Seasonal variability of living benthic foraminifera from ...

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Journal of Sea Research 59 (2008) 297 – 319 www.elsevier.com/locate/seares

Seasonal variability of living benthic foraminifera from the outer continental shelf of the Bay of Biscay Gérald Duchemin a,b,⁎, Frans J. Jorissen a,b , François Le Loc'h c , Françoise Andrieux-Loyer d , Christian Hily e , Gérard Thouzeau e a

Laboratory of Recent and Fossil Bio-Indicators (BIAF), UPRES EA 2644, Angers University, 2 Boulevard Lavoisier, 49045 ANGERS Cedex 01, France b LEBIM, Ker Châlon, 85350 Ile d'yeu, France c IRD, UR070-RAP, Centre de Recherche Halieutique Méditerranéenne et Tropicale, Avenue Jean Monnet, BP 171, 34203 Sète cedex, France d IFREMER centre de Brest, Département DEL/EC/EB, B.P. 70, 29280 Plouzané, France e Institut Universitaire Européen de la Mer, UMR 6539 CNRS, Technopole Brest-Iroise, Place Nicolas Copernic, 29 280 Plouzané, France Received 26 November 2007; received in revised form 25 March 2008; accepted 27 March 2008

Abstract Living benthic foraminiferal faunas of six stations from the continental shelf of the Bay of Biscay have been investigated during three successive seasons (spring, summer and autumn 2002). For the three investigated stations, bottom water oxygen concentration, oxygen penetration into the sediment and sediment organic carbon contents are all relatively similar. Therefore, we think that the density and the composition of the foraminiferal faunas is mainly controlled by the quantity and quality of organic input resulting from a succession of phytoplankton bloom events, occurring from late February to early September. The earliest blooms are positioned at the shelf break, late spring and early summer blooms occur off Brittany, whereas in late summer and early autumn, only coastal blooms appear, often in the vicinity of river outlets. In spring, the benthic foraminiferal faunas of central (B, C and D) and outer (E) continental shelf stations are characterised by strong dominance in the first area and strong presence in the second area of Nonionella iridea. In fact, station E does not serve as a major depocenter for the remains of phytoplankton blooms. If station E is not considered, the densities of this taxon show a clear gradient from the shelf-break, where the species dominates the assemblages, to the coast, where it attains very low densities. We explain this gradient as a response to the presence, in early spring, of an important phytoplankton bloom, mainly composed of coccolithophorids, over the shelf break. This observation is supported by the maximum particles flux values at stations close to the shelf break (18.5 g m− 2 h− 1) and lower values in a station closer to the coast (6.8 g m− 2 h− 1). In summer, the faunal density is maximum at station A, relatively close to more varied phytoplancton blooms that occur off Brittany until early June. We suggest that the dominant species, Nonion fabum, Cassidulina carinata and Bolivina ex. gr. dilatata respond to phytodetritus input from these blooms. In autumn, the rich faunas of inner shelf station G are dominated by N. fabum, B. ex. gr. dilatata, Hyalinea balthica and Nonionella turgida. These taxa seem to be correlated with the presence of coastal blooms phenomena, in front of river outlets. They may be favoured by an organic input with a significant contribution of terrestrial, rather low quality organic matter. © 2008 Elsevier B.V. All rights reserved. Keywords: Benthic Foraminifera; Seasonality; Continental Shelf; France; Bay of Biscay; Pelagic–Benthic Coupling

⁎ Corresponding author. Laboratory of Recent and Fossil Bio-Indicators (BIAF), UPRES EA 2644, Angers University, 2 Boulevard Lavoisier, 49045 ANGERS Cedex 01, France. E-mail address: [email protected] (G. Duchemin). 1385-1101/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2008.03.006

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1. Introduction The successful interpretation of the ecological patterns of benthic foraminiferal faunas found in past and present environments requires a thorough knowledge of the recent ecology of the dominant species. For this reason numerous studies have tried to interpret recent distributional patterns of foraminiferal faunas in function of the available environmental parameters. Most of these studies are based on samples collected during one single sampling campaign, and give at best a very partial picture of the distribution of living benthic foraminifera. Often, such single time distributional data are implicitly considered as typical for the average environmental conditions reigning at the sampling stations. However, an increasing amount of evidence suggests that benthic foraminifera may show a rapid response to short-term changes of the physico-chemical and biological parameters of the ecosystem. Especially seasonal and interannual variability of food input (the flux of organic material to the sea floor) may provoke a reproductive and/or growth response of the benthic foraminiferal faunas (e.g. Altenbach, 1988, 1992; Gooday, 1988, 1993, 2002; Gooday and Lambshead, 1989; Gooday and Rathburn, 1999; Kitazato et al., 2000; Jannink, 2001; Gooday and Hughes, 2002; Fontanier et al., 2003, 2005; Heinz and Hemleben, 2003). In otherwise oligotrophic environments, episodic food inputs may even be responsible for the majority of foraminiferal test production (e.g. Gooday, 1988, 1993; Gooday and Lambshead, 1989). In fact, the initial input of a fairly small amount of organic matter may trigger a foraminiferal population increase, whereas the organic flux delivers during the principal phase of the spring bloom may sustain these populations (Nomaki et al., 2005a). Such foraminiferal reproduction events especially favour some highly opportunistic species that may be rather poor in the background faunas, but proliferate in case of a massive food inputs. Therefore, these opportunistic taxa strongly dominate the fossil faunas forming at sites subject to recurrent massive food inputs. Alternatively, the faunas collected by single time sampling, outside the productive periods, are representative for the average, more oligotrophic conditions at the site, that are dominant in time. As a consequence, living faunas frequently show important differences with the fossil faunas forming at the same site that may be strongly biased by the addition of opportunistic taxa during reproductive events following massive food inputs (e.g. Jorissen and Wittling, 1999), even if taphonomic processes cause probably most of the differences between recent and fossil assemblages. Massive food inputs, such as important phytodetritus

