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Composition, abundance and distribution of Peracarida from the Southern Ocean deep sea
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Deep-Sea Research II 54 (2007) 1752–1759 www.elsevier.com/locate/dsr2
Composition, abundance and distribution of Peracarida from the Southern Ocean deep sea Wiebke Bro¨kelanda,, Madhumita Choudhuryb, Angelika Brandtb a
Forschungsinstitut Senckenberg, DZMB, Su¨dstrand 44, 26382 Wilhelmshaven, Germany Biozentrum Grindel: Zoologisches Museum, Universita¨t Hamburg, Martin-Luther-King-Platz 3, D-20146 Hamburg, Germany
b
Accepted 4 July 2007 Available online 3 August 2007
Abstract Peracarida are an important component of both the deep-sea fauna and the fauna known from the Antarctic shelf. The peracarid composition from epibenthic-sledge samples obtained during the expeditions ANDEEP I–III is shown and compared to that of other deep-sea basins and the Antarctic shelf. The Amphipoda were the most abundant taxon in the samples, with 43% of all peracarid individuals, followed by the Isopoda (35%), Cumacea (16%), Tanaidacea (5%) and Mysidacea (1%). Highest abundances were found at the shallower stations, but a clear trend towards decreasing abundances with depth could not be confirmed. Although the decrease of amphipod numbers and the increase of isopod numbers with depth in general correspond to other areas, e.g., the Angola Basin, the Southern Ocean deep sea seems to be special by harboring more amphipod specimens at medium depths, thus reflecting the situation on the Antarctic shelf. Outstanding by their number of specimens were the samples from station 133-2 at about 1500 m depth in the Powell Basin, which contained the highest number of specimens, and from station 152-6 in the Bransfield Strait, where the number of specimens was strikingly low despite the moderate depth of about 2000 m. r 2007 Elsevier Ltd. All rights reserved. Keywords: Peracarida; Composition; Abundance; Distribution; Southern Ocean; Deep sea
1. Introduction Peracarida are known to be an important component of Antarctic zoobenthos (De Broyer et al., 2003; De Broyer and Jazdzewski, 1996). On the Antarctic shelf, where benthic decapods are almost absent (Arntz et al., 1994; Clarke et al., 2004), Peracarida have undergone an extensive
Corresponding author.
E-mail addresses: [email protected] (W. Bro¨keland), [email protected] (M. Choudhury), [email protected] (A. Brandt). 0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.07.014
radiation (Brandt, 1999, 2000, 2005; De Broyer et al., 2003), especially the Amphipoda, which are thought to be a cold adapted group (Barnard and Barnard, 1983), and Isopoda, which thrive on the Antarctic shelf (Holme, 1962). Both taxa have developed a broad variety of lifestyles, ranging from predators and scavengers to filter feeders and deposit feeders, and include benthic, pelagic and bentho-pelagic forms. The other peracarid taxa, Cumacea, Tanaidacea and Mysidacea, display a smaller range of lifestyles and are less abundant and speciose. Cumacea are mostly benthic, spending their lives predominantly buried into the substrate, but with occasional pelagic phases (Zimmer, 1941).
