Maud Rise – a snapshot through the water column

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Abstract

The benthic fauna was investigated during the expedition ANT-XXIV/2 (2007/08) in relation to oceanographic features, biogeochemical properties and sediment characteristics, as well as the benthic, pelagic and air-breathing fauna. The results document that Maud Rise (MR) differs distinctly from surrounding deep-sea basins investigated during previous Southern Ocean expeditions (ANDEEP 2002, 2005). Considering all taxa, the overall similarity between MR and adjacent stations was low (∼20% Bray-Curtis-Similarity), and analyses of single taxa show obvious differences in species composition, abundances and densities. The composition and diversity of bivalves of MR are characterised by extremely high abundances of three species, especially the small sized Vesicomya spp. Exceptionally high gastropod abundance at MR is due to the single species Onoba subantarctica wilkesiana, a small brooder that may prey upon abundant benthic foraminiferas. The abundance and diversity of isopods also show that one family, Haplomunnidae, occurs with a surprisingly high number of individuals at MR while this family was not found at any of the 40 bathyal and abyssal ANDEEP stations. Similarly, polychaetes, especially the tube-dwelling, suspension-feeder fraction, are represented by species not found at the comparison stations. Sponges comprise almost exclusively small specimens in relatively high numbers, especially a few species of Polymastiidae. Water-column sampling from the surface to the seafloor, including observations of top predators, indicate the existence of a prospering pelagic food web. Local concentrations of top predators and zooplankton are associated with a rich ice-edge bloom located over the northern slope of MR. There the sea ice melts, which is probably accelerated by the advection of warm water at intermediate depth. Over the southern slope, high concentrations of Antarctic krill (Euphausia superba) occur under dense sea ice and attract Antarctic Minke Whales (Balaenoptera bonaerensis) and several seabird species. These findings suggest that biological prosperity over MR is related to both oceanographic and sea-ice processes. Downward transport of the organic matter produced in the pelagic realm may be more constant than elsewhere due to low lateral drift over MR.

Introduction

Maud Rise is a seamount, a name first used officially by the US Board on Geographic Names in 1938, when the “Davidson Seamount” was named (Pitcher et al., 2007). Since then more than 14000 seamounts have been mapped from satellite altimetry (Kitchingman et al., 2007). Some authors estimate there may be 1 000 000 seamounts globally (Pitcher et al., 2007). Seamounts are usually of volcanic origin. They are often located along the mid-ocean ridges, along tectonic plates or transform vaults, or along the “hot spots” at subduction zones (Wessel, 2007). The first seamount was biologically investigated by Hubbs (1959). According to the latest evaluation of their zoogeographical importance (Brewin et al., 2007), a total of 232 seamounts have been biologically investigated so far. (Samadi et al., 2007). Nevertheless, most of the seamounts have only been sampled partly or sporadically (e.g. Stocks and Hart, 2007), even despite their increasing importance for fisheries research (Pitcher et al., 2007, Proelß, 2007). Therefore, data on the invertebrate distribution on seamounts are scarce (Samadi et al., 2007). In general, seamounts should be investigated because they can play a role as “stepping stones” (Hubbs, 1959), for the distribution of benthic species, as well as be submarine “islands” harbouring isolated and unique fauna (de Forges et al., 2000).

Maud Rise (MR) is located in the eastern Lazarev Sea, at roughly 65°S, 3°E, and about 700 km north of the Antarctic continent (see introduction of this volume). Its elevation rises almost 3000 m from the sea floor with its crest located less than 1000 m below the sea surface. Based on the plate tectonic reconstructions, Martin and Hartnady (1986) predicted that MR was once connected to the Agulhas Plateau and the Mozambique Plateau. Its final generation then occurred through the ridge jumps associated with the enhanced volcanic activity during the South Atlantic seafloor spreading, around 84–94 Ma (Schandl et al., 1990, Martin and Hartnady, 1986, Kim et al., 2005).

