiently through the trophic chain to large species mainly responsible for POC export, compared to oligotrophic regimes, where most organic carbon is recycled in surface waters (Eppley, 1989; Wollast, 1998). Evidence of this is given by the zooplanktonic biomasses (Morel, 2000), which are actually larger at site M than at site O at a higher ratio (3.4 and 3.6 over the 0–200 and 0–1000 m water column interval, respectively) than the total primary production (2.3). The export of POC is also thought to increase with the size of the phytoplanktonic species (Boyd and Newton, 1995, 1999). Large species such as diatoms predominate in the eutrophic region over the small-sized cells that predominate in the O region (Claustre, 1994; Morel, 2000). This size gradient could therefore contribute to the observed trend, because of the regular influence of the eutrophic water masses in the M region (see Section 4.3). Moreover, a shorter residence time of the particulate matter in the water column at the M site, inferred from the drastic increase in the coccoliths/coccospheres ratio (Table 3) from M to O regions (coccospheres are thought to degrade into coccoliths as a function of time in the water column; Honjo, 1976; Balch et al., 1992; Beaufort and Heussner, 1999; Broerse et al., 2001) and by the faster settling rates (see Section 4.4), is in agreement with a more efficient transfer of POC to depth. POC fluxes at 100 m depth were calculated from 1000 and 2500 m fluxes from the Martin et al. (1987) relationship (Table 4). The corresponding E ratios at 100 m depth are 0.04–0.07 and 0.17– 0.29 at sites O and M (Table 4). These results are in good agreement with the Fp-ratio (i.e., an 2274 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282

Table 4 POC ﬂux and E ratioa Martin et al. (1987)

F2500=F1000 Site PP Trap POC ﬂux F100 E ratio (F3000=F1000)

À2 À1 À2 À1 À2 À1 (mg m d ) depth (m) (mg m d ) (mg m d )(F100/PP) Theoretical Eumeli Oligotrophic 300 1000 1.8 13 0.04 2500 1.5 24 0.08 0.46 0.84

1000 20.2 146 0.20 Mesotrophic 710 2500 16.2 256 0.36 0.46 0.97 3000 5.3 98 0.14 (0.39) (0.71) a Average POC flux at EUMELI sites (n.b. averaged sampling periods are different between depths and sites; see Table 3 and Appendix A). POC flux at 100 m depth (F100) estimated using Martin et al. (1987) empirical relationship 0:858 (Fz ¼ F100=ðz=100Þ ) and POC flux measurements at 1000, 2500, and 3000 m. E ratio calculated as F100 versus PP. Expected decrease of POC with depth from Martin’s relationship (presented as F2500=F1000 and F3000=F1000) compared to the observed values (ratios correspond to common sampling periods between compared depths).

analogue of the f -ratio, new primary production versus total PP, estimated from the phytoplankton community structure), 0.055–0.070 and 0.16–0.56 at sites O and M (Claustre, 1994), respectively, obtained during two periods (September–October 1991 and May–June 1992) of the 2-yr EUMELI program. However, it is worth noting that the decrease of the POC ﬂuxes with depth at both sites is much lower (3% and 15% between 1000 and 2500 m at sites M and O) than the one calculated from the Martin et al. (1987) relationship (>50%). This is of particular interest, since such a relationship has been used for depth corrections in several studies (Jickells et al., 1996; Lampitt and Antia, 1997). EUMELI data, at both sites O and M, do not support the validity of such a use of the Martin et al. (1987) relationship for deep POC ﬂuxes.