deposits that are generally triggered by surface water phytoplankton bloom events, may also provoke short term modifications of bottom water oxygenation and sediment redox conditions. It appears that the seasonal succession of phytoplankton blooms, and other mechanisms responsible for the transport of organic matter to the sea floor (e.g. river runoff, bottom nepheloids, suspended sediment transport, etc) may be responsible for an important temporal variability of the foraminiferal faunas. If we want to use foraminifera (or any other organisms) as indicators of anthropogenic influence, it is essential to evaluate the extent of this temporal variability, and to understand the underlying mechanisms. For this reason, in the last decade, an increasing number of authors have studied the temporal variability of foraminiferal faunas, in a wide range of environments. Studies comparing the seasonal succession of foraminiferal faunas with the fluctuations of available environmental parameters have been carried out in salt marshes (e. g. Stubbles, 1995; Murray and Alve, 2000; Swallow, 2000; Alve and Murray, 2001), in continental shelf environments (e. g. Barmawidjaja et al., 1992; Jannink, 2001) as well as in deep-sea ecosystems (e. g. Altenbach, 1992; Silva et al., 1996; Ohga and Kitazato, 1997; Gooday and Rathburn, 1999; Kitazato et al., 2000; Gooday, 2002; Fontanier et al., 2003; Heinz and Hemleben, 2003, 2006). Most of these studies show that the temporal variability of foraminiferal faunas may indeed be significant. The main aim of the present study is to better understand the ecology of the foraminiferal faunas in the “Grande Vasière” mud/silt belt. In a first paper (Duchemin et al., 2005), based on a single sampling in spring 2002, at three of our four outer shelf stations, foraminiferal faunas were strongly dominated by Nonionella iridea, which is considered as an opportunistic species, dependant on a high flux of organic carbon to the sea floor (Gooday, 1986; Mackensen et al., 1990; Gooday and Hughes, 2002). The difference in foraminiferal density and composition allowed us to distinguish two pairs of stations (stations A and B at 100 m, stations C and D at 130 m depth), with N. iridea being very rich at the 130 m stations and absent or lower frequent at the 100 m deep stations. We interpreted the high foraminiferal densities in the deeper stations, together with the strong dominance of N. iridea, as a possibly response to a phytoplankton bloom, that occurred over the shelf break in the vicinity of the two deepest stations, in March–April 2002. This interpretation was strengthened by the nitrate/nitrite profiles and fluxes that suggest a phytodetritus deposit in the weeks prior to sampling in the two deepest stations.

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Satellite images of chlorophyll-a distribution in the Bay of Biscay clearly show a seasonal variability of phytoplankton densities, with bloom events occurring above the shelf break in late winter and early spring, followed by more coastal phytoplankton blooms from late spring to early summer. It appears that opportunistic species, such as N. iridea, could respond to the seasonal variability of phytoplankton production, and especially the strong early spring bloom events could have a strong impact on the composition of the fossil assemblages. If true, N. iridea could serve as an index species for important natural or anthropogenic organic matter deposits in recent ecosystems, and also as a paleoindicator of seasonal phytoplankton bloom events in the past, since its fragile test is commonly preserved in the sediment in some areas (Gooday and Hughes, 2002). However, until now our suggestion was only based on a study of the benthic foraminiferal faunas sampled in April–May 2002. In order to confirm our hypothesis, and to obtain a better insight into the temporal variability of the foraminiferal populations, in this paper we will compare the benthic foraminiferal faunas sampled in spring 2002 with those sampled at the same stations in June–July and September 2002. The comparison with satellite images and available data on sediment fluxes to the ocean floor, will give us more insight into the linkage between hydrological features, surface water primary production events, and benthic ecosystem ecology. 2. Environmental setting A large coast-parallel body of fine-grained sediments, known as the “Grande Vasière” (“Great Clay Belt”) characterizes the northern continental shelf of the

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Bay of Biscay (French coast, Northeast Atlantic; Fig. 1). The “Grande Vasière” is about 275 km long, and situated between 47°45'N and 45°40'N at a water depth from 80 to 130 m. Its west–east width varies from 55 to 75 km. The “Grande Vasière” consists of predominantly silty sediments originating mainly from the Loire and Garonne rivers, deposited parallel to a coastal area composed of pre-littoral depressions, gravelly plains and hydraulic dunes. To the west, at a depth of around 130 m, a hard bottom separates the “Grande Vasière” from the outermost part of the continental shelf that is constituted of Ditrupa sands, down to 160 m depth (Glémarec, 1971). The “Grande Vasière” is an important French demersal and benthic fisheries area, mainly producing Atlantic hake (Merluccius sp) and Norway lobster (Nephrops norvegicus), but also sole (Solea solea), and anglerfish (Lophius sp.; Dardignac, 1988). The seasonal variability of temperature and salinity in the continental shelf environments of the Bay of Biscay have previously been described by Lazure and Jegou (1998) and Lampert (2001). In winter, the combination of minimal freshwater input and strong winds leads to a mixed water column, with homogeneous temperature and salinity, with the thermal and haline fronts parallel to the coastline. In late winter and early spring, when runoff from the Loire and Gironde rivers is substantial (2500–3500 m3 s− 1) and when there is a southern to western wind, a salinity stratification with a halocline at 20 m depth settles close to the coast, north of the major estuaries. In case of north-western or eastern winds, this salinity stratification may spread over a large part of the continental shelf. Generally, we can observe a temperature (from ∼ 10 to ∼ 12 °C) and salinity (from ∼ 30.5 to ∼ 35.5) gradient from the inner

Fig. 1. (a) Location of the “Grande Vasière” mud/silt belt (in dark grey) in the Bay of Biscay. (b) Location of our sampling stations A, B, C, D, E and G.

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continental shelf until the 100 m isobath (salinity ∼ 35.5). In summer, as a result of the increase of solar radiation and the decrease of freshwater input, the thermo-haline stratification is gradually replaced by a thermal stratification that rapidly extends over the entire continental shelf. In this period, surface water temperatures decrease from 18 °C in the south to 15 °C in the north of the Bay of Biscay, with isotherms perpendicular to the coastline. Phytoplankton production is strongly controlled by these hydrological patterns. In winter, the low temperature and minimum insolation inhibit the proliferation of phytoplankton. In spring, when the temperature difference at the thermocline rises above 0.4 °C, internal waves may form on the continental shelf, especially during high tides at spring tide conditions. These internal waves become amplified in seaward direction, to become maximal above the shelf break (Lampert, 2001). This phenomenon induces injection of cold, nutrient-rich intermediate waters into the photic zone, favouring primary production blooms on the outer continental shelf and above the shelf break (Holligan and Groom, 1986; Mazé, 1987), that are dominated by coccolithophorids (Lampert et al., 2002). The duration of these spring blooms is limited by the availability of silicates and phosphates. In summer, coastal blooms develop in front of the major river mouths; these blooms are characterized by a dominance of diatoms (∼ 40%), coccolithophorids (∼ 20%) and dinoflagellates (∼ 15%, Lampert, 2001). Nitrate and phosphate concentrations are the limiting factors now. At the end of summer, the maximum thermal stratification inhibits the input of nutrient-enriched intermediate waters (Loyer, 2001), and nutrients become exhausted. Consequently, late summer phytoplankton production is largely based on recycled nutriments. Blooms are limited to coastal areas, and are dominated by cyanobacteria, that may account for 45% of the total primary production (Lampert, 2001). In spite of these substantial seasonal changes in the composition of the phytoplankton community, diatoms are a dominant element throughout the year (20–50%, Lampert, 2001). 3. Methods 3.1. Collection of the sediment samples Stations A, B, C and D (Fig. 1, Table 1) were sampled by the R.V. Thalassa and the R.V. Côte de la

Table 1 Location of stations Stations

Longitude

Latitude

Water depth (m)