Schmitz et al. (1996) investigated the latest Palaeocene benthic extinction on the Southern Tethyan shelf and found that changes in deep-sea benthic foraminiferal assemblages were perhaps the most dramatic during the past 90 Ma. The most detailed studies of this event have been performed on MR during the Ocean Drilling Program (Hole 690B). According to Kennett and Stott (1991) the benthic foraminiferal mass-extinction took place during the initial stages of a short period (>100 ka) of unusual deep-water circulation. During this event, the temperature of MR bottom waters (∼2100 m palaeo-depth) rose from 9 °C to 16 °C, and δ13C values declined by 2.5‰ (Schmitz et al., 1996). These changes may have been related to a short-term change in the loci of global deep-water formation from the southern high latitudes to the subtropical regions. During the onset of rapid cooling of Antarctica in the Eocene, Southern Ocean (SO) deep water lay at intermediate depths of MR. While tracing the Pacific seawater with a radiogeneric εND, Scher and Martin (1996) suggested that the Weddell Gyre was absent or little organised at that time, a proto-front existed near MR partially separating it from water masses of the Agulhas Ridge. Diester-Hass and Zahn (1996) describe large variations in the palaeo-productivity and microfossil distribution explaining these patterns by the formation of such a proto-front. Also, during the middle and late Eocene a significant rise in productivity and organic carbon burial is reported (Scher and Martin, 1996). Scher and Martin (1996) correlate this with a possible early and successive deep opening of the Drake Passage, which may have caused a change in the ocean circulation and enhanced upwelling.

Despite ongoing changes in the SO oceanographic climate, these early events most likely had a major influence on the development of the faunal composition and distribution found at MR today.

MR has attracted much interest in the oceanographic community since the mid 1970s as a potential primary cause for a large polynya discovered through satellite images of the sea-ice cover in the Weddell Sea (the Weddell Polynya) (Zwally and Gloersen, 1977, Carsey, 1980, Comiso and Gordon, 1987). The large Weddell Polynya first formed in the vicinity of MR and then slowly propagated westward into the central Weddell Sea during the next two winters. While a polynya of the size of the Weddell Polynya has never been seen since, sea-ice coverage in the MR region is typically reduced and seasonally the first in the Weddell Sea to dissipate (De Steur et al., 2007).

MR stands in the way of the south-eastern limb of the Weddell Gyre (Gordon et al., 1981), with which relatively warm and salty Circumpolar Deep Water (CDW) is advected from the Antarctic Circumpolar Current polewards into the Weddell Sea together with the re-circulating Warm Deep Water (WDW) (Schröder and Fahrbach, 1999). Potential vorticity conservation forces the deep-reaching ocean currents to circumvent MR. This forms a so-called “Taylor column”, a rather stagnant water column about 150–200 km wide, lying on top of the seamount. Interaction of the mean flow with topography paves the way for an upward flux of heat and salt from the CDW/WDW (temperature maximum) between 200 and 500 m depth (Bagriantsev et al., 1989). This ultimately reduces the sea-ice coverage in the MR region (Bersch et al., 1992). A warm halo consisting largely of CDW and WDW is associated with the Taylor column (Muench et al., 2001). According to De Steur et al. (2007), the halo features elevated isopycnals and strong subsurface velocities (up to 20 cm s−1) in the northern MR, as well as semi-permanent warm pools to the west. An isopycnic ocean model was applied to a variety of parameters including the shape of the seamount, inflow parameters and vertical stratification. The model shows “(1) that the dynamics of the warm-water halo with a shallow mixed layer are related to the formation of a jet, surrounding the Rise and the overlying Taylor column, and (2) that eddies of alternating sign (cyclones and anticyclones) are formed from instability of the jet-like flow structure, being subsequently shed from the western flanks of the Rise” (De Steur et al., 2007: 871). This finding is consistent with Holland (2001), who explained the Weddell Polynya by oceanic flow past the MR seamount, which causes a horizontal cyclonic eddy to be shed from its northwest flank, as well as in general agreement with the studies conducted by Beckmann et al. (2001) and Lindsay et al. (2008). The ultimate reason for the large Weddell Polynya occurrence is probably deep convection. Evidence of deep convection extending to below 3000 m, associated with a mesoscale eddy, roughly 10 degrees of longitude west of MR, was indeed found in 1977 (Gordon, 1978), but never since.

According to its location within the south-eastern limb of the Weddell Gyre, with its south-westward currents, the sedimentation at MR is characterised by pelagic phytodetritus. Antarctic ice-rafted detritus plays only a minor role. In relation to the central Weddell Sea, biogenic surface production, highest in the MR region, is associated with the local hydrographic features (van Bennekom et al., 1988, Nöthig et al., 1991). Surface productivity over MR remains into late autumn and early winter (Spiridonov et al., 1996). In contrast, sedimentation rates are rather low with 0.4 - 0.9 cm/1000a, and in surface sediments siliceous oozes from diatoms and radiolarians dominate (Barker et al., 1988).