Acknowledgements

We thank T. Jickells and his team for providing ICP-AES facilities (see section 2, Procedure 1). A. Bory thanks P.P. Newton for his invaluable support during the early stage of this study. The manuscript beneﬁted from the helpful comments of P. Biscaye and three anonymous reviewers. LSCE contribution 526. Appendix

Table 5

Oligotrophic site 250 m trap 1000 m trap 2500 m trap Beginning Sampling Swimmers Mass >1 mm POC PIC Lithogenic Swimmers Mass >1 mm POC PIC Lithogenic Swimmers of duration number flux fraction flux flux flux number flux fraction flux flux flux number sampling (d) (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 ) 2/18/91 1

2/19/91 4 2591 3.3 4.5 0.3 69 2251–2282 (2001) 48 I Research Deep-Sea / al. et Bory A. 2/23/91 10 4569 14.3 6.3 0.9 1.0 2.8 158 7.8 1.1 0.4 4 3/5/91 10 20140 39.7 3.1 2.0 2.6 5.2 122 21.6 2.6 1.0 8 3/15/91 10 30.8 4.6 1.6 2.3 6.8 121 19.5 1.4 0.9 1.5 4.1 14 3/25/91 10 23.4 1.5 1.2 1.9 4.0 88 26.5 1.5 1.4 10 4/4/91 10 18.3 4.1 1.0 1.3 4.1 107 15.4 4.0 0.9 10 1/14/91 10 28.0 4.5 1.5 2.1 6.0 159 17.7 2.4 0.9 11 4/24/91 10 26.5 4.4 1.7 2.0 5.5 135 24.8 3.3 1.0 1.4 8.2 12 5/4/91 10 60.6 5.3 4.1 4.4 11.4 101 39.9 5.8 2.7 2.5 11.6 18 5/14/91 10 39.7 5.8 1.8 3.1 7.6 124 29.3 2.8 1.5 1.9 8.1 14 5/24/91 10 34.8 6.2 1.7 2.8 6.3 128 32.5 3.1 1.1 2.4 7.5 13 6/3/91 10 38.4 3.8 2.1 2.1 12.0 144 36.7 2.4 1.5 2.7 9.2 22 6/13/91 140 59.6 4.0 3.6 4.4 11.4 120 43.2 8.4 2.4 3.3 9.3 14 6/23/91 10 48.6 2.7 2.5 4.0 7.5 110 64.2 4.4 2.5 5.2 12.7 11 7/3/91 10 26.3 3.1 1.1 2.1 4.2 90 45.5 1.7 1.8 3.3 10.7 16 7/13/91 10 21.0 2.2 0.8 1.6 3.8 150 29.6 9.9 1.1 2.2 7.1 15 7/23/91 10 32.1 2.5 2.2 1.4 10.6 132 35.9 1.7 1.9 2.4 9.8 12 8/2/91 10 35.7 3.2 2.4 2.2 9.6 116 44.4 2.9 2.7 3.1 11.8 18 8/12/91 10 19.5 2.0 1.2 1.1 6.1 112 39.0 1.9 1.1 2.6 10.4 15 8/22/91 10 23.5 0.7 1.6 6.8 143 41.3 2.1 1.3 2.7 12.1 13 9/1/91 10 23.6 2.5 1.4 1.5 6.6 142 45.1 3.8 1.3 3.1 12.7 17 9/11/91 7 11.0 5.1 1.0 0.5 2.9 84 35.6 2.1 1.7 2.3 11.1 15 9/18/91 1 8.3 1.1 18 30.7 5.5 1.9 3

