Macro- and megabenthic assemblages in the bathyal and abyssal Weddell Sea (Southern Ocean)

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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 Ebbee, Vincent Heterierf, Dorte Janusseng, Pablo J. Lo´pez Gonza´lezh, Myriam Schu¨llerf, Enrico Schwabec, Michael R.A. Thomsoni a

British Antarctic Survey, Natural Environmental Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK b Zoologisches Institut und Museum, Universita¨t Hamburg, Martin-Luther-King Platz 3, D-20147 Hamburg, Germany c Zoologische Staatssammlung Mu¨nchen, Mu¨nchhausenstr. 21, D-81247 Mu¨nchen, Germany d Royal Belgian Institute of Natural Sciences, Rue Vautier 29, B-1000 Bruxelles, Belgium e Forschungsinstitut Senckenberg, DZMB-CeDAMar, c/o Forschungsmuseum Ko¨nig, Adenauerallee 160, D-53113 Bonn, Germany f Universite´ Libre de Bruxelles, Laboratoire de Biologie Marine, CP 160/15, 50 av. F.D. Roosevelt, B-1050 Bruxelles, Belgium g Forschungsinstitut und Naturmuseum Senckenberg, Sektion Marine Evertebraten I, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany h Departamento de Fisiologı´a y Zoologı´a, Facultad de Biologı´a, Universidad de Sevilla, Avda. Reina Mercedes 6, E-41012 Sevilla, Spain i School 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. r 2007 Elsevier Ltd. All rights reserved. Keywords: Macrofauna; Megafauna; Benthos; Deep-sea; Antarctica; South Atlantic

Corresponding author. Tel.: +44 1223 221 631; fax: +44 1223 221259.

E-mail address: [email protected] (K. Linse). 0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.07.011

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1. Introduction In the last three decades, since the discoveries of abyssal hydrothermal vents and manganese nodules, scientific and commercial interest in studying the global deep oceans has increased greatly (e.g., Bluhm, 1994; Decraemer and Gourbault, 1997; Lambshead et al., 2002; Tyler et al., 2002; Van Dover et al., 2003; Van Dover and Lutz, 2004). Sites in the deep North Atlantic and Pacific oceans have especially become the focus of long-term projects, and what started as descriptive research there has moved into process-orientated investigations (Bett et al., 2001; Billett et al., 2001; Narayanaswamy et al., 2005). Much less is known about the deep-sea assemblages of the Arctic, Indo-Pacific and Southern oceans (Bluhm et al., 2005; Brandt et al., 2004a; Ingole, 2003; Kro¨ncke, 1998; Wlodarska-Kowalczuk et al., 2004). About half the world’s surface is abyssal yet only tiny areas have been visited and we know very little of the biodiversity and abundance of animals there (Rex et al., 2006). One of the leastknown abyssal areas surrounds Antarctica, the deep Southern Ocean. For more than a century, deep-water samples have occasionally been taken in the Southern Ocean. Most of these studies, such as the Russian expeditions with R.V.s Ob, Akademik Kurchatov and Dmitriy Mendeleev (Malyutina, 2004 and references therein) and American expeditions with USNS Eltanin and R.V. Hero (Dell, 1990), concentrated on describing and discovering species. Assessments of macro- or megafaunal abundances, community structure or richness levels were seemingly not considered. The recent ANDEEP expeditions to the Antarctic and South Atlantic have greatly increased our knowledge of faunal abundances in the deep sea (Brandt et al., 2004b). During the ANDEEP I and II expeditions, benthic fauna was sampled in bathyal and abyssal depths (1121–6348 m) of the Shackleton Fracture Zone, the northern Weddell Sea Basin, and the South Sandwich Islands. However, most studies have been restricted to specific taxonomic groups (Brandt et al., 2004b; Cornelius and Gooday, 2004; Linse, 2004) or meiofauna (Gutzmann et al., 2004; Vanhove et al., 2004) and macrofauna (Blake and Narayanaswamy, 2004). Information about deep megabenthic assemblages, communities and abundances across taxa is still scarce (Brandt, 2005). In contrast to the nearly unknown deep sea, the Antarctic shelf fauna and its community composi-