Cordes (1990) described sedimentology and palaeomagnetism of MR from gravity core samples (station PS14/58-1). His core samples were taken at 2480 m depth, 73 km north from the position of our data (Station PS71/39). According to Cordes (1990), the surface sediment was dominated by 60–80% foraminiferous oozes with sub-fractions of diatom ooze. The sandy sediment exhibits a moderate porosity with values around 0.5 throughout the first 60 cm. Between 1 and 3 m sediment depth, the porosity drops to values below 0.4 and increases to >0.8 underneath. The sediment is well-mixed by bioturbation at the surface. Burrow tubes and relicts of such structures ascribed to planolites (Cordes, 1990) were found at 20 cm depth and below. The organic carbon content is relatively low with 0.1–0.3% decrease from the sediment surface downward.

Enrichment of pelagic systems near seamounts may occur through water mixing, leading to an increased productivity and can be observed throughout the food chain (e.g. Yen et al., 2004, Ballance et al., 2006, Bourne, 1992, Doksaeter et al., 2008). Phytoplankton blooms potentially sustaining a locally increased abundance of pelagic life are favoured by the advection from the Weddell Gyre in combination with advection round the northern flank of MR seamount (Holland, 2001). For instance, evidence for increased abundances in the vicinity of MR have been reported for copepods (Bathmann et al., 1993), Antarctic krill (Euphausia superba; Atkinson et al., 2009, Meyer et al., 2010), and mesopelagic fish and their larvae (Flores et al., 2008). An additional aim of this study is to link concentrations of pelagic and benthic communities to the hydrographic peculiarity of MR.

Until recently, little to nothing has been known about neither the benthos from MR nor its features from surface down to the seafloor. This study presents the first description and characterisation of MR's oceanography, sedimentology, and fauna from the near-surface down to the benthos. Based on experience from seamounts worldwide and the peculiar oceanography of the MR, and despite the often wide distribution of Antarctic deep-sea species, we hypothesise a unique faunal composition for MR in comparison to the deep Weddell Sea. In addition, we analyse the communities of selected higher taxa at MR in the context of knowledge on the SO deep-sea benthos accumulated during the last decades (e.g., projects ANDEEP I-III and EASIZ).

By relating benthos community data to local hydrography and sedimentology as well as the composition and density of pelagic life from the surface to bathypelagic depths, this paper aims to evaluate the biological peculiarity of MR compared to the surrounding deep sea, and discusses potential connections between the benthic and pelagic systems.

Section snippets

Sampling of the different marine realms

Sampling has been summarised in detail in Bathmann (2010), and a station map is included in the general introduction to this volume.

Circulation and water mass distribution

Currents measured with the VM-ADCP along the 3°E, 0°E and 3°W transects revealed two current cores with maximum jets of∼20 cm s−1 at the northern and the north-western flanks of MR (Fig. 1). These jets were >30 nm (>50 km) wide and appeared centred at the deepest part of the topographic slope, approximately oriented with the 4500 m depth contour. Elsewhere in the area the flow was either sluggish or apparently dominated by mesoscale eddies. One of those eddy-like features, of cyclonic nature and of

Oceanography

The oceanographic observations made during the Polarstern expedition ANT-XXIV/2 agree with the majority of the earlier findings regarding MR hydrography, in particular with the formation of a Taylor column above the seamount (e.g., Muench et al., 2001). Our measurements confirm the existence of a topographically steered jet at the northern and north-western flanks, with which warm water is advected around the seamount and thus forms the warm halo. The water on top of the rise was less affected

Conclusions and future directions

This paper is the first attempt of an overall biodiversity assessment through size classes of the most frequently occurring faunal taxa in an Antarctic deep-sea ecosystem. Maud Rise has been selected because of some obvious faunistic peculiarities. The holistic approach of a synoptic sampling at Maud Rise, including physical and biological oceanography, ice-associated fauna from the top predators through to phyto- and zooplankton, from different water layers, down to the benthic species

Acknowledgments

The authors are grateful to the BMBF, the DFG, and the Helmholtz Foundation and the Marx-foundation for the support of the expedition with RV Polarstern, including travelling costs for D.J. We thank the captain and crew of RV Polarstern during ANT-XXIV/2 for all their support. Dr. K. George is thanked for general discussions with regard to the biology of seamounts. Participation of IMARES in this project was funded by the Netherlands AntArctic Programme (NAAP; project nr ALW-NWO 851.20.011),

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