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Tanaidacea are restricted to benthic life and are either epibenthic, epizoic or tube dwellers (Larsen, 2005). The appearance of Mysidacea, with their pelagic and hyperbenthic lifestyle (Tattersall and Tattersall, 1951; Brandt et al., 1998), in epibenthic sledge (EBS) samples is coincidental rather than representative of their actual distribution. The fact that they occur in swarms might explain their low species richness and patchy occurrence in the samples (Brandt, 1993). In the deep sea, like in the Antarctic, Peracarida are an important component of the benthic fauna. Especially, the Isopoda are known to be abundant and speciose in abyssal environments (Hessler and Sanders, 1967; Kussakin, 1967; Hessler, 1974; Gage and Tyler, 1991; Rex et al., 1993; Brandt et al., 2004a, 2005). The Southern Ocean deep-sea peracarid composition was hardly known prior to the ANDEEP expeditions (Brandt et al., 2004b). This paper gives an overview over the peracarid composition in the ANDEEP samples from the EBS, based on abundance data only as most peracarid orders are not yet sorted to species level. 2. Methods The data were collected during the ANDEEP I–III (ANT XIX/3-4, ANT XXII/3) expeditions with R.V. Polarstern to the Southern Ocean. Samples were taken by means of an EBS (after Rothlisberg and Pearcy, 1977; Brandt and Barthel, 1995; Brenke, 2005) in depths between 774 and 6348 m. Stations were located in the Drake Passage, off Elephant Island, near the South Shetland Islands, in the Weddell Sea, the Powell Basin, off the South Sandwich Islands, off Kapp Norvegia, and in the Cape Basin in the South Atlantic. In total, 40 stations were sampled with the EBS (Table 1) covering an area of 129,895 m2. The EBS has an epi-net and a supra-net with a mesh size of 500 mm. It was trawled in a standardized way across the seafloor (Brandt et al., 2004a). Nevertheless, the trawling distances varied between 711 and 6464 m due to the different depths of deployment; therefore the numbers of specimens were standardized to 1000-m hauls for the comparative analysis. Trawling distances were calculated on the basis of velocity of ship and winch during trawling and heaving until the sledge left the ground. The EBS collects mainly epibenthic, but also some endobenthic and suprabenthic organisms
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randomly. The samples from the cod ends were fixed immediately in 96% pre-cooled EtOH and kept in the cold storage for 48 h. The samples were sorted on board and later in the laboratory in the Zoological Museum of the University of Hamburg into the major taxonomic groups. Only the specimens from the cod ends were counted and used for this study. 3. Results In total, 38,010 peracarids were sorted from the EBS samples, comprising 16,359 Amphipoda (43% of all Peracarid specimens), 13,446 Isopoda (35%), 6060 Cumacea (16%), 1729 Tanaidacea (5%) and 416 Mysidacea (1%). The high number of Amphipoda is mainly due to station 133-2 in the Powell Basin, where 7602 specimens were collected. If this station is excluded, isopods become the most numerous group (39%; Amphipoda 32%). The number of specimens varies markedly between stations (Fig. 1; compare also figure of stations in the introduction of this volume). The deepest sample from 142-6 (6384 m; ANDEEP II) contained only five (1.18/1000 m2) specimens, while at station 133-2 (1584 m; ANDEEP III) 10,735 (9222.50/1000 m2) specimens were collected. There is no continuous decline in abundance with depth, but stations below 4000 m generally have abundances below 300 specimens, while abundances at stations between 2000 and 4000 vary between about 16 and 1350 specimens. Among the stations above 2000 m are those with highest abundances (74-6 and 133—2; ANDEEP III). Station 152-6 in the Bransfield Strait in about 2000 m was characterized by the strikingly low number of 12 specimens (uncorrected number). The three most abundant peracarid taxa were Amphipoda, Isopoda and Cumacea. In some of the seven geographical regions (Drake Passage, Elephant Island, South Shetland Island, Powell Basin, South Sandwich Islands, Weddell Sea and Cape Basin), the relative abundance of these taxa shows a correlation with depth (Fig. 2). In the Weddell Sea, the relative number of isopods increases with depth, whereas the relative number of Cumacea decreases; Amphipoda show only a slight decrease with depth. The increase of Isopoda with depth is also visible in the Powell Basin and the South Sandwich Islands, as is the decrease of Amphipoda, while the Cumacea show no clear trend. In contrast, no such trend can be observed in the region of the Drake Passage, Elephant Island and the South Shetland Islands.