A B C D E G

47°14 N 46°56 N 47°09 N 46°53 N 46°55 N 47°35 N

3°40 W 3°30 W 3°56 W 3°42 W 4°30 W 4°08 W

100 100 130 130 140 80

Manche in late April–early May 2002 (termed “spring”), in late June–early July 2002 (“summer”) and in mid-September 2002 (“autumn”). Additional stations (E and G, Fig. 1, Table 1) were sampled in spring and autumn. Samples were taken with a classical Barnett multi-tube corer (Barnett et al., 1984), providing one core with an inner diameter of 9.4 cm/station and per season. Because of bad weather, in summer the multicorer could not be deployed, and samples were taken with the tube of a multi-tube corer in a Reineck box core (170 cm2), at stations A, B, C and D. Except for station A, all box cores still contained ambient sea water when they arrived on board. We studied the benthic foraminiferal faunas, bottom and pore water oxygen concentrations, nitrate and nitrite concentrations, and the percentage of organic matter. 3.2. Surface water chlorophyll-a concentrations and suspended particulate matter In order to compare the seasonal variability of the foraminiferal faunas with surface water primary production patterns, on-line data (SeaWiFS), elaborated by means of a five channel chlorophyll concentration algorithm (Gohin et al., 2002) have been used to estimate the chlorophyll-a concentrations in our study area. Free-floating multiple sample programmable sediment traps (Pro-Trap) were deployed at all locations in spring and autumn. Each Pro-Trap system consisted of four PVC sediment tubes, each of 0.018 m2, mounted in a stainless steel frame. Carousels mounted underneath two of the tubes were each fitted with 11 polypropylene sample jars, which were advanced by rotation of the carousels at pre-set intervals (2 series of 11 sample jars). Depth and angle sensors allowed defining the position of the traps in the water column, while light scattering was measured by a Sea Tech LS sensor. A VALEPORT 800-0 series electromagnetic current meter provided 2-

Fig. 2. Satellite picture of the Chlorophyll-a concentrations on the Bay of Biscay (SeaWiFS data and SeaDAS calculation), with an indication of the helf break (yellow line, 100 m isobath), and the position of chlorophyll-a maxima (red lines).

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axis flow velocity measurements at the trap aperture. The mooring consisted of a spherical float (425 l buoy), five 22-litres VINYFLOAT buoys at a nominal depth of 15–30 m, and one (in spring) or two (in autumn) ProTrap/Valeport positioned under or just over the thermocline and 1.6 m off the sea-floor. In our paper, we only use the data from the bottom Pro-Trap. Sampling duration (12 h) and partitioning (T = 66 min) were chosen to record short-term variations in sedimentation events in relation to water mass advection and biological activities of planktonic organisms. Given the sampling durations, poison or preservative were not added to the sample jars. Trap samples were treated on board in order to measure mass flux (Total Particulate Matter, Particulate Organic Carbon, Particulate Inorganic Carbon, Chlorophyll-a, Phaeopigments). For each sediment trap deployment, a profiler was used, which allowed us to collect the bottom water (1 to 3 m from sea-floor) using a carousel water sampler. Suspended Particulate Matter, Particulate Organic Carbon and Particulate Inorganic Carbon were measured on the water samples. The material collected in each cup of the sediment traps was re-suspended and homogenised before subsamples were taken for various analyses. The samples collected either by sediment traps or Niskin bottles have undergone the same treatments. Suspended Particulate Matter and Total Particulate Matter were filtered on pre-combusted and pre-weighted (Whatmann GF/F) filters (47 mm diameter) and stored at − 20 °C prior analysis. The filters were rinsed with demineralised water to remove salt and dried at 60 °C for 48 h before weighting. Then, the samples were analysed for Particulate Total Carbon content by using a Carlo Erba NC 2500 Element Analyser (Majeed, 1987). 3.3. Early diagenesis Organic matter and nitrate/nitrite concentrations have only be measured for the spring and autumn campaigns. For summer, we used data from another sampling cruise, performed in June 2002. At each station and each season, one core was sliced in half cm slices until 3 cm, and in cm slides deeper down. Organic C was measured using a Leco CHN-800 elemental analyser, after removal of inorganic C with phosphoric acid (Cauwet, 1975). The oxygen concentration of the free waters, overlying the undisturbed sediment, was measured by the Winkler titration method. Profiles of pore water oxygen were obtained on board by using a miniaturized Clarktype oxygen sensor (Unisense OX500) coupled with a

picoammometer (Unisense PA2000) and a micromanipulator (Unisense MM33). The imprecision of this method is due to the subjectivity involved in the Winkler titration method, the precise determination of the sediment-water interface and the stirring sensitivity (= 1.5%) of the microelectrode. 3.4. Foraminifera For foraminiferal analysis, at each station, one core was sliced every 0.5 cm until 4 cm depth. Individual samples were stored in a mixture of 95% ethanol and 1 g l− 1 rose Bengal. In the laboratory, the samples were washed through 63 µm and 150 µm sieves and dried at 50 °C. In order to concentrate the foraminiferal faunas, which were extremely diluted by the important quantity of sediment grains, flotation was carried out using trichloroethylene, which is largely applied for shelf foraminifera (e.g. Murray, 1969, 1985, 1992; Sen Gupta, 1971; Boltovskoy and Totah, 1985; Scott et al., 2003). After flotation, foraminifera were stored in 95% ethanol. In a previous paper dealing with the composition and microhabitat of foraminiferal faunas in 10 cm deep cores sampled during the spring 2002 campaign (Duchemin et al., 2005), foraminifera were largely restricted to the first 4 cm. Therefore, stained specimens of the N 150 µm fractions were sorted until 4 cm depth, whereas, because of the extremely time consuming character, stained specimens of the 63–150 µm were only sorted until 2 cm depth. An additional reason to concentrate on the superficial depth intervals is the fact that intermediate and deep infaunal taxa appear to be less responsive to fresh organic matter input than the more opportunistic superficially living taxa (Fontanier et al., 2003; Nomaki et al., 2005a,b, 2006). The taxonomical framework is similar to the one used by Duchemin et al. (2005); pictures of characteristic taxa are accessible on the web site http://ead.univ-angers.fr/ ~geologie/atlas/Biscay.html. 3.5. Statistical analyses We used a statistical approach to compare the foraminiferal in function of sampling seasons. All sediment layers of both grainsize fractions were summed before the statistical analyses, that was performed separately for both size fractions (63– 150 µm and N150 µm). We based our analysis on absolute abundances. We performed a Principal Components Analysis (using Statistica), which allows us to interpret the distribution of stations in function of the foraminiferal composition.

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4. Results 4.1. Surface water chlorophyll-a concentrations In April 2002, an extensive chlorophyll-a maximum (about 1.5 mg Chl-a m− 3) developed over the deeper parts of the Bay of Biscay, and extended until the shelf break (Fig. 2a, area 1). In May 2002, this chlorophyll-a maximum has largely disappeared in the central part of the Bay of Biscay, but has intensified over the shelf break (Fig. 2b, area 1, about 2.5 mg Chl-a m− 3). A second chlorophyll-a maximum (of about 2.5 mg Chl-a m− 3), west of the western part of Brittany, extends until station G (Fig. 2b, area 2). In early June 2002, the shelfbreak chlorophyll-a maximum has almost disappeared, whereas the second chlorophyll-a maximum extends until the vicinity of stations G, C and A (Fig. 2c, area 2). This second maximum dissipates at the end of June. In July (Fig. 2d), oligotrophic conditions can be observed everywhere around our sampling stations. In August, a strong coastal chlorophyll-a maximum (Fig. 2e, area 3) develops close to our station G. In the beginning of September (Fig. 2f), also these phytoplankton blooms have largely disappeared. 4.2. Suspended particulate matter: bottom water concentrations and fluxes to the seafloor Suspended particulate matter fluxes were determined in spring and autumn. At station E, unfortunately, the sediment trap could not be moored in spring, because of a heavy swell.