9/25/91 4 15.6 0.7 0.6 57 10.0 0.3 0.5 12 9/29/91 10 32.5 1.1 1.3 2.3 8.6 119 20.6 2.1 1.0 1.5 5.4 16 10/9/91 10 23.7 0.7 1.1 1.6 6.8 143 23.0 1.9 0.8 1.6 7.1 22 10/19/91 10 10.6 1.2 0.6 0.7 2.8 114 19.0 1.6 0.8 1.3 5.7 12 10/29/91 10 22.3 1.8 1.0 1.6 5.8 101 15.8 0.6 0.7 1.1 4.6 13 11/8/91 10 35.3 1.8 1.6 2.4 10.6 70 17.5 0.5 0.8 1.2 5.2 16 11/18/91 10 88.5 3.2 3.7 7.1 19.8 52 39.3 1.9 1.3 3.4 7.7 11 11/28/91 10 35.2 0.8 1.6 2.6 8.6 69 47.0 0.7 1.9 3.6 12.0 15 12/8/91 10 29.1 1.2 1.4 1.8 9.2 69 31.0 0.7 1.1 2.2 9.2 12 12/18/91 10 29.3 1.9 1.3 1.9 9.1 85 30.4 0.6 0.9 2.1 9.5 13 12/28/91 10 45.4 2.8 2.7 2.9 13.8 49 36.5 2.1 1.3 2.6 10.7 4 1/7/92 10 24.7 0.7 1.0 1.7 7.4 66 42.2 0.7 1.5 2.8 13.8 14 1/17/92 10 33.7 0.7 1.5 2.2 10.3 50 32.1 2.0 1.1 2.2 10.9 12 1/27/92 10 12.2 0.4 0.6 0.8 3.8 52 28.6 0.7 1.0 1.9 9.2 12 2/6/92 10 29.3 0.3 1.6 2.0 8.5 43 25.4 0.9 0.9 1.7 8.2 13 2275 (continued on next page) 2276

Table 5 (continued) Oligotrophic site 250 m trap 1000 m trap 2500 m trap Beginning Sampling Swimmers Mass >1 mm POC PIC Lithogenic Swimmers Mass >1 mm POC PIC Lithogenic Swimmers of duration number flux fraction flux flux flux number flux fraction flux flux flux number sampling (d) (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 ) .Br ta./De-e eerhI4 20)2251–2282 (2001) 48 I Research Deep-Sea / al. et Bory A. 2/16/92 10 49.3 0.3 2.9 3.4 5.6 40 59.4 1.0 2.5 4.1 19.3 9 2/26/92 10 51.3 0.4 2.7 4.1 9.6 46 48.1 0.5 2.0 3.6 12.3 10 3/7/92 10 27.9 1.3 1.1 2.1 6.9 62 44.0 0.9 1.7 3.2 12.6 18 3/17/92 10 55.4 0.6 2.5 3.8 16.1 62 35.8 1.5 1.8 2.5 11.1 7 3/27/92 10 53.6 0.1 2.2 3.8 15.1 56 40.0 2.4 1.7 2.7 12.7 13 4/6/92 10 85.0 0.3 4.0 6.0 24.8 62 64.3 3.1 3.1 4.5 20.7 10 4/16/92 10 50.5 0.8 2.4 3.4 15.5 54 85.4 1.7 4.0 6.1 26.9 14 4/26/92 10 36.0 0.3 1.9 2.5 11.8 89 46.1 2.2 1.6 3.1 15.6 18 5/6/92 1 38.9 2.0 7 36.3 2.7 1.5 0