1849

tion are much better known (e.g., Arnaud et al., 1998; Arntz et al., 1994, 2005; Dayton et al., 1994; Ramos, 1999; VoX, 1988). To date most studies of abundance in shelf communities and assemblages have focussed on gaining quantitative assessments of soft-bottom habitats (Gambi and Bussotti, 1999; Gerdes et al., 1992, 2003; Lovell and Tregi, 2003; Piepenburg et al., 2002; Saiz-Salinas and Ramos, 1999; Saiz-Salinas et al., 1997). Macrobenthic community abundance assessments using semiquantitative methods (dredges, sledges and trawls) have been undertaken by VoX (1988) in the Weddell Sea, by Arnaud et al. (1998) in the South Shetland Islands, and by Rehm et al. (2006) in the Ross Sea. Barry et al. (2003) analysed the shelf and upper slope assemblages in the Ross Sea by using towed camera footage. Linse et al. (2002) investigated the suprabenthic fauna in the Weddell Sea and the South Shetland Islands. On many Antarctic benthic expeditions, the relative abundances of macro- and megabenthic taxa were assessed on variable point classifications from absent to very abundant (Allcock et al., 2003; Arnaud et al., 1998; Arntz and Gutt, 1997, 1999; Arntz and Brey, 2003; Arntz et al., 2006) but no numerical data were collected. During ANDEEP III, the faunal assemblages collected by Agassiz trawl were assessed by higher taxon classification and numerical data taken allowing comparison with faunal assemblages from the Antarctic shelf. This paper is the first attempt to describe deep-sea mega- and macrobenthic assemblages of the Weddell Sea and their abundances. 2. Material and methods 2.1. Study area Four study regions were selected, but the main focus was on the Powell Basin and the Weddell Basin of the Weddell Sea, and their slopes (Fig. 1). Two comparative samples were taken further north in the adjacent Agulhas and southern Cape Basins, which are separated from each other by the Agulhas Ridge. The major South Atlantic deep-sea basins started forming during Jurassic and Cretaceous times in connection with the Gondwana break-up and seafloor spreading (Brandt et al., 2004a, 2007; Lawver and Gahagan, 2003). The Weddell Basin is separated from the northerly basins by the Southwest India Ridge (LaBrecque, 1986). The Powell Basin on the western side of the Weddell Sea was formed in the Tertiary by geological processes

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K. Linse et al. / Deep-Sea Research II 54 (2007) 1848–1863

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 Scotia Sea (Lawver and Gahagan, 2003; Mitchell et al., 2000). The oceanography of the deep South Atlantic seafloor is defined by its prominent water mass, the Antarctic Bottom Water (Tomczak and Godfrey, 2001). The Antarctic Bottom Water expands northwards into the Atlantic basins east and west of the Mid-Atlantic Ridge, like the Agulhas Basin, but can only enter the basins north of the Walvis Ridge (e.g., Cape Basin) via the northerly Romanche Fracture Zone. The Weddell Sea Bottom Water (WSBW), defined by a temperature of 0.7 1C and a salinity of 34.64 ppt (Orsi et al., 1993), is the main water mass above the Weddell Sea benthos (Fahrbach et al., 2001). The WSBW flows from the western Weddell Sea into the Scotia Sea and South Sandwich Forearc, and its circulation is driven by the Weddell Sea gyre (e.g., Fahrbach et al., 1994; Orsi et al., 1993, 1995). The sediments in the bathyal and abyssal Weddell and Powell Basins are dominated by silt and clay (Howe et al., 2004, unpublished data). 2.2. Collection and treatment of samples A 3-m wide Agassiz trawl (AGT) was deployed at two locations in the South Atlantic and 14 locations

in the Southern Ocean during the PFS Polarstern expedition ANT XXII/3 WECCON 2005— ANDEEP III in January–April 2005 (Fahrbach, 2006) (Table 1; Fig. 1). The sample depths ranged from 1047 to 4931 m, sampling continental slopes of the eastern Weddell Sea (off Kapp Norvegia) and western Weddell Sea and the South Orkney Islands, and deep Cape, Agulhas, Weddell and Powell Basins (Fig. 1). At the stations 074-7, 078-11 and 081-9, the cod end mesh size was 10 mm, while at all other stations, an inlet of 500 mm mesh size was inserted. The 500 mm mesh size was used because of smaller adult size of deepsea macrobenthos compared to shelf macrobenthos (Gray, 2002). The deployment protocol was standardised to 10 min trawling at 1 knot with 1.5  cable length to water depth to facilitate comparability between the different sites. At station 059-10, the AGT was trawled for 20 min. The haul distances were calculated from the time the Agassiz trawl travelled on the ground. The tension meter of the winch clearly indicated when the AGT left the seabed. Haul length varied from 731 to 3841 m (Table 1). Sample volumes were estimated and the general sediment composition was noted (Table 1). Sediment data analysis from core samples taken at the same sample locations was done by John Howe

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Table 1 Details of Agassiz trawl (AGT) stations of the Southern Ocean cruise, ANDEEP III Area

AGT

Station

Date

Depth (m)

Latitude

Longitude

Start

End

Start

End

Haul length (m)

Volume (L)

Sediment sand/silt/ clay (%) 4/54/42 17/68/15 Soft sediment 5/70/25, dropstones Dropstones Soft sediment, dropstones 16/58/26, dropstones No sediment 2/64/34 Soft sediment 1/47/52 Soft sediment, dropstones Soft sediment 3/66/31 Soft sediment Soft sediment