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In spring, the bottom water concentration of suspended particulate matter (Niskin bottles) ranges from 7.2 g DW m− 3 at station D to 14.2 g DW m− 3 at station A. In autumn, it ranges from 4.5 g DW m− 3 at station E to 10.0 g DW m− 3 at station G, with a systematical increase from open sea to the coastal area (Table 2). The particulate organic carbon concentration shows a very similar pattern with the highest spring values at station A (141.9 mg C m− 3), and the lowest values at station D (70.2 mg C m− 3). In autumn, the lowest values are found at the outer shelf stations (B, C, D and E; 49.4–63.9 mg C m− 3) and highest value at station G (104.8 mg C m− 3; Table 2). The highest spring concentrations of particulate inorganic carbon are measured at stations A, C and D (44.5–50.1 mg C m− 3), and the lowest values at stations B and D (10.5 and 18.7 mg C m− 3, respectively). In autumn, the highest values are found at station G (50.6 mg C m− 3), the lowest values at station B (4.0 mg C m− 3), and mean values at stations A, C, D and E (18.6– 32.6 mg C m− 3). The daily fluxes recorded by sediment traps moored on the sea floor show much more geographical and temporal variability. In spring, the total particulate matter flux is maximal at station D (18.5 g m− 2 h− 1), is high at station B (15.4 g m− 2 h− 1) and minimal (but much higher than in autumn) at stations G, A and C (6.8–8.9 g m− 2 h− 1; Table 2). The flux of particulate inorganic carbon is maximal at stations D and B, whereas a minimal value is recorded at station G. The particulate organic carbon flux shows maximum values at stations G (172.3 mg C m− 2 h− 1) and D (140.7 mg C m− 2 h− 1). Also Chlorophyll-a and phaeopigments fluxes are maximal at these two stations.

Table 2 Particulate concentrations in bottom water and particulate flux sampled in bottom moored sediment trap (ND = No Data) Bottom water Suspended particulate matter

Spring

Station A Station B Station C Station D Station E Station G Autumn Station A Station B Station C Station D Station E Station G

Particulate flux (sediment trap) Particulate organic carbon

Particulate inorganic carbon

Total particulate matter

Particulate organic carbon

Particulate inorganic carbon

Chlorophyll-a Phaeopigments

(g DW m− 3) (mg C m− 3) (mg C m− 3) (g m− 2 h− 1) (mg C m− 2 h− 1) (mg C m− 2 h− 1) (µg m− 2 h− 1)

(µg m− 2 h− 1)

14.2 8.0 11.5 7.2 ND 8.7 9.4 7.3 7.7 6.9 4.5 10.0

90.7 99.7 64.9 159.3 ND 274.7 73.5 20.1 27.1 40.0 73.4 130.9

141.9 95.9 97.3 70.2 ND 77.8 87.7 63.9 49.4 56.3 50.5 104.8

49.4 10.5 50.1 18.7 ND 44.5 32.6 4.0 22.9 18.6 23.0 50.6

6.8 15.4 8.7 18.5 ND 8.9 2.6 2.4 1.5 1.6 2.8 10.6

64.1 94.5 74.2 140.7 ND 172.3 32.3 16.9 13.8 18.2 30.8 220.0

94.0 248.1 181.5 259.5 ND 44.1 15.1 13.3 10.1 48.1 35.5 110.9

12.4 13.9 8.4 22.9 ND 50.5 10.6 2.4 3.7 4.1 6.2 13.2

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In autumn, the particulate organic and inorganic carbon fluxes are maximum at station G (220.0 mg C m − 2 h− 1 and 110.9 mg C m − 2 h − 1 , respectively). Also chlorophyll-a and phaeopigments fluxes are maximal at station G, although the values are significantly lower than in spring. At station A, the total particulate matter flux is much lower, but chlorophyll-a and phaeopigment fluxes are comparable to the values observed in spring. At stations B, C and D, finally, all flux values are much lower than in spring. 4.3. Oxygen concentration and organic carbon content Bottom water oxygen concentrations (Fig. 3) vary from 3.6 (station C, autumn) to 5.7 ml l− 1 (stations D and G, spring), and are systematically lower during the autumn cruise (3.6–4.7 ml l− 1 ) than during the spring and summer cruises (4.9–5.7 ml l − 1). At all stations, the limit between oxic and dysoxic conditions (as defined by Tyson and Pearson, 1991) is reached within the first centimetre of the sediment, whereas the depth of the zero oxygen boundary varies from only 0.3 to 1.7 cm depth (Fig. 3). Stations C (in spring and autumn), D (in autumn) and E (in spring) present irregular oxygen profiles, with a first sub-surface minimum followed by an increase, before arriving at the zero oxygen level. The highest sedimentary organic carbon contents are measured at stations G and in the topmost layer of station E (11.5–14.0‰ and 11.5‰ respectively; Fig. 4). Compared to stations E and G, the organic carbon contents at stations A, B, C and D are very low, about 2– 3‰. Only the top 2 cm depth of station A show slightly higher values (5 to 7.5‰). 4.4. Foraminifera 4.4.1. Foraminiferal density In the N150 µm fraction, seasonal variations are rather similar for stations B, C and D, and probably also for stations E and G, for which we do not dispose of summer data (Fig. 5a). Standing stocks of the upper 5 cm are maximal in spring (∼ 470–500 specimens per 50 cm2), tend to be slightly lower in autumn (∼250–480 specimens per 50 cm2), and minimal in summer (∼ 150– 430 specimens per 50 cm2). Station A, on the contrary, shows a minimum value in spring (∼ 300 specimens per