5/28/92 4 34.5 1.2 2.1 1.9 9.3 93 20.0 0.7 1.1 1.2 5.8 10 6/1/92 8 48.1 0.5 2.9 2.6 14.9 159 37.5 1.6 1.5 2.0 10.7 6 6/9/92 8 42.2 1.1 2.1 2.6 14.2 189 42.1 0.8 1.6 2.2 13.1 14 6/17/92 8 35.4 1.1 1.4 2.1 10.2 169 39.1 1.5 1.3 2.6 12.3 21 6/25/92 8 37.6 1.3 1.8 2.2 11.9 147 33.9 0.4 1.1 2.5 11.1 13 7/3/92 8 28.6 0.7 1.6 1.7 9.1 104 30.8 1.2 1.0 1.9 11.9 12 7/11/92 8 39.9 1.1 1.6 2.2 11.5 68 36.3 3.4 1.2 2.0 9.9 16 7/19/92 8 35.1 1.0 1.8 2.3 11.7 76 39.8 1.4 1.3 2.3 11.0 17 7/27/92 8 50.0 0.9 3.3 3.3 19.5 86 56.0 1.4 2.0 3.4 16.7 16 8/4/92 8 43.0 0.5 2.4 2.0 14.5 97 38.5 1.4 1.4 2.4 12.8 11 8/12/92 8 52.3 1.2 2.5 3.4 21.0 99 38.3 1.2 1.5 2.3 9.1 8 8/20/92 8 40.3 1.8 2.1 2.5 14.7 111 44.7 1.4 1.9 2.9 15.7 7 8/28/92 8 38.6 0.9 1.7 2.6 12.3 76 45.5 1.3 1.7 2.7 15.8 6 9/5/92 8 40.6 2.4 2.2 2.1 10.8 69 28.9 0.8 1.3 1.4 8.8 2 9/13/92 8 62.7 2.5 2.5 2.9 23.0 99 42.5 1.4 2.2 2.5 9.9 17 9/21/92 8 43.0 0.4 2.0 2.4 13.6 78 51.1 1.5 1.9 3.3 16.1 14 9/29/92 8 34.4 0.6 1.5 1.7 14.0 81 29.5 1.3 1.3 1.6 10.4 6 10/7/92 8 54.4 1.9 3.0 3.6 19.5 100 39.3 0.1 1.7 2.4 12.7 16 10/15/92 8 37.7 0.7 1.6 2.1 14.0 81 40.3 1.0 1.5 2.2 14.2 12 10/23/92 8 36.2 0.6 1.7 1.6 12.8 106 46.5 0.3 2.0 2.8 17.4 13 10/31/92 8 12.1 0.2 0.6 0.9 5.5 110 43.5 0.6 1.7 2.7 14.5 13 11/8/92 8 3.7 1.4 0.3 0.2 1.7 68 70.1 0.3 2.0 5.2 18.2 7 11/16/92 8 4.8 0.4 0.3 0.3 1.6 57 51.4 1.2 1.6 3.2 11.5 4 11/24/92 8 7.3 0.9 0.5 0.4 2.4 99 50.6 1.0 1.7 3.5 17.1 9 Table 6 Mesotrophic site 250 m trap 1000 m trap 2500 m trap 3000 m trap Beginn- Sampling1mm SwimmersMass >1 mm POC PIC Litho- SwimmersMass >1 mm POC PIC Opal Litho- SwimmersMass POC ing of duration fraction number flux fraction flux flux genic number flux fraction flux flux flux genenic number flux flux sampling(d) flux flux (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 ) 2/12/91 1 55.6 609 67.8 25.0 3.7 55 56.7 17.7 2.5 11 2/13/91 10 23.9 3534 114.0 16.0 5.3 6.3 39.5 485 99.1 11.3 4.4 5.1 8.4 40.1 74 2/23/91 10 23.9 1917 392.8 16.5 20.7 26.4 99.7 424 308.8 12.9 16.2 17.2 28.6 96.9 120 3/5/91 10 41.4 8764 747.9 17.3 73.9 44.9 125.3 453 680.8 16.8 61.9 45.7 49.3 98.5 99 2251–2282 (2001) 48 I Research Deep-Sea / al. et Bory A. 3/15/91 10 26.2 1937 309.8 12.9 25.7 18.5 45.6 224 577.5 17.3 64.9 34.1 23.1 102.6 224 3/25/91 10 87.5 9405 638.8 29.6 73.2 42.9 64.8 261 602.6 32.8 116.3 41.8 15.8 77.8 112 4/4/91 10 38.5 5609 204.2 27.7 24.7 11.8 33.9 153 318.3 31.3 40.8 19.5 19.1 37.4 210 4/14/91 10 6.7 2080 55.8 8.6 5.4 1.5 21.6 222 108.9 12.9 7.2 6.0 9.5 38.3 59 4/24/91 10 25.0 2707 211.3 14.8 39.9 10.7 27.4 156 178.4 16.4 15.0 11.2 12.3 42.7 54 5/4/91 10 20.0 1790 121.3 15.1 11.5 6.1 39.9 174 115.0 20.6 8.9 6.6 7.4 35.1 41 5/14/91 10 15.6 1502 110.1 5.9 26.6 2.6 26.3 129 120.7 10.9 10.9 6.3 8.3 40.2 35 5/24/91 10 16.5 1367 652.6 12.4 161.9 32.9 46.3 69 508.1 11.6 124.8 26.5 30.7 39.9 114 6/3/91 10 8.6 1874 521.8 25.1 102.4 25.7 52.5 336 467.4 6.5 107.8 26.9 56.8 52.9 132 6/13/91 10 22.7 1836 476.1 39.1 35.8 28.6 92.5 504 504.6 61.0 31.6 37.9 24.0 80.4 198 6/23/91 10 185.2 9.7 13.2 10.7 46.6 453 264.8 18.7 12.6 19.6 25.4 56.4 149 7/3/91 10 157.7 9.0 15.3 6.7 56.0 371 167.6 8.0 9.4 10.4 17.9 48.7 60 7/13/91 10 48.6 10.5 3.4 2.7 14.6 347 95.7 6.9 4.5 5.5 11.1 33.2 61 7/23/91 10 24.5 4.8 3.0 0.9 7.6 185 85.0 5.5 5.0 4.9 7.7 28.7 83 8/2/91 10 34.0 3.8 5.2 1.3 9.4 230 132.0 12.8 8.6 7.2 10.8 46.2 52 8/12/91 10 17.2 3.0 1.9 0.6 5.6 216 86.3 4.9 5.3 5.1 7.8 28.9 45 8/22/91 10 17.6 5.5 2.7 0.2 6.6 239 85.2 3.0 5.2 4.9 7.0 31.4 58 9/1/91 10 20.9 1.6 1.8 0.5 9.1 238 177.1 16.3 9.0 8.4 14.2 79.5 56 9/11/91 1 57.6 6.1 4.4 28 149.4 10.9 6.6 5.5 76.5 7 9/12/91 1 27.1 8.1 1.6 25 97.9 5.0 5.5 3.6 50.6 8