CB AB WS

1 2 3

PS67/016-11 PS67/021-8 PS67/057-2

26.01.05 29.01.05 10.02.05

4699–4730 4579–4579 1819–1822

4117.460 S 47139.190 S 69124.500 S

4117.420 S 47139.030 S 69124.620 S

9155.110 E 4116.500 E 5119.370 W

9154.920 E 4116.510 E 5119.680 W

3577 3525 1436

20 30 4200

WS

4

PS67/059-10

15.02.05

4648–4648

67130.370 S

67130.270 S

013.740 E

014.340 E

2619

50

WS WS

5 6

PS67/074-7 PS67/078-11

20.02.05 21.02.05

1055–1047 2147–2147

71118.480 S 7119.390 S

71118.400 S 7119.350 S

13158.550 W 13159.330 W

13158.140 W 13158.810 W

813 1588

50 4200

WS

7

PS67/080-6

22.02.05

3006–2978

70140.230 S

70140.420 S

14143.780 W

14143.830 W

1977

4200

0

0

0

0

WS WS WS

8 9 10

PS67/081-9 PS67/088-11 PS67/094-11

24.02.05 27.02.05 02.03.05

4390–4392 4930–4931 4893–4894

70132.94 S 6813.580 S 66138.050 S

70133.15 S 6813.570 S 66138.100 S

14134.40 W 20124.580 W 2715.900 W

14134.10 W 20124.220 W 2715.460 W

2743 3641 3488

1 150 o200

WS WS

11 12

PS67/102-11 PS67/110-2

06.03.05 09.03.05

4794–4797 4701–4704

65135.400 S 6510.790 S

65135.510 S 6510.850 S

36129.000 W 4310.410 W

36128.830 W 4310.250 W

3841 3298

4300 4300

WS

13

PS67/121-7

14.03.05

2616–2617

63134.920 S

63134.650 S

50141.970 W

50141.680 W

2424

4500

0

0

0

0

PB PB

14 15

PS67/142-6 PS67/150-7

18.03.05 20.03.05

3403–3404 1970–1954

6219.93 S 61148.320 S

6219.80 S 61148.200 S

49130.47 W 47128.450 W

49130.59 W 47128.640 W

2323 2064

4500 100

PB

16

PS67/151-1

20.03.05

1181–1188

61145.460 S

61145.340 S

4717.570 W

4717.780 W

731

100

The area abbreviations are: AB, Agulhas Basin; CB, Cape Basin; PB, Powell Basin; WS, Weddell Sea.

(SAMS, UK) (www.cedamar.org, ANDEEP III sediment data). When the trawl reached the deck, each sample (for volumes, see Table 1) was separated on a 500-mm sieve. Mega- and larger macrofauna were separated by eye on deck and the residues in the sieves were fixed in pre-cooled 96% ethanol. After 48 h fixation at +8 1C, the sieve residue was sorted under stereomicroscope. The taxa of each trawl sample were identified to morphospecies level. The number of morphospecies and specimens were counted to determine the abundance and species richness of major taxonomic groups. For faunal analysis, organisms were assigned to 1 of 44 taxonomic groups (Table 2). To enable comparisons between stations, the number of individuals were standardised to 1000 m2 trawled area hauls. The times and positions when the AGT reached and left the seafloor were used to calculate trawl length to compensate for the fact that the trawl cannot be closed. Biomass measurements were not taken. Comparisons of community compositions between stations were done using Bray–Curtis similarities (Bray and Curtis, 1957). Bray–Curtis scores of

the relative abundance of each taxon were analysed as a dendrogram using PRIMER 5 (Clarke and Warwick, 2001). The relative abundances were used to compensate for the semi-quantitative nature of the AGT data. 3. Results In the abyssal basins of the Southern Ocean and South Atlantic, more than 5900 specimens belonging to 12 phyla, at least 26 classes and at least 44 orders, were sampled from 16 AGT catches (Tables 2 and 3). There was a significant positive correlation between morphospecies richness and abundance at stations (t-test: p ¼ 0.001, T ¼ 3.596, d.f. ¼ 30). The stations with the highest account of morphospecies and abundance levels were 057-2, 074-7 and 121-7 (all Weddell Sea). The major six taxa (Cnidaria, Mollusca, Annelida, Crustacea, Echinodermata and Chordata) occurred at all stations, but only echinoderms, crustaceans and molluscs dominated the species composition. Examples for high species richness in relation to abundance were 58 crustacean morphospecies in

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Table 2 Morphospecies richness of macro- and megazoobenthic taxa in AGT samples Phylum

Porifera Cnidaria

Class

CB AB WS WS WS WS WS WS WS WS WS WS WS PB PB PB 016- 021- 057- 059- 074- 078- 080- 081- 088- 094- 102- 110- 121- 142- 150- 15111 8 2 10 7 11 6 9 10 11 11 2 7 6 5 1 0 0 0 0

0 0 0 0

2 0 0 0

3 0 0 0

20 0 0 0

6 0 0 1

4 0 0 0

0 0 1 0

4 1 0 0

5 0 1 0

7 0 0 0

1 0 0 0

17 0 0 0

2 0 1 0

1 0 1 0

1 0 0 0

0

0

0

0

0

2

0

0

0

0

0

0

0

1

0

0

0 1 0 0 0 0 0 Nemertea 1 Mollusca Bivalvia 16 Gastropoda Prosobranchia 7 Opisthobranchia 5 Polyplacophora 0 Scaphopoda 5 Cephalopoda Octopoda 0 Teuthida 0 Annelida Polychaeta Sedentaria 4 Errantia 6 Sipunculida 0 Echiurida 0 Crustacea Ostracoda 7 Cirripedia Thoracica 0 Malacostraca Amphipoda 0 Tanaidacea 0 Cumacea 0 Isopoda 0 Mysidacea 1 Natantia 1 Chelicerata Pycnogonida 0 Tentaculata Bryozoa 1 Brachiopoda 0 Echinodermata Ophiuroidea 3 Asteroidea 2 Echinoidea Regularia 1 Irregularia 0 Crinoidea 0 Holothuroidea 9 Chordata Ascidiacea 0 Pisces 1