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Table 1 Station list of ANDEEP I–III including abundance data of the peracarid taxa Depth (m)
Latitude
Longitude
Haul length (m)
Isopoda
Amphipoda
Cumacea
Tanaidacea
Mysidacea
ANDEEP I 41-3 42-2 43-8 46-7 99-4 105-7 114-4 129-2
26.01.2002 27.01.2002 03.02.2002 30.01.2002 12.02.2002 12.02.2002 17.02.2002 22.02.2002
2370 3689 3962 3894 5191 2308 2921 3640
59122.240 S–59122.570 S 59140.300 S–59140.320 S 60127.130 S–60127.190 S 60138.330 S–60138.060 S 61106.400 S–61106.400 S 61124.160 S–61124.250 S 61143.540 S–61143.510 S 59152.210 S–59152.200 S
60104.060 W–60104.050 W 57135.420 W–57135.640 W 56105.120 W–56104.810 W 53157.380 W–53157.510 W 59116.570 W–59117.610 W 58151.560 W–58151.560 W 60144.210 W–60144.430 W 59158.750 W–59158.630 W
4928 4766 4782 5639 5336 2881 4482 4076
43.83 189.89 41.20 256.63 5.60 2.34 99.73 21.59
22.32 114.77 46.84 547.61 2.25 9.02 20.97 22.57
7.31 99.87 15.89 480.42 0.56 3.47 19.19 10.06
2.23 22.66 5.86 35.87 0.75 0.35 4.02 6.13
2.44 3.77 0.42 27.78 0 0.69 2.68 1.22
ANDEEP II 131-3 05.03.2002 132-2 06.03.2002 133-3 07.02.2002 134-3 09.03.2002 135-4 10.03.2002 136-4 12.03.2002 137-4 14.03.2002 138-6 17.03.2002 139-6 20.03.2002 140-8 21.03.2002 141-10 23.03.2002 142-6 24.03.2002 143-1 25.03.2002
3053 2086 1121 4069 4678 4747 4976 4542 3950 2970 2312 6348 774
65119.830 S–65119.990 S 65117.750 S–65117.620 S 65120.170 S–65120.080 S 65119.200 S–65119.050 S 65100.050 S–65159.970 S 64101.540 S–64101.510 S 63144.980 S–63144.740 S 62158.080 S–62157.990 S 58114.100 S–58114.150 S 58115.980 S–58116.280 S 58125.070 S–58124.630 S 58150.780 S–58150.440 S 58144.690 S–58144.450 S
51131.610 W–51131.230 W 53122.810 W–53122.860 W 54114.300 W–54114.340 W 48103.770 W–48102.920 W 43103.020 W–43100.820 W 39106.880 W–39106.880 W 38147.750 W–38148.230 W 27154.100 W–27154.280 W 24121.200 W–24121.210 W 24153.730 W–24154.090 W 24100.780 W–24100.740 W 23157.750 W–23157.590 W 25110.280 W–25110.660 W
3553 2523 1314 4553 2773 5306 4581 4147 6464 4183 3094 4221 1441
258.09 13.48 564.69 11.20 149.3 6.97 15.28 46.06 9.90 32.99 11.83 0.47 40.25
117.93 12.68 244.29 4.61 10.82 0.94 3.93 17.84 2.94 21.57 73.69 0.47 131.58
18.01 1.19 416.29 3.30 3.61 0.94 1.09 1.69 4.18 16.50 29.73 0.24 9.02
7.04 0 57.08 0.88 2.52 0.57 1.53 3.62 1.24 12.67 9.37 0 9.02
10.41 5.95 9.89 0 0.72 0 0 0.48 0.31 0.24 11.64 0 5.55
ANDEEP III 16-10 26.01.2005 21-7 29.01.2005 59-5 14.02.2005 74-6 20.02.2005 78-9 22.02.2005 80-9 23.02.2005 81-8 24.02.2005 88-8 27.02.2005 94-14 02.03.2005 102-3 06.03.2005 110-8 10.03.2005 121-11 14.03.2005 133-2 16.03.2005 142-5 18.03.2005 150-6 20.03.2005 151-7 21.03.2005 152-6 23.03.2005 153-7 29.03.2005 154-9 30.03.2005