9/19/91 10 4869 130.7 0.6 4.1 4.8 34.4 44 9/29/91 10 5718 114.1 0.5 5.2 4.9 47.2 40 10/9/91 10 4454 92.4 3.7 4.9 4.1 41.8 40 102.2 4.5 10/19/91 10 18792 115.0 10. 5.3 4.9 45.9 26 80.4 4.1 10/29/91 10 3978 104.0 1.8 4.5 4.8 41.3 37 92.1 4.4 11/8/91 10 3986 106.3 3.1 5.2 4.7 35.7 30 74.9 3.7 11/18/91 10 2166 123.2 3.5 6.7 5.1 53.2 72 94.1 4.4 11/28/91 10 5392 122.1 12.5 4.4 7.1 41.8 38 108.3 4.7 12/8/91 10 4301 108.0 3.0 4.5 5.9 28.9 45 138.4 5.6 12/18/91 10 3432 162.0 10.7 6.9 6.3 56.8 38 109.5 4.3 12/28/91 10 3372 115.9 12.2 4.8 7.0 26.1 41 178.9 7.1 1/7/92 10 a lot 116.2 33.0 5.4 6.2 40.5 55 122.4 5.0 1/17/92 10 a lot 102.6 4.3 1/27/92 10 a lot 120.2 4.9 2/6/92 10 a lot 147.6 6.6

2/16/92 10 8336 132.7 6.4 2277 (continued on next page) 2278

Table 6 (continued) Mesotrophic site 250 m trap 1000 m trap 2500 m trap 3000 m trap Beginn- Sampling1mm SwimmersMass >1 mm POC PIC Litho- SwimmersMass >1 mm POC PIC Opal Litho- SwimmersMass POC ing of duration fraction number flux fraction flux flux genic number flux fraction flux flux flux genenic number flux flux sampling(d) flux flux (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 (mg mÀ2 dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 )dÀ1 ) 2251–2282 (2001) 48 I Research Deep-Sea / al. et Bory A. 2/26/92 10 a lot 147.8 7.1 3/7/92 10 843 164.4 6.8 3/17/92 10 1496 113.7 4.0 3/27/92 10 263.7 9.7 4/6/92 10 143.0 6.0 4/16/92 10 91.1 3.7 4/26/92 10 81.5 3.7 5/6/92 1 70.1 3.6

