Southern Ocean) Katrin Linsea’*, Angelika Brandtb, Jens M
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Available onlinewww.seiencedirect.com at ------------------------------ *%" ScienceD irect deepsea research P a r t I I ELSEVIER Deep-Sea Research II 54 (2007) 1848-1863 www.elsevier.com/locate/dsr2 Macro- and megabenthic assemblages in the bathyal and abyssal Weddell Sea (Southern Ocean) Katrin Linsea’*, Angelika Brandtb, Jens M. Bohnc, Bruno Danisd, Claude De Broyerd, Brigitte Ebbe6, Vincent Heterierf, Dorte Janussen8, Pablo J. López González11, Myriam Schüller1/ Enrico Schwabe6, Michael R.A. Thomson1 aBritish Antarctic Survey, Natural Environmental Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK bZoologisches Institut und Museum, Universität Hamburg, Martin-Luther-King Platz 3, D-20147 Hamburg, Germany c Zoologische S taats Sammlung München, Münchhausens tr. 21, D-81247 München, Germany dRoyal Belgian Institute of Natural Sciences, Rue Vautier 29, B-1000 Bruxelles, Belgium eForschungsinstitut Senckenberg, DZMB-CeDAMar, c/o Forschungsmuseum König, Adenauerallee 160, D-53113 Bonn, Germany fUniversité Libre de Bruxelles, Laboratoire de Biologie Marine, CP 160/15, 50 av. F.D. Roosevelt, B-1050 Bruxelles, Belgium gForschungsinstitut und Naturmuseum Senckenberg, Sektion Marine Evertebraten I, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany hDepartamento de Fisiología y Zoología, Facultad de Biología, Universidad de Sevilla, Avda. Reina Mercedes 6, E-41012 Sevilla, Spain íSchool of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Accepted 6 July 2007 Available online 3 August 2007 Abstract The assemblages inhabiting the continental shelf around Antarctica are known to be very patchy, in large part due to deep iceberg impacts. The present study shows that richness and abundance of much deeper benthos, at slope and abyssal depths, also vary greatly in the Southern and South Atlantic oceans. On the ANDEEP III expedition, we deployed 16 Agassiz trawls to sample the zoobenthos at depths from 1055 to 4930 m across the northern Weddell Sea and two South Atlantic basins. A total of 5933 specimens, belonging to 44 higher taxonomic groups, were collected. Overall the most frequent taxa were Ophiuroidea, Bivalvia, Polychaeta and Asteroidea, and the most abundant taxa were Malacostraca, Polychaeta and Bivalvia. Species richness per station varied from 6 to 148. The taxonomic composition of assemblages, based on relative taxon richness, varied considerably between sites but showed no relation to depth. The former three most abundant taxa accounted for 10-30% each of all taxa present. Standardised abundances based on trawl catches varied between 1 and 252 individuals per 1000 m2. Abundance significantly decreased with increasing depth, and assemblages showed high patchiness in their distribution. Cluster analysis based on relative abundance showed changes of community structure that were not linked to depth, area, sediment grain size or temperature. Generally abundances of zoobenthos in the abyssal Weddell Sea are lower than shelf abundances by several orders of magnitude. © 2007 Elsevier Ltd. All rights reserved. Keywords: Macrofauna; Megafauna; Benthos; Deep-sea; Antarctica; South Atlantic * Corresponding author. Tel.: + 44 1223 221 631; fax: +441223 221259. E-mail address: [email protected] (K. Linse). 0967-0645/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.07.011 K. Linse et aí / Deep-Sea Research I I 54 (2007) 1848-1863 1849 1. Introduction tion are much better known (e.g., Arnaud et al., 1998; Arntz et al., 1994, 2005; Dayton et al., 1994; In the last three decades, since the discoveries of Ramos, 1999; Voß, 1988). To date most studies of abyssal hydrothermal vents and manganese no abundance in shelf communities and assemblages dules, scientific and commercial interest in studying have focussed on gaining quantitative assessments the global deep oceans has increased greatly (e.g., of soft-bottom habitats (Gambi and Bussotti, 1999; Bluhm, 1994; Decraemer and Gourbault, 1997; Gerdes et al., 1992, 2003; Lovell and Tregi, 2003; Lambshead et al., 2002; Tyler et al., 2002; Van Piepenburg et al., 2002; Saiz-Salinas and Ramos, Dover et al., 2003; Van Dover and Lutz, 2004). Sites 1999; Saiz-Salinas et al., 1997). Macrobenthic in the deep North Atlantic and Pacific oceans have community abundance assessments using semi- especially become the focus of long-term projects, quantitative methods (dredges, sledges and trawls) and what started as descriptive research there has have been undertaken by Voß (1988) in the Weddell moved into process-orientated investigations (Bett Sea, by Arnaud et al. (1998) in the South Shetland et al., 2001; Billett et al., 2001; Narayanaswamy Islands, and by Rehm et al. (2006) in the Ross Sea. et al., 2005). Much less is known about the deep-sea Barry et al. (2003) analysed the shelf and upper assemblages of the Arctic, Indo-Pacific and South slope assemblages in the Ross Sea by using towed ern oceans (Bluhm et ah, 2005; Brandt et ah, 2004a; camera