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50 cm2), and maximum values in summer and autumn (∼ 470–510 specimens per 50 cm2). In the 63–150 µm fraction, the foraminiferal densities in the topmost 2 cm vary from about 110 to 1080 specimens per 50 cm 2 (Fig. 5b). Station A presents a maximum density during the summer cruise (990 specimens per 50 cm 2 ) and a minimum during spring and autumn (340–470 specimens per 50 cm 2 ). Densities at stations B and C progressively decrease from spring to autumn (from 960–1060 in spring to 750–610 in summer, to 440–290 specimens per 50 cm 2 in autumn, respectively). Station D has a minimum value during the summer cruise (780 specimens per 50 cm 2 ) and maximum values in spring and autumn (about 1050 specimens per 50 cm 2 ). Stations E and G, that were only sampled in spring and autumn present a maximum value in spring (990 and 1080 specimens per 50 cm 2 , respectively) and a minimum value in autumn (110 and 530 specimens per 50 cm 2 , respectively). 4.4.2. Foraminiferal composition The faunas of the N150 µm fraction tend to be dominated by Nonion fabum, Cassidulina carinata and Bulimina marginata (Table 3 and Fig. 6). N. fabum (3.2 to 45.5%, ∼ 10–260 specimens per 50 cm2) is frequent (N5%) at all stations in all seasons, except for station D, where its percentage is low in summer and autumn. Also B. marginata (0.4 to 16.6%, 2–75 specimens per 50 cm2) is frequent at most stations, but it shows much lower percentages in autumn than in other seasons. C. carinata is much more variable; at stations A and C it attains very high percentages in summer and especially in autumn, but it is less frequent at other sites and in spring. At stations of the Grande Vasière (A, B, C and D), the faunas collected in spring are characterized by elevated percentages of Hanzawaia boueana, Nonionella turgida (except for station A) and Textularia agglutinans. In summer, several agglutinated taxa (Bigenerina nodosaria, Cribrostomoides scitulum, Eggerella medius, Haplophragmoides subglobosum, Liebusella goesi and Siphotextularia flintii) are relatively abundant, especially at stations C and D (cumulative percentage 36.0 and 41.1%, 65 and 115 specimens per 50 cm 2 , respectively). However, these taxa stay low frequent (5.2%, 25 specimens per

Fig. 3. Dissolved oxygen concentrations (ml l− 1) in the six cores in spring (late April–early May), summer (late June–early July) and autumn (September) in the top 2 cm of sediment (top 3 cm for station E).

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Fig. 5. Densities of the total live foraminiferal faunas, in the four cores, standardized for a surface area of 50 cm2, (a) from 0 to 4 cm depth on the N150 µm fraction, and (b) from 0 to 2 cm depth on the 63–150 µm fraction.

50 cm2 ) in the faunas of station A, that are also in summer strongly dominated by N. fabum (32.3%, 150 specimens per 50 cm 2 ). In autumn, the faunas at Grande Vasière stations are dominated by C. carinata at stations A and C (25.5 and 31%, 270 and 75 specimens per 50 cm 2 , respectively), by N. fabum at stations B (31%, 150 specimens per 50 cm 2 ) and by S. flintii at station D (21.6%, 90 specimens per 50 cm 2 ). At stations B, C and D, Uvigerina peregrina (8.6– 12.6%, 30–60 specimens per 50 cm2 ) is an important faunal element. At station D, the autumn faunas are further characterised by increased percentages of B. nodosaria (14.5%, 60 specimens per 50 cm 2 ), Melonis barleeanus (11.8%, 50 specimens per 50 cm 2 ) and

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Angulogerina angulosa (8.8%, 35 specimens per 50 cm2 ). The faunas of the inner shelf station G are in spring and autumn dominated by N. fabum (45.5% and 25.6%, 260 and 140 specimens per 50 cm2, respectively), whereas Hyalinea balthica has an important contribution in autumn (20.9%, 110 specimens per 50 cm2). They are further characterised by the presence of fair amounts of agglutinated species, such as Nouria polymorphinoides (8.8–7.4%, 50–40 specimens per 50 cm2 in spring and autumn), Eggerella scabra (6.1– 8.3%, 35–45 specimens per 50 cm2 in spring and autumn) and L. goesi (5.9–12.0%, 35–65 specimens per 50 cm2 in spring and autumn). At the outer shelf station E, faunas are fairly similar to those found in the “Grande Vasière” (stations A, B, C and D). In autumn, however, the faunas of station E are dominated by Globobulimina affinis forma hoeglundi (26.2%, 45 specimens per 50 cm2), a taxon that was absent from the spring sample, and has only been observed at this station. The faunas of the 63–150 µm fraction are dominated by N. iridea, Bolivina ex. gr. dilatata, Adercotryma wrighti, C. carinata and B. marginata (Table 4 and Fig. 7). Bolivina ex. gr. dilatata and B. marginata (2.4– 28.8%, 15–280 specimens per 50 cm2 and 0.8–11.1%, 5–115 specimens per 50 cm2, respectively) are present at all stations, at all cruises. In spring, N. iridea is a dominant taxon in the faunas of stations B, C, D and E (11.0–62.6%, 135–620 specimens per 50 cm2). Rather surprisingly, it is totally absent at station A and rare at station G (2.4%, 30 specimens per 50 cm2), where it is also low frequent in autumn (4.2%, 25 specimens per 50 cm2). Bulimina marginata shows elevated percentages at all stations (5.2–11.1%, 5–115 specimens per 50 cm2). C. carinata is an important faunal component at stations A, D and E (7.3–11.9%, 40–150 specimens per 50 cm2). The fauna of inner shelf station G is characterised by high densities of Textularia porrecta (21.5%, 265 specimens per 50 cm2) and Epistominella vitrea (8.4%, 105 specimens per 50 cm2). In summer, stations A, B, C and D show elevated percentages of N. iridea (5.8–62.1%, 55–480 specimens per 50 cm2), B. ex. gr. dilatata (6.2–28.8%, 45– 280 specimens per 50 cm2) and B. marginata (2.7– 7.1%, 20–50 specimens per 50 cm2). The faunas at

Fig. 4. Organic carbon contents (mg g− 1) in the six cores in spring (late April–early May), summer (late June–early July) and autumn (September).

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Table 3 Percentage of main foraminiferal taxa of the N150 µm fraction

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The grey boxes represent dominant taxa with a relative proportion of 5% at least one of the stations. The last column represents the percentage of each species if we sum all stations.

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Fig. 6. Densities of main species of foraminifera (number of specimens per 50 cm2) of the larger fraction (N150 µm) at the six stations in spring and autumn and at stations A, B, C and D in summer.

station D are strongly dominated by N. iridea, C. carinata is a major faunal element at stations A and C (18.5%, 180 specimens per 50 cm2 and 25.5%, 155 specimens per 50 cm2, respectively), whereas N. turgida (18.2%, 135 specimens per 50 cm2), Bolivina striatula

(14.5%, 110 specimens per 50 cm2), and Stainforthia fusiformis (14.3%, 105 specimens per 50 cm2) are particularly frequent at station B. In autumn, B. ex. gr. dilatata is still a major faunal constituent at all stations (7.3–24.1%, 15–130 specimens

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Table 4 Percentage of main foraminiferal taxa of the 63–150 µm fraction

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The grey boxes represent dominant taxa with a relative proportion of 5% at least one of the stations. The last column represents the percentage of each species if we sum all stations.