6/5/92 1 934 15.7 0.5 1.0 4.4 85 34.7 0.4 1.3 20 6/6/92 3 2834 52.7 14.6 3.1 3.1 16.9 250 78.0 3.9 3.2 5.4 21.2 37 34.3 1.2 6/9/92 8 23033 90.8 28.5 4.3 6.9 34.7 250 94.7 11.9 4.3 5.3 36.6 47 111.6 4.5 6/17/92 8 20740 100.2 2.9 5.4 4.7 38.5 297 89.9 9.1 3.5 5.1 31.1 59 139.6 4.9 6/25/92 8 108072 293.0 4.2 14.8 16.5 70.4 163 191.5 9.6 7.0 12.3 45.0 37 200.0 6.6 7/3/92 8 16900 180.6 34.9 8.2 14.8 47.9 248 194.0 6.9 8.5 14.0 33.1 47 227.3 4.4 7/11/92 8 10818 295.2 43.8 5.1 31.3 18.3 205 332.3 22.4 7.6 31.5 31.0 36 373.1 8.2 7/19/92 8 6387 198.6 148.0 5.6 17.2 35.4 174 212.0 92.9 4.4 16.5 50.1 52 300.3 5.5 7/27/92 8 469.0 40.3 6.5 48.2 41.6 287 426.1 27.2 6.0 45.5 13.4 44 489.4 7.3 8/4/92 8 169.7 24.2 4.9 16.2 48.2 602 159.9 1.1 3.5 12.2 37.0 34 185.9 4.2 8/12/92 8 206.4 13.0 5.1 21.6 52.6 268 138.2 1.2 3.6 10.0 40.2 33 131.1 3.9 8/20/92 8 313.2 10.5 5.0 30.4 56.6 234 374.7 11.5 6.4 32.7 60.6 38 338.0 4.9 8/28/92 8 114.6 17.9 3.1 8.6 39.9 394 155.4 1.0 3.8 10.6 44.8 48 180.3 4.1 9/5/92 8 113.3 10.3 4.5 7.6 43.1 240 167.7 7.0 5.1 10.0 42.8 28 208.3 6.3 9/13/92 8 208.7 3.4 7.2 12.2 97.7 196 115.4 4.6 2.6 6.6 44.3 27 124.8 4.0 9/21/92 8 231.6 11.2 8.9 13.1 115.6 134 152.4 2.9 4.5 9.3 62.2 26 109.6 3.2 9/29/92 8 172.7 17.2 7.3 9.4 69.0 151 107.1 12.3 3.5 6.2 44.0 27 109.1 3.2 10/7/92 8 248.5 18.8 10.1 16.9 86.7 102 183.6 12.7 5.7 12.1 44.3 19 220.3 7.1 10/15/92 8 305.7 11.5 10.3 16.3 111.4 132 155.6 2.6 4.4 8.1 48.8 22 171.4 4.9 10/21/92 8 171.1 5.1 5.4 11.8 83.0 147 137.4 9.5 4.7 8.7 50.0 17 126.3 3.8 10/31/92 8 257.5 11.1 15.5 15.7 63.2 99 166.4 11.4 4.2 10.1 49.7 21 181.5 5.0 11/8/92 8 254.8 9.9 17.2 17.0 61.2 68 291.6 12.4 11.8 20.6 60.8 17 255.0 7.9 11/16/92 8 157.3 2.2 8.4 10.6 63.0 159 208.9 19.6 7.1 13.3 75.2 30 206.8 6.7 11/24/92 8 154.1 9.2 7.0 8.8 42.6 158 170.3 14.0 6.6 9.5 69.0 24 165.4 5.9 A. Bory et al. / Deep-Sea Research I 48 (2001) 2251–2282 2279

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