footage. Linse et al. (2002) investigated the Ingole, 2003; Kröncke, 1998; Wlodarska-Kowalczuk suprabenthic fauna in the Weddell Sea and the et al., 2004). About half the world’s surface is South Shetland Islands. On many Antarctic benthic abyssal yet only tiny areas have been visited and we expeditions, the relative abundances of macro- and know very little of the biodiversity and abundance megabenthic taxa were assessed on variable of animals there (Rex et ah, 2006). One of the least- point classifications from absent to very abundant known abyssal areas surrounds Antarctica, the deep (Allcock et al., 2003; Arnaud et al., 1998; Arntz and Southern Ocean. Gutt, 1997, 1999; Arntz and Brey, 2003; Arntz et al., For more than a century, deep-water samples 2006) but no numerical data were collected. have occasionally been taken in the Southern During ANDEEP III, the faunal assemblages Ocean. Most of these studies, such as the Russian collected by Agassiz trawl were assessed by higher expeditions with R.V.s Ob, Akademik Kurchatov taxon classification and numerical data taken and Dmitriy Mendeleev (Malyutina, 2004 and allowing comparison with faunal assemblages from references therein) and American expeditions with the Antarctic shelf. This paper is the first attempt to USNS Eltanin and R.V. Hero (Dell, 1990), con describe deep-sea mega- and macrobenthic assem centrated on describing and discovering species. blages of the Weddell Sea and their abundances. Assessments of macro- or megafaunal abundances, community structure or richness levels were see 2. Material and methods mingly not considered. The recent ANDEEP expeditions to the Antarctic and South Atlantic 2.1. Study area have greatly increased our knowledge of faunal abundances in the deep sea (Brandt et al., 2004b). Four study regions were selected, but the main During the ANDEEP I and II expeditions, benthic focus was on the Powell Basin and the Weddell fauna was sampled in bathyal and abyssal depths Basin of the Weddell Sea, and their slopes (Fig. 1). (1121-6348m) of the Shackleton Fracture Zone, the Two comparative samples were taken further north northern Weddell Sea Basin, and the South in the adjacent Agulhas and southern Cape Basins, Sandwich Islands. However, most studies have been which are separated from each other by the Agulhas restricted to specific taxonomic groups (Brandt Ridge. The major South Atlantic deep-sea basins et al., 2004b; Cornelius and Gooday, 2004; Linse, started forming during Jurassic and Cretaceous 2004) or meiofauna (Gutzmann et al., 2004; times in connection with the Gondwana break-up Vanhove et al., 2004) and macrofauna (Blake and and seafloor spreading (Brandt et al., 2004a, 2007; Narayanaswamy, 2004). Information about deep Lawver and Gahagan, 2003). The Weddell Basin is megabenthic assemblages, communities and abun separated from the northerly basins by the South dances across taxa is still scarce (Brandt, 2005). In west India Ridge (LaBrecque, 1986). The Powell contrast to the nearly unknown deep sea, the Basin on the western side of the Weddell Sea was Antarctic shelf fauna and its community composi formed in the Tertiary by geological processes 1850 K. Linse et a!. / Deep-Sea Research I I 54 (2007) 1848-1863 80°W 70" 60" 50° 40" 30° 20° 10° 0° 10° 20° 30°E South Africa C ape Basin 40° S - *016-11 - 40° S Aghulas Ridge *021-8 Agulhas Basin 50° - - 50° Southwest Indian Ridge Scotia Sea 60° - 60° 150-7 * • 151-1 142-6 ; . 121-7* Weddell Basin 081-9 70° S 0 8 0 -6I # - 70° S 078-11 Dronning Maud Land 80°W 70° 60° 50° 40° 30°/ 20° 10° 0° 10° 20° 30°E Fig. 1. Locations of the Agassiz trawl stations sampled during ANDEEP III in the Southern Ocean and South Atlantic. opening the Drake Passage and tectonic movements in the Southern Ocean during the PFS Polarstern in the Scotia Sea (Lawver and Gahagan, 2003; expedition ANT XXII/3 WECCON 2005— Mitchell et al., 2000). ANDEEP III in January-April 2005 (Fahrbach, The oceanography of the deep South Atlantic 2006) (Table 1; Fig. 1). The sample depths ranged seafloor is defined by its prominent water mass, the from 1047 to 4931 m, sampling continental slopes of Antarctic Bottom Water (Tomczak and Godfrey, the eastern Weddell Sea (off Kapp Norvegia) 2001). The Antarctic Bottom Water expands north and western Weddell Sea and the South Orkney wards into the Atlantic basins east and west of the Islands, and deep Cape, Agulhas, Weddell and Mid-Atlantic Ridge, like the Agulhas Basin, but can Powell Basins (Fig. 1). At the stations 074-7, only enter the basins north of the Walvis Ridge 078-11 and 081-9, the cod end mesh size was (e.g., Cape Basin) via the northerly Romanche 10 mm, while at all other stations, an inlet of Fracture Zone. The Weddell Sea Bottom Water 500 pm mesh size was inserted. The 500 pm mesh (WSBW), defined by a temperature of —0.7 °C and size was used because of smaller adult size of deep- a salinity of 34.64 ppt (Orsi et ah, 1993), is the sea macrobenthos compared to shelf macrobenthos main water mass above the Weddell Sea benthos (Gray, 2002).