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Fig. 7. Densities of main species of foraminifera (number of specimens per 50 cm2) of the smaller fraction (63–150 µm) at the six stations in spring and autumn and at stations A, B, C and D in summer.

per 50 cm2). The percentage of N. iridea varies between 4.2 and 19.1% (10–95 specimens per 50 cm2), except for station D, where it still strongly dominates the faunas

(62.4%, 740 specimens per 50 cm2). The “Grande Vasière” stations A, B, C and D show increased percentages of the agglutinated taxa Recurvoides trochaminiformis (24.8

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Fig. 8. PCA ordination diagrams on the first two axes of sampling stations based on the selected N150 µm species. (a) Plot of the factor loadings. Aang = Angulogerina angulosa; Bnod = Bigenerina nodosaria; Bmar = Bulimina marginata; Ccar = Cassidulina carinata; Csci = Cribrostomoides scitulum; Emed = Eggerella scabra; Esca = Eggerella scabra; Gaff = Globobulimina affinis; Hbou = Hanzawaia boueana; Hsub = Haplophramoides subglobosus; Hbal = Hyalinea balthica; Lgoe = Liebusella goesi; Nfab = Nonion fabum; Ntur = Nonionella turgida; Npol = Nouria polymorphinoides; Sfli = Siphotextularia flintii; Tagg = Textularia agglutinans; Uper = Uvigerina peregrina. (b) Plot of the factor scores. The first letter corresponds to the station letter, whereas two others are an abbreviation of the season (sp = spring; su = summer; au = autumn).

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Fig. 9. PCA ordination diagrams on the first two axes of sampling stations based on the selected 63–150 µm species. (a) Plot of the factor loadings. Awri = Adercotryma wrighti; Bdil = Bolivina dilatata; Bmar = Bulimina marginata; Bstr = Bolivina striatula; Ccar = Cassidulina carinata; Evit = Epistominella vitrea; Hbra = Haplophragmoides bradyi, Msub = Miliolinella subrotunda; Nfab = Nonion fabum; Niri = Nonionella iridea; Ntur = Nonionella turgida; Rtro = Recurvoides trochaminiformis; Sfli = Siphotextularia flintii; Sfus = Stainforthia fusiformis; Tpor = Textularia porrecta; Uper = Uvigerina peregrina. (b) Plot of the factor scores. The first letter corresponds to the station letter, whereas two others are an abbreviation of the season (sp = spring; su = summer; au = autumn).

and 7.9% at stations B and A) and S. flintii (25.4, 9.3 and 7.6% at stations C, A and D). As is the case for the N 150 µm fraction, the fauna of outer shelf station E has

a comparable composition as the Grande Vasière stations, but stands out by the increased density of E. vitrea (9.2%, 10 specimens per 50 cm2). The fauna of inner shelf station

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G is characterised by an increased percentage of N. turgida (16.8%, 105 specimens per 50 cm2). 4.5. PCA analyses About 75.0% of the total variability of the N 150 µm data set is explained by the first two axes (46.0% and 29.0% for axes 1 and 2, respectively; Fig. 8a). N. fabum (+ 59.0 and − 33.1 for axes 1 and 2, respectively) and C. carinata (− 43.9 and − 50.0 for axes 1 and 2, respectively) strongly contribute to axes 1 and 2. When the sample scores are plotted on the factorplane, stations D, B, C and E are in all seasons positioned in the central part, D and E showing the highest positive values on factor 2 (Fig. 8b). Station G plots both in spring and autumn at the positive side of factor 1, due to a high density of N. fabum and low densities of the species with negative loadings on this axis. Station A has a peculiar behavior: in spring it plots in the vicinity of other “Grande Vasière” stations, in summer it shift to the negative side of factor 2 (high densities of N. fabum and C. carinata, whereas in autumn is plots at the lower left, due to the strong dominance of C. carinata. In the PCA carried out on the 63–150 µm fraction, about 83% of the variability is explained by the first two axes (71.8% and 11.3% respectively for axes 1 and 2; Fig. 9a). The first axis is strongly dominated by N. iridea (+ 251.1), whereas the second axis translates density variations of B. ex. gr. dilatata (− 76.7), A. wrighti (− 40.6), C. carinata (− 27.9), T. porrecta (− 27.6) and B. marginata (− 24.1). Three groups of samples are clearly separated on the axial plot (Fig. 9b). Group A regroups all samples in which the fauna is strongly dominated by N. iridea: all seasons for station D and the spring sample of station B. Group B contains the spring samples of stations G, E and C, and the summer sample of station A. The faunas are enriched in B. ex. gr. dilatata and various agglutinated taxa. Group C contains all autumn samples (except station D), the summer samples of stations B and C, and the spring sample of station A. The faunas are characterized by low densities of N. iridea and B. ex. gr. dilatata. 5. Discussion 5.1. Sampling bias Most studies on the temporal variability of benthic foraminifera are based, as is also the case in this paper, on a single sample per station and per season. Confusion

between temporal and micro/meso-scale spatial variability is a potential problem, which in most studies has not been taken into account. The very few existing studies (e.g. Gooday, 1988, 1993; Fontanier et al., 2003) on spatial variability suggest that small (decimetric– metric) and medium (decametric) scale patchiness of foraminiferal faunas may be significant. Such spatial variability can easily be interpreted erroneously as temporal variability. In the absence of data on smallscale spatial variability, data on the temporal succession of foraminiferal faunas should therefore be interpreted with much reserve. Because of bad weather, we sampled with a box corer in summer, while we used a multi-tube corer in spring and autumn. A quantitative comparison between these two methods has revealed that meiobenthos collected by box corers may be affected quantitatively and qualitatively by the loss of the water overlying the sediment, resulting in a displacement of surface sediments and any superficial detritus layer (Bett, 1994). The magnitude of this potential bias cannot be quantified because it depends on the efficiency of the individual box corer to retain overlying water, of the presence or absence of phytodetritus, or on the nature of the sediment. However, the use of box corer samples may probably lead to an underestimation of the foraminiferal density, especially of taxa living at or close by the sediment– water interface. 5.2. Foraminiferal densities related to oxygen and organic matter availability Foraminiferal densities, whatever the size fraction, present maximal values in spring (∼1270–1700 specimens per 50 cm2) and minimal values in autumn (∼280–1070 specimens per 50 cm2), except at station D, which shows a maximum in spring and autumn (∼1590–1660 specimens per 50 cm2) and a minimum in summer (1050 specimens per 50 cm2), and station A, with a maximum in summer (∼ 1450 specimens per 50 cm2), and minimum values in spring and autumn (∼650–1050 specimens per 50 cm2). Oxygen content and labile organic matter availability are the two main factors explaining foraminiferal differences in term of densities and composition (i.e. Jorissen et al., 1995). In our study area, bottom oxygen concentrations are only slightly different, with lower values in autumn (3.6–4.7 ml l− 1) than in spring (4.9– 5.7 ml l− 1). The lower autumn values are probably the result of the intensified stratification of the water column that limits oxygen renewal at the sea floor. Moreover, all oxygen profiles are rather similar, if we take the