4720 4577 4655 1032 2149 3100 4382 4931 4891 4801 4695 2659 1584 3405 1984 1183 1998 2096 3803
41107.550 S–41107.020 S 47139.870 S–47138.520 S 67130.750 S–67129.810 S 71118.420 S–71118.330 S 71109.520 S–71109.340 S 70138.450 S–70139.180 S 70131.080 S–70132.230 S 68103.840 S–68103.640 S 66139.080 S–66137.160 S 65133.180 S–65134.320 S 64159.200 S–64100.910 S 63138.270 S–63137.310 S 62146.730 S–62146.330 S 62111.360 S–62111.360 S 61149.130 S–61148.520 S 61145.670 S–61145.420 S 62120.640 S–62119.910 S 63119.820 S–63119.180 S 62132.520 S–62131.310 S
09155.940 E–09154.850 E 04115.790 E–04114.940 E 00100.230 W–00101.940 E 13158.210 W–13157.650 W 14100.760 W–13158.850 W 14142.860 W–14143.430 W 14134.820 W–14134.900 W 20131.390 W–20127.490 W 27109.260 W–27110.130 W 36133.240 W–36131.050 W 43102.050 W–43102.100 W 50137.160 W–50138.040 W 53102.570 W–53104.140 W 49127.620 W–49129.570 W 47127.510 W–47128.160 W 47107.190 W–47108.070 W 57153.120 W–57153.680 W 64136.440 W–64137.530 W 64139.450 W–64138.660 W
3198 2923 2878 711 2376 1778 2935 3488 3476 3283 2904 1945 1164 2251 1567 1383 2113 1954 2525
70.36 23.61 37.53 1037.97 172.56 346.46 128.45 76.83 26.47 28.33 172.18 192.8 2441.58 41.31 199.11 78.81 2.84 110.03 10.30
27.52 5.47 14.94 1486.64 175.08 290.21 66.44 26.95 20.14 6.40 18.94 165.55 6530.93 43.98 172.3 284.89 1.42 136.13 43.17
18.45 4.11 3.82 1383.97 269.78 88.86 17.04 5.73 2.30 1.22 4.82 56.04 75.6 15.99 72.11 34.38 0 77.79 21.00
7.19 1.71 6.60 551.34 33.25 15.19 32.03 8.89 1.44 1.83 7.58 44.73 168.38 1.33 17.87 28.92 1.42 36.85 8.32
0.63 0 2.43 43.60 8.00 5.62 1.02 0 0.29 0.30 1.03 4.11 6.01 0 14.68 12.29 0 3.07 0.40
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AII-143/774 m AIII-74/1032 m AII-133/1121 m AIII-151/1183 m AIII-133/1584 m AIII-150/1984 m AIII-152/1998 m AII-132/2086 m AIII-153/2096 AIII-78/2149 m AI-105/2308 m AII-141/2312 m AI-41/2370 m AIII-121/2659 m AI-114/2921 m AII-140/2970 m AII-131/3053 m AIII-80/3100 m AIII-142/3405 m AI-129/3640 AI-42/3689 m AIII-154/3803 m AI-46/3894 m AII-139/3950 m AI-43/3962 m AII-134/4069 m AIII-81/4382 m AII-138/4542 m AIII-21/4577 m AIII-59/4655 m AII-135/4678 m AIII-110/4695 m AIII-16/4720 m AII-136/4747 m AIII-102/4801 m AIII-94/4891 m AIII-88/4931 m AII-137/4976 m AI-99/5191 m AII-142/6348 m
7000
6000
5000
4000
3000
2000
1000
0
stations
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Fig. 1. Number of peracarid specimens obtained at ANDEEP stations and station depth. Station 74-6 and 133-2 are deleted to enhance the resolution of the remaining stations.