1 1 0 0 1 0 0 0 10 1 1 0 5 0 0 1 1 0 0 0 0 1 0 0 0 1 0 0 1 0 5 2 0 0 0 5 0 3

0 2 0 1 1 1 0 2 5 11 2 0 4 1 0 19 9 1 0 1 0 3 3 4 8 1 1 0 0 1 4 6 0 1 3 11 1 1

1 1 1 0 0 0 1 0 5 0 0 0 0 1 0 1 0 0 0 0 1 1 1 0 1 0 0 1 0 0 4 2 0 0 0 4 1 2

1 4 0 0 1 0 0 1 2 1 0 1 0 1 0 2 0 0 0 0 0 0 0 0 3 1 1 2 3 0 7 5 0 0 2 11 0 3

0 0 1 1 0 0 0 1 4 1 2 0 4 0 0 11 4 1 0 0 0 1 0 0 2 1 1 0 5 1 3 4 2 1 0 9 0 3

0 1 0 0 0 0 0 0 6 9 0 0 1 0 0 4 0 0 0 1 0 0 4 0 4 0 0 0 0 0 7 2 0 0 0 0 2 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1

1 1 0 0 0 0 1 0 7 1 0 0 1 0 0 4 4 0 0 2 0 1 4 1 6 0 0 0 0 0 1 2 0 0 0 2 2 1

1 1 0 0 0 0 1 0 7 1 0 0 1 0 0 2 3 1 0 9 0 2 1 1 11 1 1 0 0 0 2 6 0 0 0 10 1 2

1 1 0 0 0 0 1 0 8 3 0 0 3 0 0 5 2 1 0 1 0 1 1 1 2 2 0 0 1 0 1 1 0 0 0 3 1 0

0 1 0 0 0 0 1 0 4 1 0 0 1 0 0 4 0 0 0 1 0 1 0 1 5 0 0 1 0 0 2 0 0 0 0 0 0 1

0 0 0 0 0 0 0 1 4 2 0 0 1 2 0 39 10 5 1 0 1 17 14 1 15 0 0 3 3 0 3 2 1 1 0 3 1 1

1 0 4 0 0 0 1 0 0 0 0 0 0 0 0 0 12 4 11 7 2 0 0 0 2 1 0 0 1 0 24 5 10 4 1 1 1 1 1 5 1 0 19 9 8 1 2 3 23 12 0 2 0 1 0 2 0 0 0 0 8 5 5 1 1 10 3 0 2 1 7 7 1 7 1 2

0 2 0 0 1 0 0 0 3 1 0 0 0 1 0 4 3 2 1 1 0 3 1 0 1 0 1 3 0 0 5 1 1 0 1 7 1 2

Totals

40

110 32

72

72

45

6

47

71

47

25

148 122 84

40

Hydrozoa Scyphozoa Anthozoa

Alcyonacea (soft cor.) Alcyonacea (gorg.) Pennatulacea Actiniaria Corallimorpharia Scleractinia Zoanthidea Ceriantharia Antipatharia

72

The area abbreviations are: AB, Agulhas Basin; CB, Cape Basin; PB, Powell Basin; WS, Weddell Sea.

272 crustacean specimens at station 121-7 (Western Weddell Sea) and 49 polychaete species in 727 individuals. The mean number of species over all stations was 59, the averaged number of specimens.

A positive effect of the small-sized (500 mm) inner net and cod end on the collection quantify was observed. Macro- and megafaunal groups like molluscs, crustaceans, poriferans and polychaetes

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Table 3 Numbers of specimens per macro- and megazoobenthic taxon collected in AGT samples Phylum

Porifera Cnidaria

Class

CB AB WS WS WS WS WS WS WS WS WS WS WS PB PB PB 016- 021- 057- 059- 074- 078- 080- 081- 088- 094- 102- 110- 121- 142- 150- 15111 8 2 10 7 11 6 9 10 11 11 2 7 6 5 1 0 0 0 0

0 0 0 0

2 0 0 0

3 0 0 0

50 0 0 0

15 0 0 2

4 0 0 0

0 0 1 0

4 1 0 0

6 0 1 0

90 0 0 0

100 0 0 0

52 0 0 0

3 0 1 0

1 0 1 0

1 0 0 0

0

0

0

0

0

4

0

0

0

0

0

0

0

1

0

0

0 1 0 0 0 0 0 Nemerteans 3 Mollusca Bivalvia 117 Gastropoda Prosobranchia 10 Opisthobranchia 67 Polyplacophora 0 Scaphopoda 6 Cephalopoda Octopoda 0 Teuthida 0 Annelida Polychaeta Sedentaria 10 Errantia 7 Sipunculida 0 Echiurida 0 Crustacea Ostracoda 9 Cirripedia Thoracica 0 Malacostraca Amphipoda 0 Tanaidacea 0 Cumacea 0 Isopoda 0 Mysidacea 6 Natantia 5 Chelicerata Pycnogonida 0 Tentaculata Bryozoa 4 Brachiopoda 0 Echinodermata Ophiuroidea 100 Asteroidea 2 Echinoidea Regularia 1 Irregularia 0 Crinoidea 0 Holothuroidea 50 Chordata Ascidiacea 0 Pisces 1