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imprecision of the measurements into account (i.e. positioning of the sediment–water interface, stirring sensitivity = 1.5%). The irregular patterns of some oxygen profiles (stations C and E in spring, stations C and D in autumn) are probably due to the presence of large macrofaunal burrows (Duchemin et al., 2005). Therefore, it appears that the oxygen content, which is relatively stable through the year, cannot explain the variation of foraminiferal densities. The sedimentary organic carbon contents at stations A, B, C, D and E are very low (2–3‰); and maximum value is found at station G (11.5–14.0‰). Station A presents higher values (5 to 7.5‰) in the top 2 cm in summer, when the foraminiferal density shows a maximum. A similar correspondence is found at station E in spring. Therefore, the foraminiferal density appears to be at least partially explained by the input of organic matter from column water into the benthic system. However, Duchemin et al. (2005) suggest that most labile organic carbon resulting from short primary production events is consumed very rapidly after being deposited. This is supported by the very low oxygen penetration depth, indicative of high consumption of sedimentary oxygen in the days prior to sampling. 5.3. Phytoplankton bloom events As described in the chapter on surface water chlorophyll-a concentrations, the middle and outer shelf environments of the Bay of Biscay experience several chlorophyll-a maxima, that are the result of a series of consecutive phytoplankton bloom events taking place from late winter/early spring to autumn. These consecutive bloom events are characterised by different absolute primary production values, and by different phytoplankton groups. The input of detrital material resulting from successive phytoplankton blooms is probably a major factor structuring the continental shelf benthic ecosystems. The distance to the successive phytoplankton blooms may be related to the quantity of the organic input, whereas the dominant phytoplankton groups may result in different qualities of organic matter being transported to the sea floor. We think that the combination of these two parameters is responsible for a large part of the spatial and seasonal variability of the foraminiferal faunas observed at our stations. Stations D and E are closest to the late winter–early spring chlorophyll-a maximum, situated close to the shelf break. In the Bay of Biscay, the late winter to early spring phytoplankton blooms are strongly dominated by

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coccolithophorids (Lampert, 2001). The middle shelf stations C and A and the inner shelf station G, on the contrary, are closer to the location of late spring to early summer chlorophyll-a maxima off Brittany, which are the consequence of phytoplankton blooms dominated by dinoflagellates and coccolithophorids (pers. comm. F. Gohin). The inner shelf station G (and in a lesser degree also station A), is closer to the chlorophyll-a maximum which appears close to the coast in late summer and autumn, and which represent phytoplankton blooms rich in cyanobacteria (Lampert, 2001). In this context, of a successive appearance of blooms of different phytoplanktonic organisms at different sites, it is important to realise that the remains of these successive phytoplankton blooms will no be deposited directly under the sites where the blooms occur. In the case of the late winter to spring phytoplankton bloom situated at the shelf-break, the sea-bottom at these sites consists of coarse-grained sands, without any recent sedimentation. It appears that the remains of the bloom events are deposited elsewhere, notably in areas with active sedimentation, where the sediment is characterised by a much smaller grain size. This may be in muddy sediment of the upper continental slope, or in the silty sediments of the “Grande Vasière”. It is therefore evident that particle transport will have an important lateral component, and that seasonal variability observed in the benthic ecosystem cannot be compared directly with the productivity patterns in the overlying surface waters. 5.4. Benthic foraminifera and particulate fluxes The present data on particulate fluxes to the sea floor, and on the seasonal variability of the benthic foraminiferal at six mid to outer shelf stations, allow us to verify the suggestion of our earlier paper (Duchemin et al., 2005), that the strong dominance in spring of the opportunistic taxon N. iridea, at stations D, B and C, could be a response to the late winter to early spring production maximum situated at the shelf-break. Our present data show that in spring, stations D and G present maximum fluxes of particulate organic carbon, chlorophyll-a and phaeopigments. However, there is a large difference between both stations in the particulate inorganic carbon flux, which is very high at station D (like in stations B and C), but shows a minimum value at station G. This suggests that station D (and to a lesser degree also stations B and C) receives an important flux of coccolithophorid remains (explaining the high inorganic carbon flux). Station G, on the contrary, could be under the influence of the late spring

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primary production cell off Brittany, apparently (because of the much lower inorganic carbon flux) being less dominated by coccolithophorids. 5.4.1. Spring maximum of N. iridea The very strong correspondence between the high spring inorganic carbon flux at stations D, B and C, and the strong dominance of N. iridea in the benthic foraminiferal faunas at these stations (in spring) strongly suggests a causal relationship between these two phenomena. In Fig. 10, where the relative proportion of inorganic carbon (with respect to the total, organic + inorganic, carbon flux) is plotted against the percentage of N. iridea, it can be seen that this species only dominates the faunas when the inorganic particulate flux heavily outweighs the organic particulate carbon flux. It appears therefore that this taxon is strongly favoured by a very specific type of phytodetritus remains, more precisely, by the relatively organic-poor remains of massive coccolithophorid blooms. N. iridea has previously been reported alive surrounded by a sediment cyst, and has been considered as dependent on a high organic carbon flux (Gooday, 1986; Mackensen, 1988). On the continental shelf of the Weddell Sea, it is related to a high sedimentary organic carbon content (Mackensen et al., 1990), while it is strongly linked to phytodetritus deposits in bathyal environments of the NE Atlantic (Gooday and Hughes, 2002). Moreover, at bathyal sites, it settles around the sediment surface, just below the phytodetritus aggregates (Gooday and Lambshead, 1989; Gooday and Hughes, 2002). At the outer shelf station E, also in the vicinity of the shelf-break chlorophyll-a maximum in spring, the sediment shows a much coarser grain size than in the “Grande Vasière” (stations A-D). The significantly lower percentage of N. iridea (11%) found at this station (in spring) suggests that this site is largely

Fig. 10. Graphical comparison between the relative abundance of Nonionella iridea and the percentage of particulate inorganic carbon from the particulate carbon flux.

bypassed by sedimentation processes, and does not serve as a major depocenter for the remains of the late winter to early spring phytoplankton blooms. The elevated percentages of the agglutinated taxa A. wrighti, S. flintii and T. agglutinans (in spring as well as in autumn) appear to be a response to the coarser sediment, and probably also to a much lower organic input as at our other outer shelf stations. The middle shelf station A shows relatively low flux values in spring. N. iridea is totally absent here, and the relatively poor fauna is rather equilibrated, without strongly dominant taxa. Also the inner shelf station G shows a very low percentage of N. iridea in spring (2.4%), suggesting that also this station is not significantly influenced by the spring phytoplankton blooms at the shelf break, which are strongly dominated by coccolithophorids. We think that the very high particulate organic particle flux observed at this station in spring, could be the result of the primary production maximum off Brittany (Fig. 2b, zone 2). The benthic foraminiferal fauna at station G is in spring strongly dominated by N. fabum and Textularia porrecta. The relatively low inorganic carbon flux suggests that these taxa are favoured by an introduction of phytodetritus with a lower contribution of coccolithophorids. N. fabum (= N. scaphum), in the southern part of the Bay of Biscay at 150 m water depth, appear to be typical for settings where labile organic matter is introduced into strongly dysoxic or anoxic environments (Fontanier et al., 2002). At the same site, Langezaal et al. (2006) suggest that this species could migrate toward fresh organic matter input. Textularia porrecta may be resilient to intermittent periods of bottom water dysoxia, typical of clay-belt settings in other coastal seas (van der Zwaan and Jorissen, 1991). 5.4.2. Composition of the foraminiferal faunas in summer and autumn Unfortunately, we do not dispose of flux data for summer. Although the outer shelf primary production maximum has disappeared, N. iridea is still the dominant faunal element at station D (but no longer at stations B and C). We suspect that the continuously high densities of this taxon at station D (it remains dominant in autumn) are the result of a long-term (∼ 6 months) eutrophication, resulting from the massive input of phytoplankton remains in spring, at this site relatively close to the surface water primary production maximum. In autumn, all flux levels are much lower for the “Grande Vasière” stations D, B, C and A. However, at station D the particulate inorganic carbon flux is 3 to 5 times higher than at stations B, C and A. The fact that N.

