Spatial and temporal patterns of benthic macrofaunal communities on the deep continental margin in the Gulf of Guinea

Abstract

Density, taxonomic composition at higher taxon level and vertical distribution of benthic macrofaunal communities and sediment characteristics (pore water, nitrogen, organic carbon, sulfur, C/N ratio, n-alcohol biomarkers) were examined at three deep sites on the Congo–Gabon continental margin. This study was part of the multidisciplinary BIOZAIRE project that aimed at studying the deep benthic ecosystems in the Gulf of Guinea. Sampling of macrofaunal communities and of sediment was conducted during three cruises (January 2001, December 2001 and December 2003) at two downslope sites (4000 m depth), one located near the Congo submarine channel (15 km in the south) and the other one far from the channel (150 km in the South). The third area located 8 km north of the Congo channel in the surroundings of a giant pockmark at 3160 m depth was sampled during one cruise in December 2003.

At these three locations the macrofaunal communities presented relatively high densities (327–987 ind. 0.25 m⁻²) compared with macrofaunal communities at similar depths; that is due to high levels of food input related to the Congo river and submarine system activities that affect the whole study area. The communities were different from each other in terms of taxonomic composition at higher taxon level (phylum, class, order for all the groups except for the polychaetes classified into families). The polychaetes dominated the communities and were responsible for the increase in densities observed at both deep sites (4000 m) between January 2001 and December 2003 whereas the tanaidaceans, the isopods and the bivalves were the other most abundant taxa responsible for the spatial differences between these sites. The community at 3150 m differed from the two deep communities by higher abundances in bivalves, nemerteans and holothuroids. The composition of the polychaete community also differed among sites.

In the vicinity of the Congo channel, the expected positive effect of the additional organic matter transported through the turbiditic currents on to the surrounding benthic communities was not observed, as the increase in densities during the study period was higher at the site located away from the Congo channel than near the channel (80% vs 30%). That may be due to the low food value of the organic matter of terrestrial origin carried through the turbidites, and/or to the disturbance caused by these turbidites. Conversely, far from the channel the macrofaunal communities benefit from organic matter of higher energetic value originating mainly from marine sources, but also from continental sources, carried by the Congo plume or by near-bed currents across or along the continental slope. Spatial and temporal variability in trophic and physical characteristics of the sediment habitat at both deep sites also affected the vertical distribution of the macrofaunal communities.

The activities of the very active Congo system structure the deep macrofaunal communities on a large area in terms of densities, composition and vertical distribution. The food input is enhanced at regional scale as well as the heterogeneity of the sediment characteristics, mainly in terms of organic matter quality (marine vs terrigenous). In turn, the densities are enhanced as well as the regional diversity of the macrofaunal communities in terms of taxonomic composition and distribution.

Introduction

The general objectives of the multidisciplinary BIOZAIRE project, developed by Ifremer in partnership with the French oil company Total, were to discover, describe and understand the then unknown benthic ecosystems in the deep Gulf of Guinea (Sibuet et al., 2002).

The main feature of the Gulf of Guinea (west equatorial Africa) is the presence of one of the world's largest active submarine systems, the Congo system. The canyon is directly connected to the mouth of the Congo river, which is the second world river in terms of water discharges to the ocean, and that is responsible for a mean organic carbon export to the Atlantic Ocean of about 7% of the total world river exportations, with limited interannual variability (Coynel et al., 2005). The Congo submarine canyon deeply and sinuously incises the Congo–Angola continental shelf and slope (Babonneau et al., 2002; Savoye et al., 2000), westward down to the abyssal plain where it ends, forming a large deep sea fan 760 km from the river mouth at a water depth of 4800 m.

Canyons are regular features along most ocean margins and they appear to be important as sites of enhanced productivity and as conduits of continental detritus to the deep sea (Vetter and Dayton, 1998). The role of canyons in providing high quantities of organic material to the benthic communities has been investigated in many areas from shelves and slopes to the deep sea (Aller and Stupakoff, 1996; Duineveld et al., 2001; Gage et al., 1995; Houston and Haedrich, 1984; Lamont et al., 1995; Maurer et al., 1994; Rowe et al., 1982; Soetaert et al., 1991; Vetter and Dayton, 1998) mainly by comparing communities in canyons and in the surroundings. These authors found dense and diverse communities in canyons compared with the adjacent areas.

The Congo system is expected to influence the surrounding deep benthic ecosystems by providing trophic inputs originating from the continent and carried in the upper layer of water by its plume and in the benthic boundary layer by the episodic turbiditic currents that overflow the flanks of the channel (Khripounoff et al., 2003). The Congo plume is permanent with probably weak interannual and seasonal variations as observed for the Congo river discharge (Coynel et al., 2005); it has a large extension affecting the whole study area (Treignier et al., 2006; van Bennekom and Berger, 1984). Nutrient concentrations in the river plume are high due to direct river influence and to river-induced upwelling (van Bennekom et al., 1978). However, primary production in the river plume is reduced because of the large quantities of suspended matter clouding the waters. The turbiditic currents in the Congo submarine system are episodic. They directly affect the benthic habitat at least on a few kilometres on both sides of the Congo channel by bringing onto the seafloor high quantities of particulate matter of continental origin that have a low food value (Khripounoff et al., 2003; Treignier et al., 2006).

Moreover, Picaut (1983) and Voituriez and Herbland (1982) demonstrated that a highly seasonal coastal upwelling affects the study area and that it extends far offshore, enhancing the primary productivity in the photic layer in the whole area. In many deep ocean areas, seasonal or episodic pulses of fresh food to the deep sea have been shown to result in increasing benthic population numbers (Billett et al., 2001; Drazen et al., 1998; Galéron et al., 2001; Vanreusel et al., 2001), increasing total community metabolism and megafaunal activity (Bett et al., 2001), in variations in vertical distribution of infaunal communities (Galéron et al., 2001; Jumars et al., 1990; Witte, 2000), and in changes in the benthic community structure (Billett et al., 2001; Vanreusel et al., 2001).

In deep ocean basins, the organic matter fuelling the benthic communities has a predominantly marine origin as is provided mainly by the primary production in the photic water layer whereas continental margins receive organic matter of both marine and terrestrial origin. The origin (marine vs terrigenous) of the sedimentary organic matter is thought to have a pronounced influence on the benthic fauna (Dauwe et al., 1998).

In the present study, the influence of the Congo submarine system on the benthic communities is evaluated by assessing the spatial and temporal patterns of the macrofaunal community structure and vertical distribution in relation to the sediment composition at two downslope sites at 4000 m depth and at a third site at 3150 m depth, located in the vicinity of the Congo channel and of a very active pockmark system.