1 1 0 0 1 0 0 0 70 1 4 0 11 0 0 1 6 0 0 0 0 2 0 0 0 1 0 0 2 0 50 5 0 0 0 16 0 4

1 0 0 9 0 1 0 1 0 0 8 0 1 0 0 0 0 0 0 0 0 5 3 0 7 20 54 1 1 18 0 2 0 1 0 0 0 42 8 1 0 0 0 0 0 3 26 6 0 9 0 0 6 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 4 0 0 0 6 6 6 5 3 0 290 153 0 10 0 0 7 5 0 0 3 0 129 5 78 7 9 3 0 14 0 0 2 0 30 0 0 49 72 0 0 0 4 3 3 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 2 1 0 0 0 0 0 1

2 2 0 0 0 0 5 0 35 1 0 0 4 0 0 4 9 0 0 2 0 5 8 1 11 0 0 0 0 0 1 4 0 0 0 2 2 1

2 2 0 0 0 0 2 0 86 1 0 0 2 0 0 3 4 2 0 28 0 8 4 2 30 2 1 0 0 0 19 6 0 0 0 20 1 2

3 2 0 0 0 0 15 0 45 6 0 0 23 0 0 10 2 3 0 2 0 1 1 1 3 4 0 0 5 0 2 1 0 0 0 7 1 0

Hydrozoa Scyphozoa Anthozoa

Alcyonacea (soft cor.) Alcyonacea (gorg.) Pennatulacea Actiniaria Corallimorpharia Scleractinia Zoanthidea Ceriantharia Antipatharia

0 2 8 1 0 1 6 0 1 0 2 0 0 3 2 0 67 7 30 0 70 0 0 0 118 0 1 1 0 0 137 1 54 0 21 0 0 0 3 0 0 1 29 1 18 1 39 0 19 2 7 0 20 0 0 1 0 0 2 0 25 9 26 6 0 0 1 0 4 0 69 34 1 1 4 2

0 0 1 0 0 1 0 6 0 2 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 9 0 0 0 0 0 1 0 0 0 23 10 184 6 11 1 4 37 13 3 0 0 3 0 0 0 0 0 0 0 4 31 15 4 0 0 3 0 0 2 0 0 1 0 0 5 664 111 8 8 0 63 23 6 3 0 23 1 1 2 0 4 1 1 1 1 0 1 5 1 0 4 3 0 0 1 107 31 12 4 0 95 31 1 1 1 1 9 5 0 11 67 66 14 1 0 0 0 7 0 0 0 0 133 51 1 4 0 2 4 0 3 0 0 0 0 0 0 0 0 2 22 148 5 22 0 3 50 1 2 0 10 50 10 33 0 2 57 0 0 0 0 30 4 3 0 5 44 22 72 0 2 1 3 1 1 1 1 4 4

Abbreviations: AB, Agulhas Basin; CB, Cape Basin; PB, Powell Basin; WS, Weddell Sea.

were high in richness and abundance. The smallsized inner net also collected many typically larger faunal elements like sponges, cnidarians and fishes.

3.1. Taxon richness The numbers of preliminary identified species and morphospecies found per station ranged from 6 at

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was dominated by the class Demospongiae, with 30 species so far identified, especially by the families Cladorhizidae (carnivore sponges) and Polymastiidae. However, 14 species of Hexactinellida, especially of the family Rossellidae (glass sponges) and 3 species of the class Calcarea, all probably new to science, were also found (a preliminary list of the sponge species from ANDEEP I-III is given by Janussen and Tendal, 2007). Within the Mollusca, turrid gastropods and taxodont bivalves of the families Nuculanidae and Yoldiidae, respectively, were most speciose. There was no distinct gradient in taxonomic richness with increasing depth from the upper continental slope to the abyssal basins (Fig. 3). Taxon frequencies were changed considerably between stations as well as between depths. At most stations, malacostracan crustaceans, polychaetes and bivalves were most species-rich accounting each for 10–30% of the present taxa (30–60% together). Sponges were most dominant with 28% at the shallowest station (074-7, eastern Weddell Sea) at 1055 m, but represented just 2–9% of the taxa at the other stations.

station 081-9 (eastern slope of Weddell Sea) to 148 at the western Weddell Sea slope station 121-7 (Fig. 2; Table 1). Highest species numbers were found along the continental slopes in depths between 1800 and 3400 m. Morphospecies richness in the abyssal plain stations (4300–4900 m) was in general lower than in the slope stations with the exceptions of the stations 016-11 in the Cape Basin and 094-11 in the Weddell Basin, where more than 70 species were found (Fig. 2). The most frequent taxon were ophiuroids occurring in all 16 stations. Bivalves, polychaetes and asteroids were found at 15 stations (Table 2). Sedentary polychaetes were the most speciose taxon at a single station with 39/24 morphospecies found at the western Weddell Sea station 121-7 and the Powell Basin station 142-6, followed by isopods (23 species at 142-6 and 17 species at 121-7) and sponges (20 species at 074-7 in the eastern Weddell Sea at Kapp Norvegia). Among the polychaetes, the families Cirratulidae, Malanidae and Paraonidae were richest. The richest isopod families were those with small-sized species, such as the families Acanthaspidiidae, Munnopsidae, Desmosomatidae and Haploniscidae. Sponge richness 150