Cape Basin
AIII-21/4577 m AIII-16/4720 m
100%
AIII-74/1032 m AII-133/1121 m AII-132/2086 m AIII-78/2149 m AIII-121/2659 m AII-131/3053 m AIII-80/3100 m AII-134/4069 m AIII-81/4382 m AII-138/4542 m AIII-59/4655 m AII-135/4678 m AIII-110/4695 m AIII-102/4801 m AIII-94/4891 m AIII-88/4931 m AII-137/4976 m
80%
AII-143/774 m AII-141/2312 m AII-140/2970 m AII-139/3950 m
60%
AIII-151/1183 m AIII-133/1584 m AIII-150/1984 m AIII-142/3405 m
40%
AIII-153/2096 AI-105/2308 m AI-41/2370 m AI-114/2921 m AI-129/3640 AI-42/3689 m AIII-154/3803 m AI-46/3894 m AI-43/3962 m
20%
0%
Isopoda
Weddell Sea
Amphipoda
Drake Passage Powell South Elephant Island Basin Sandwich South Shetland Islands Islands stations
Cumacea
Fig. 2. Relative abundance of Amphipoda, Isopoda and Cumacea from ANDEEP I–III. Stations sorted according to geographical region and depth. Stations with less than 10 individuals (99-4, 136-4, 142-6, 152-6) are omitted.
relative number of specimens
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80 67.8
70 relative abundance of peracarid taxa %
Isopoda Amphipoda Cumacea Tanaidacea Mysidacea
60 54.7
50 43.0
40 35.5
34.4
35.4
37.1
32.2
30
27.1 24.3
20
22.8 19.2
15.9 12.5
10
7.3 1.1
0 total
5.8
5.0
4.5
1.8
0.5
1000-2000 m
2000-3000 m
3.1
1.5
3000-4000 m
6.5 0.6
4000-5000 m
depth interval Fig. 3. Relative abundance of peracarid taxa at ANDEEP stations from different depth intervals and at all ANDEEP stations.
The increase of Isopoda with depth is also obvious when depth ranges of 1000 m are considered (Fig. 3). Between 1000 and 2000 m, they comprise 27% of the four major peracarid taxa, increasing to 68% at depth between 4000 and 5000 m. The amphipods display a different pattern, their proportion decreasing from 55% between 1000 and 2000 m to 19% between 4000 and 5000 m. Cumacea have highest abundance (25%) at medium depth ranges (2000–3000 m), while Tanaidacea and Mysidacea show no distinct depth-related pattern. 4. Discussion For deep-sea sampling of the small macrofaunal organisms, like Peracarida, the EBS proved to be a very successful gear, as it yields many more organisms for different studies than other samplers, for example, the box corer. The mesh size of 500 mm with a 300-mm cod end to retain the specimens is well suited to catch this fraction of the benthic fauna. Besides, the specimens are mostly in a very good condition and therefore suitable for taxonomic studies. Despite these advantages, the EBS is not the best gear for quantitative sampling due to the varying
trawling distances and other sampling errors (Brenke, 2005). Nevertheless, the patchy distribution of deep-sea organisms makes a comprehensive assessment by means of quantitative gears, e.g., boxcorer or multicorer, difficult because of the small area sampled by these gears (Brandt, 1995), and because of the bow-wave effect, which also disturbes ‘‘quantitative’’ box-corer samples (Holme and McIntyre, 1971). Therefore, EBS samples may be used for the evaluation of the biodiversity only if the flaws of the gear in quantitative measurements are considered with due care (Brenke, 2005). The general rule that abundance decreases with depth due to decreasing food availability (Hessler, 1974; Gage and Tyler, 1991) is not fully confirmed here. Overall the peracarid abundances at stations below 3000 m tend to be lower than at shallower stations, but there is no regular decline in abundance with depth, indicating that food availability is not dependant on depth alone. Besides, the abovementioned citations refer to deep-sea areas in nonpolar regions and our results were obtained in the Weddell Sea, an area of intense deep-water production, a fact that might explain the patterns described above. Due to the deep-water formation, food can be deposited quickly even at abyssal depths (Brandt,