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iridea still dominates the faunas at station D (but no longer at stations B and C) once more suggests a linkage between this taxon and the remains of coccolithophorid blooms positioned at the shelf-break. At station G, the autumn flux values are much higher than at all other stations, suggesting that unlike the other 5, more offshore stations, this coastal station is influenced by coastal bloom events (Fig. 2f). The relatively poor faunas at this station have important contributions of N. fabum, H. balthica, Bolivina ex gr. B. dilatata and N. turgida. With the exception of H. balthica, these species have all been described in prodelta environments characterised by the input of important amounts of river-derived organic matter (e.g. Jorissen, 1987; Barmawidjaja et al., 1992, etc.). H. balthica has been described in sediments with a high organic matter content (Qvale and Van Weering, 1985). In the Canyon of Cap Breton, which receives an important advected flux of relatively low quality organic carbon this species is one of the dominant faunal elements (Hess et al., 2005). The relatively low autumn chlorophyll-a flux could indicate that also at our station G, the very elevated particulate organic carbon flux could at least partially be due to an admixture with continental, relatively low quality organic matter. At station A, the faunal density is maximum in autumn and summer. In these seasons, the fauna is strongly dominated by C. carinata and B. ex. gr. dilatata. Our flux data show that although the autumn particle fluxes are much lower than the spring fluxes, the chlorophyll-a, phaeopigments and particulate organic carbon fluxes are higher at station A than at the other mid shelf stations (B, C and D). This could indicate that station A still experiences the impact of the chlorophylla maximum off Brittany, which was maximum in early June (Fig. 2c). Cassidulina species have earlier been considered as opportunistic (Nees and Struck, 1999), and C. carinata, which in summer and autumn is dominant at stations A and C, has been described in areas with sustained food input on the continental shelf and open slope east of New Zealand (Hayward et al., 2002). At a French upper middle bathyal station (southern part of the Bay of Biscay, at 550 m water depth), this species appears to respond a reproductive event to a labile organic matter input following the spring phytoplankton bloom (Fontanier et al., 2003). Bolivina ex gr. dilatata shows fairly high relative densities at almost all stations, for the three investigated seasons. Also the latter species has been described as opportunistic (Schmiedl et al., 1997) and seems to be able to adapt to rapidly fluctuating organic matter fluxes and oxygen concentrations in bottom and pore waters

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(Barmawidjaja et al., 1992; Jorissen et al., 1992). Our data suggest that C. carinata and B. ex gr. dilatata may proliferate in settings where large amounts of phytodetritus, which a relatively limited contribution of coccolithophorids, are transported to the sea floor. 6. Conclusion The new data presented in this paper, concerning the particulate fluxes to the sea floor in spring and autumn, and the composition of the foraminiferal faunas in summer and autumn (and at two new stations, E and G), allow us to reconsider the possible coupling between surface water primary production events and the characteristics of the benthic foraminiferal faunas. On the basis of our present data set (spring and autumn flux data, spring, summer and autumn foraminiferal data), three different types of faunas can be recognized, that appear to respond to three types of primary production: (1) In spring, a major part of the benthic environments of the outer and middle continental shelf (our stations D, B, C and to a minor extent also E) appear to be influenced by phytodetritus input resulting from coccolithophorid blooms over the shelf break. The faunas in this area show a maximum density, and are strongly dominated by N. iridea. At station D this imprint remains visible until autumn. (2) Only station G presents a high particle flux in autumn. The faunas dominated by N. fabum, H. balthica, Bolivina ex gr. B. dilatata and N. turgida, could be indicative of the input of a relatively low quality organic matter, partially of continental origin. (3) At station A, faunal density is maximum in summer and autumn, when the faunas show high percentages of B. ex. gr. dilatata and C. carinata. These taxa could respond to phytodetritus input resulting from the late spring to early summer bloom off Brittany, less dominated by coccolithophorids. (4) Combining these observations, it appears that phytoplankton primary production and subsequent transport of phytodetritus the sea floor may be one of the principal parameters causing the variation of foraminiferal density and composition. Opportunistic species like N. iridea, C. carinata and B. ex. gr. dilatata are most reactive to the phytodetritus deposits. A first step occurs in late winter–early spring with the presence of phytoplankton blooms, mostly

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composed of coccolithophorids, over the shelf break. The remains of these blooms, relative rich in particulate inorganic carbon, appear to be highly profitable for N. iridea, that presents maximum densities at stations where the flux data indicate significant impact of the bloom remains. From late spring to early summer, phytoplankton blooms occur off Brittany, and their detritus could influence the more coastal areas close to stations G and A. These phytoplankton blooms are composed of dinoflagellates, coccolithophorids and diatoms. Their detrital remains appear to favour C. carinata and B. ex. gr. dilatata. In autumn, only station G appears to be under the influence of coastal bloom phenomena. N. fabum, H. balthica and N. turgida, who dominate the autumn faunas at this station, apparently profit from the admixture of continental organic matter. Acknowledgements Ocean colour data used in this study were produced by the SeaWiFS Project at Goddard Space Flight Centre. The data were obtained from the Goddard Earth Sciences Distributed Active Archive Centre under the auspices of the National Aeronautics and Space Administration. Use of this data is in accord with the SeaWiFS Research. Authors want also to thank Dr. Francis Gohin (IFREMER) for access to modified data of the Goddard Space Flight Centre. The authors have also special thanks to the captain and the crew the RV Thalassa and RV Côte de la Manche for their assistance during cruises. Two anonymous reviewers are thanked for their valuable comments. References Altenbach, A.V., 1988. Deep sea benthic foraminifera and flux rate of organic carbon. Rev. Paléobiol. 2, 719–720 Spec. Vol. Altenbach, A.V., 1992. Short term processes and patterns in the foraminiferal response to organic flux rates. Mar. Micropaleontol. 19, 119–129. Alve, E., Murray, J.W., 2001. Temporal variability in vertical distributions of live (stained) intertidal foraminifera, southern England. J. Foraminiferal Res. 31, 12–24. Barmawidjaja, D.M., Jorissen, F.J., Puskaric, S., Van der Zwaan, G.J., 1992. Microhabitat selection by benthic foraminifera in the Northern Adriatic Sea. J. Foraminiferal Res. 22, 297–317. Barnett, P.R.O., Watson, J., Connely, D., 1984. A multiple corer for taking virtually undisturbed sample from shelf, bathyal and abyssal sediments. Oceanol. Acta 7, 399–408. Bett, B.J., 1994. Sampler bias in the quantitative study of deep-sea meiobenthos. Mar. Ecol. Prog. Ser. 104, 197–203.

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