Species numbers per class

120

90

60

30

Porifera Echiurida

Cnidaria Crustacea

Nemerteans Chelicerata

Mollusca Tentaculata

Annelida Echinodermata

088-10

094-11

102-11

110-2

016-11

059-10

021-8

081-9

142-6

080-6

121-7

078-11

150-5

057-2

151-1

074-7

0

Sipunculida Chordata

Fig. 2. Species richness by phylum and AGT station. The AGT stations are ranked by depth, from shallowest on the left.

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074-7 1055 m

151-1 1181 m

057-2 1819 m

150-5 1970 m

078-11 2147 m

121-7 2616 m

080-6 3006 m

142-6 3403 m

021-8 4579 m

059-10 4648 m

016-11 4699 m

110-2 4701 m

102-11 4794 m

094-11 4893 m

088-10 4930 m

Porifera

Hydrozoa

Scyphozoa

Anthozoa

Nemerteans

Bivalvia Polychaeta Pycnogonida Echinoidea

Gastropoda Sipunculida Bryozoa Crinoidea

Polyplacophora Echiurida Brachiopoda Holothuroidea

Scaphopoda Cirripedia Ophiuroidea Ascidiacea

Octopoda Malacostraca Asteroidea Pisces

Fig. 3. Proportions of morphospecies by higher taxa in the abyssal Southern Ocean and South Atlantic sorted by depth.

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the Brachiopoda only inarticulate forms of the genus Pelagodiscus were found. Echinoids were represented by the eurybathic regular taxa Sterechinus agassizii Mortensen, 1910, Ctenocidaris nutrix Mortensen, 1928 and Aporocidaris milleri Mortensen, 1909 and the deep-water irregular taxa Antrechinus, Plexechinus and Echinosigra. Holothuroidea were diverse with at least 40 morphospecies including, cosmopolitan species like Psychropotes longicauda The´el, 1882 and Scotoplanes globosa (The´el, 1879) and as yet unidentified species. Ascidians were represented by colonial and stalked taxa. Fish were represented by tripod fish in the African basins and grenadiers (Macrouridae) in the Weddell Basin.

Among the 45 species of sponges were 14 species of Hexactinellida, and eurybathic Polymastidae and Myxillidae as well as 3 species of predatory Cladorhizidae (all Demospongiae) and three calcareous species. Caulophacus (Oxydiscus) weddelli Janussen, Tabachnick and Tendal, 2004 was collected for the first time since its initial discovery (Janussen et al., 2004), and the biggest and only complete specimen of Malacosaccus coatsi Topsent, 1910 was collected. Among the anthozoan morphospecies identified were 10 Octocorallia and 26 Hexacorallia of which the actiniarians were most diverse with 16 species. The anthozoan fauna at depths below 4000 m were mainly represented by Galatheantheumum profundale (Carlgren, 1956), Umbellula cf. thomsoni Kolliker, 1874 and Antipatharia gen.1. A total of 53 gastropod morphospecies were identified, often represented by single specimens like the newly described Bathylepeta linseae Schwabe, 2006. Bivalves were represented by 43 species and scaphopods by 7 species, and 4 species of ooctopodiform cephalopodes also were found in the samples. Polyplacophora were represented by the sole record of Stenosemus simplicissimus (Thiele, 1906) at the shallowest Station 074-7. The peracarids dominated the crustaceans, especially small-sized isopods and amphipods, but also larger taxa like serolids of the genus Acanthoserolis and the amphipod Epimeria cf. inermis Walker, 1903 were found. Natant decapods were represented by the deep-water genus Nematocarcinus. Among

3.2. Abundance Malacostracan crustaceans were the most abundant taxon with more than 1300 individuals (Table 3). This was influenced by the occurrence of the shrimp Nematocarcinus, which found at seven stations and accounted for 653 specimens. The next most abundant groups were the polychaetes (1183 specimens) and bivalves (742 specimens). Hydrozoans and polyplacophorans were present with only a single specimen each at the Weddell Basin station 88-10 (the former) and station 074-7 on the eastern Weddell Sea slope. Zoobenthic composition based on relative abundances per taxon revealed differences between the stations, but no

Relative abundance

100% 80% 60% 40% 20%

Porifera Ophiuroidea

Bivalvia Holothuroidea

Polychaeta Cephalopoda

151-1

150-5

142-6

121-7

110-2

102-11

094-11

088-10

081-9

080-6

078-11

074-7

059-10

057-2

021-8

016-11

0%

Malacostraca others

Fig. 4. Relative abundance of macro- and megazoobenthic taxa. Taxa with minor abundances are pooled in ‘‘others’’: Hydrozoa, Scyphozoa, Anthozoa, Nemerteans, Gastropoda, Polyplacophora, Scaphopoda, Sipunculida, Echiurida, Cirripedia, Pycnogonida, Bryozoa, Brachiopoda, Asteroidea, Crinoidea and Pisces.

