Live benthic foraminiferal faunas from the Bay of Biscay: faunal density, composition, and microhabitats

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Abstract

In the meso-oligotrophic Bay of Biscay, a diminishing downward organic matter flux with depth is accompanied by an important decrease of the live foraminiferal density. Although bottom water oxygenation is not directly influenced by organic matter input, the oxygenation of interstitial waters and the primary redox fronts do change in response to variations of the organic matter flux. The occurrence of deep and intermediate infaunal taxa can be linked to fundamental redox fronts and putative associated bacterial consortia. Our data are in agreement with the TROX-model, which explains the benthic foraminiferal microhabitat as a function of organic flux and benthic ecosystem oxygenation. Both the depth of the principle redox fronts and the microhabitat of deep infaunal species show important increases with depth. At the deepest oligotrophic stations, deep infaunal faunas become relatively poor. Therefore, the exported flux of organic matter appears to be the main parameter controlling the composition and the vertical distribution of benthic foraminiferal faunas below the sediment-water interface. The oxygenation of pore waters plays only a minor role. A species-level adaptation of the TROX-model is presented for the Bay of Biscay.

Introduction

Benthic foraminifera are an important component of the meiofaunal community of deep-sea detritus feeders. In deep-sea environments, they commonly represent more than 50% of the total biomass (Gooday et al., 1992). Thanks to their extraordinary potential of adaptation, benthic foraminifera are able to survive and proliferate in a wide range of marine environments, including extreme ecosystems, such as oligotrophic abyssal plains (Tietjen, 1971; Coull et al., 1977) or hydrothermal vents (Sen Gupta and Aharon, 1994). Because of their potentially important role in deep ocean environments, they are at present studied intensively for a better understanding of their role in the benthic ecosystem and for a more precise definition of their contribution to the recycling of organic matter at the ocean floor.

There is a general consensus that the faunal composition of heterotrophic benthic foraminiferal faunas is strongly linked to the quantity and quality of the organic detritus reaching the ocean floor, and to the oxygenation at the sediment-water interface and of the interstitial waters in the first cm of the sediment (e.g. Van der Zwaan, 1982; Altenbach and Sarnthein, 1989; Loubere et al., 1993; Fariduddin and Loubere, 1997; Jorissen et al., 1998). In this context, this paper describes the variability of live (Rose Bengal stained) benthic foraminiferal assemblages along a transect in the Bay of Biscay comprising five stations from 140 to 2000 m water depth (Table 1, Fig. 1). Since 1997, a number of stations in this depth range have been sampled periodically in order to follow the temporal and spatial succession of the benthic foraminiferal faunas.

The Bay of Biscay is a typical temperate meso-oligotrophic environment with two annual bloom periods. The first one, in late winter or early spring, is associated with a shallowing of the mixed layer and with a strengthening of the rather shallow thermocline. Moreover, internal waves associated with strong spring neap tides cause nutrient injection into the photic zone and subsequent phytoplankton blooms at the shelf break in the northern part of the basin (Pingree et al., 1986). The second main bloom event, less marked than the first one, is commonly recorded in autumn, when the summer thermocline starts to be eroded by a deepening of mixing. The advantage of a study of benthic foraminiferal ecosystems in this mesotrophic-oligotrophic context is the fact that the oxygenation at the sediment-water interface is not seriously influenced by the seasonal variability of the organic matter input. The bottom water oxygen concentrations at the five stations are always relatively high (Table 1), and the seasonal monitoring of the oxygenation indicates that the concentrations do not vary significantly throughout the year (Anschutz et al., 1999; Hyacinthe et al., 2001). Therefore, ecosystem variability is probably caused mainly by changes in the organic flux reaching the ocean bottom. The downward organic flux varies with water depth and in response to the temporal and spatial oscillations of primary production in the surface waters. The five stations discussed in this paper were all sampled in late autumn-early winter (of the years 1997 and 1998; Table 1). The stations were selected in order to better understand the influence of the spatially variable organic matter flux reaching the ocean bottom on benthic foraminiferal faunas in an open slope setting. Our study focuses on the faunal density and composition and on foraminiferal microhabitats. The microhabitat is defined as the vertical distribution of a taxon in the first cms of the sediment, which is controlled by the composite action of all physical, chemical and biological processes (Corliss, 1985). Understanding the foraminiferal microhabitat is important, because it allows the precise trophic and oxic requirements of each species in the total live benthic foraminiferal assemblage to be known.

The main objectives of this paper are (1) to present and discuss variation of the foraminiferal density with increasing water depth and thus with calculated diminishing organic fluxes, (2) to better explain the compositional changes observed along the five stations bathymetrical/trophic transect, and (3) to explain the microhabitat changes in response to the trophic conditions at the sediment-water interface.

Section snippets

Study area, material and methods

The Bay of Biscay is a semi-enclosed basin at the eastern side of the North Atlantic Ocean, bathed by rather homogeneous oceanic waters belonging to the North Atlantic thermohaline and geostrophic circulation and more precisely, to the North Atlantic Drift. The Bay of Biscay is bordered by the Irish shelf in the north, the Armorican and Aquitaine shelves in the east, and by the Iberian shelf in the south. The hydrographical structure is relatively well known (Ogawa and Tauzin, 1973). The

Results

The total density of the live foraminiferal fauna were determined by integrating the numbers of live individuals picked in all levels from 0 to 10 cm depth. It is expressed as the number of live foraminifera found in and below a 72 cm2 surface area (Fig. 2). Concerning vertical profiles, specific foraminiferal densities were normalised for each layer to a 50 cm3 sediment volume; we generally regrouped the four uppermost slices of each core into two 0.5 cm thick samples. The percentages of the

Benthic ecosystem redox conditions

Bottom water oxygen concentrations for our five stations vary between 4.36 and 5.85 ml/l (Table 1). These values agree very well with the data which have been published for the various water masses in the Bay of Biscay (Ogawa and Tauzin, 1973). The minimum values encountered at station A are typical of the Mediterranean outflow waters. Measurements of bottom water oxygenation during 10 successive sampling campaigns show only minor seasonal and interannual changes for our five stations (Anschutz

Conclusions

  • 1.

    The dissolved oxygen concentration of the bottom waters is not seriously influenced by the exported organic flux. The interstitial waters, on the contrary, show a clear linkage: at the shallowest site (D), high and relatively constant organic matter inputs result in a redox front close to the sediment-water interface. At the deeper sites, the zero oxygen level is found much deeper in the sediment, as a direct result of the decreasing organic flux.

  • 2.

    The flux of organic matter to the ocean floor is

Acknowledgements

We thank the crews of Côte de la Manche for good collaboration during the OXYBENT cruises. We are grateful to two anonymous reviewers for their helpful comments on the original manuscript. This work has been carried out within the framework of the French national program PROOF.

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    1

    Present address: Angers University, Dept. Geology, UPRES EA 2644, 2 Boulevard Lavoisier, 49045 Angers, France.

    2

    Present address: Alfred Wegener lust. for Polar and Marine Research, Columbusstrasse, D-27515 Bremerhaven, Germany.

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