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general consistent pattern was found (Fig. 4). The proportion of bivalves increased with increasing depth (t-test po0.001, T ¼ 9.473, d.f. ¼ 30). Malacostraca dominated stations along the slope, but were also important at some of the deepest stations. Ophiuroids were most important at stations between 3000 and 4500 m (e.g., stations 080-6, 142-6, 021-8). The importance of holothurians (which had the highest biomass, estimated by sample volume) varied between stations and depth. Abundances per 1000 m2 ranged from 0.9 individuals (hereafter abbreviated ind) at 081-9 to 252 ind at 074-7, both at the eastern Weddell Sea slope (Fig. 5). The stations on the two continental slopes (074-7 to 142-6 in Fig. 5, 1055–3403 m (depth) showed significantly higher abundances (median 118.5 ind m2) than the stations in the basins (081-9 to 088-10, 4579–4930 m depth; median 16.5 ind m2). The two transects taken down the continental slopes at Kapp Norvegia/eastern Weddell Sea and Powell Basin/western Weddell Sea presented contrasting patterns. Whilst at Kapp Norvegia species richness and abundance decreased with increasing depth, the opposite trend was observed in the Powell Basin (Fig. 6). The cluster analysis showed a separation of stations into clusters at a similarity of about 70% (Fig. 7), with exception of the eastern Weddell Sea station, 081-9 station, which had just 38% similarity. The stations in the Cape and Agulhas Basins formed a group as did those in Powell Basin.

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4. Discussion 4.1. Taxon richness The results of the current study suggest that higher taxon richness of the bathyal and abyssal Weddell Sea (e.g., at phylum, class and order levels) can be as diverse as that of other Antarctic and subAntarctic shelf habitats (e.g., Arnaud et al., 1998; Arntz and Brey, 2003; Arntz et al., 2005, 2006; Ramos, 1999; Rehm et al., 2006; VoX, 1988). The zoobenthos compositions of the ANDEEP III AGT collections we report here show a higher taxon diversity than similarly collected AGT data of ANDEEP I and II (Allcock et al., 2003). These differences may be explained by important changes in the AGT deployment between the cruises. During ANDEEP I and II, a 1-m wide trawl with 10-mm cod end was used, whilst during ANDEEP III, a 3-m wide trawl with 500-mm cod end was used. Both taxon composition and quantity on the recent cruise increased compared to ANDEEP I and II (Allcock et al., 2003; Fu¨tterer et al., 2003). Especially species of sizes less than 10 mm were caught more frequently and in higher specimen numbers. Casual observations suggest that the megafauna that was collected with the 3-m trawl with 500-mm cod end contained many more holothurians and cnidarians. Considerably more small-sized sponge species, particularly important deep-sea taxa, such Cladorhizidae and abyssal Calcarea, were collected during this cruise compared to earlier cruises (Janussen, 2006; Janussen et al., 2004; Janussen and Tendal,

Number of ind/1000m2

300.0 250.0 200.0 150.0 100.0 50.0

088-10

094-11

102-11

110-2

016-11

059-10

021-8

081-9

142-6

080-6

121-7

078-11

150-5

057-2

151-1

074-7

0.0

Stations: from shallow to deep Fig. 5. Macro- and megabenthos abundance per 1000 m2. The grey line marks the slope stations.

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(A) 300

2 Individuals per 1000 m

250 200 150

252

100

131

106

50

87

44

PB

32

0 1000

2000

KN/WS

3000

Depth (m) (B) 140

Species numbers

120 100 122

80 84

60 72

40

40

72

20 45

0 1000 2000 Depth (m)

PB 3000

KN/WS

Fig. 6. Patterns in (A) abundance per 1000 m2 and (B) species richness along two vertical transects at Kapp Norvegia and in the Powell Basin. Abbreviations: KN/WS, Kapp Norvegia/Weddell Sea; PB, Powell Basin.

Fig. 7. Station dendrogram from the Cluster analysis. Brey–Curtis Index, group average method.

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2007). On the other hand, amphipods that occurred at most of the former stations during ANDEEP I and II were very rare this time. On high taxonomic levels macro- and megafaunal composition of abyssal Antarctic soft-bottom habitats is comparable to those of deep-sea and Arctic (e.g., Bluhm et al., 2005; Deubel, 2000; Gage, 1978; Kro¨ncke, 1998). Polychaetes, the most speciose taxon in this study, are often a dominant element of the deep benthic faunas in the Antarctic (Hilbig, 2001, 2004; Montiel et al., 2005), the Atlantic and Pacific (Glover et al., 2001, 2002; Hilbig and Blake, 2006) and Arctic (Bluhm et al., 2005; Kro¨ncke, 1998; Narayanaswamy et al., 2005). Malacostracan crustaceans and bivalves, speciose in the Antarctic samples, are also known to be species rich in the deep Atlantic and Arctic oceans (Brandt, 1995; Brandt et al., 2005a; Olabarria, 2005; Rex et al., 2000; Richling, 2000). Sponges, the dominant and characteristic group of the Antarctic shelf (Arntz et al., 1994; Barthel and Tendal, 1994), are less prominent in the deep but still speciose, and especially the glass sponges are more diverse on higher taxonomic levels (genera and families) (D. Janussen et al., 2004, unpublished data). The number of morphospecies reported in the ANDEEP III AGT samples, ranging from 6 to 148 per trawl, is lower than that of Antarctic shelf sites. Arntz et al. (2005) reported between 99 and 306 species in 17 trawls taken on the eastern Weddell Sea shelf in 230–855 m depth. Trawls collected at the isolated sub-Antarctic island of Bouvet reported 46–98 species per sample (Arntz et al., 2006). The decrease in species numbers with increasing water depth towards the abyssal basins (44000 m) observed in the current study fits the common knowledge on bathymetric trends in deep-sea fauna (Carney, 2005; Gage and Tyler, 1991). More specific information on the species composition of selected taxa can obtained from the of ANDEEP III cruise report (Fahrbach, 2006). 4.2. Abundance Most of the abundance assessments of Antarctic macrobenthos have been carried out using grabs and corers (e.g., Gerdes et al., 1992; Montiel et al., 2005; Piepenburg et al., 2002 and references therein). The use of trawled devices like AGTs, dredges and sledges for abundance studies has been criticised for being of semi-quantitative nature (Eleftheriou and Holme, 1984). On the other hand, the

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trawls are more efficient to assess macro- and megafaunal diversity in an area (Rehm et al., 2006). Various methods have been used to quantify trawl catches. One method is to use devices that close when they leave the seafloor (Brandt and Barthel, 1995; Brenke, 2005). Another method for bottom and Agassiz trawls is to take subsamples of either representative volume per catch (VoX, 1988), of 5-L volume per catch (Arnaud et al., 1998) or 5-L volume per catch (Arntz et al., 1996, 2006). Here, we analysed the complete trawl catches and calculated the trawl length between the points when the trawl reached and left the seafloor. This is the first study on the abundance of macroand megafaunal assemblages in the Antarctic deepsea. Similar studies on the relative abundances of the Antarctic shelf and the Arctic shelf and deep-sea zoobenthos used lower taxonomic resolution, either phylum level (Arntz et al., 2006; Bluhm et al., 2005; Feder et al., 2005; Rehm et al., 2006) or a mixture of phyla and classes (Kro¨ncke, 1998) or pooled stations (Arnaud et al., 1998; Ingole, 2003). Comparisons with these studies therefore can only be made to their levels and then the relative range of taxon abundances in our study is similar to theirs. The standardised abundances per 1000 m2 (1 and 252 ind 1000 m2) decrease with increasing depth and were very low at depths over 4500 m. Other benthos studies have previously found a decline in abundance with increasing depth (e.g., Rex et al., 2006; Saunders and Hessler, 1969; Soltwedel, 2000). The vertical transects collected at the continental slopes at Kapp Norvegia/eastern Weddell Sea and in the Powell Basin/western Weddell Sea showed opposing patterns in abundance. At Kapp Norvegia abundances decreased with increasing depth whilst in the Powell Basin no obvious decrease was found. Such findings support previously suggested (suprabenthos) abundance increases with depth in some areas of the Weddell Sea and decreases with depth in other areas (e.g., Halley Bay and in the Bransfield Strait, see Linse et al., 2002). At depths of 1000–3500 m on continental slopes, abundances of macrobenthos are more variable and seem to be very patchy across scales measured to date (Brandt et al., 2005b; Kaiser et al., 2007). Previously, patchy distribution patterns have been suggested for bivalves (Linse, 2004) and isopods (Brandt et al., 2004a) from analysis of ANDEEP I and II expeditions. Compared to macrofaunal abundances from the Antarctic shelf collected by grabs (16–14.483 ind m2, see Arntz et al., 2005) the

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deep-sea abundances are orders of magnitude lower. It is likely that these results are linked to limited and patchy food availability for deep-sea benthos (Schwinghammer, 1985; Rice et al., 1990; Smith et al., 1997; Soltwedel, 2000). The ANDEEP II expedition made first insights possible into the deep macro- and megabenthic assemblages of Antarctic waters. The results reported here agree with the well-documented dominances of polychaetes, malacostracan crustaceans, bivalves and ophiuroids in the deep sea and on softbottom habitats supplemented by holothurians, gastropods and sponges. Further investigations of the Antarctic deep-sea habitats are needed for a more detailed faunal inventory, species level community and diversity analyses, and a better understanding of the ecological processes.

Acknowledgements We are grateful to the German Science Foundation and all national funding agencies for the financial support given to us for our participation in ANDEEP III. Thanks are due to Eberhard Fahrbach, Chief Scientist (AWI) on ANT XXII/3, and to the captain and crew of PFS Polarstern for help and support on board. We are grateful to David Barnes (BAS) for helpful comments on the manuscript. Peter Fretwell (BAS) provided the initial ANDEEP III station map. Vonda Cummings (NIWA) and Sven Thatje (NOCS) are thanked for providing helpful criticisms of the manuscript. This is ANDEEP publication #70 and a contribution to the SCAR EBA programme.

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