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1997; Linse et al., 2002) and has been frequently reported on underwater images of the SO deep-sea stations (Brandt et al., 2007a, b; Howe et al., 2007). This assumption is further supported by the fact that stations 133-2 and 152-6 (ANDEEP III) differ very much in abundance and species richness, although their depth difference is only 500 m. Station 133-2 in the Powell Basin, with its strikingly high abundance (10,735; uncorrected number), is probably located in an area of deep-water formation, where cold, saline water cascades down the continental slope into the deep sea, a source of Antarctic Bottom Water. The down-welling water mass influencing station 133-2 might serve as a perfect conduit for the passive transportation of shelf organisms downslopes, as well as for plankton or ice algae being produced in surface waters, possibly allowing the high numbers of specimens and species to coexist at this station (Brandt et al., 2007a, b). Station 152-6 in the Bransfield Strait, in contrast, is more isolated, not shaped by deep-water production (Fahrbach, 2006), and shows the second lowest abundance observed. Despite the moderate depth of about 2000 m, only 12 peracarid specimens were found at station 152-6. The sediment at this station is poorly sorted dark greenish mud and contains abundant diatoms and radiolarians. Monosulphide spots were common in the sediment cores and bottom currents are not visible at all (Howe, 2006). Among the deeper stations (below 2000 m) station 46 is conspicuous by high numbers of Amphipoda (2168; uncorrected number), followed by Cumacea (1902) and Isopoda (1016). This station was located north east of Elephant Island in the Scotia Sea. The high number of Amphipoda resembles the shelf fauna. Among the Isopoda the families Paramunnidae, Munnidae, Serolidae and Anthuridae, taxa with a distribution center on the shelf, were unusually abundant (see Brandt et al., 2007a, b), suggesting a strong faunal connection with the Antarctic shelf. While in other deep-sea areas, e.g., the Angola Basin (Brandt et al., 2005), Amhipoda are usually less abundant than Isopoda, they were the most abundant peracarid order in the ANDEEP samples, comprising 43% of all peracarid individuals. The depth-related pattern of peracarid composition (Fig. 3) generally corresponds to that in other ocean basins, with isopod numbers increasing with depth and amphipod numbers decreasing (Hessler, 1974; Hessler and Jumars, 1974). However, the relatively
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high proportion of Amphipoda in samples below 2000 m seems to be a peculiarity of the Southern Ocean deep sea. Even at the deeper stations (4001–5000 m), they are more abundant (19%) than in the Angola Basin. This is probably a result of the tremendous success of Amphipoda on the Antarctic shelf (Arntz et al., 1994; De Broyer and Jazdzewski, 1996), the lack of a thermocline, and the deep shelf, encouraging the submergence of amphipod species. Studies on the Antarctic shelf have shown Amphipoda to be the most abundant peracarid taxon in most cases (Brandt, 2001; Rehm et al., 2006, 2007) In addition, the coarse, glacial-marine sediments of the slope stations (Howe et al., 2007) might favour the occurrence of Amphipoda in the Southern Ocean in comparison to other areas at similar depths. Contrary to the mainly soft-bottomed deepsea areas of other world oceans, the deep-sea floor of the Southern Ocean is characterized by a relatively high proportion of hard-bottom created by drop stones (Thomson, 2006). Cumacea do not show strong differences in abundance with depth and seem to be well adapted to different sediment types. The peracarid composition at stations below 2000 m shows distinct differences to the peracarid composition on the Antarctic shelf. The Isopoda are the most abundant taxon at these stations (for increasing with depth; Dahl, 1954), although Amphipoda still comprise a relatively high number of individuals (Table 1). The relatively shallow stations (above 2000 m depth) show a higher resemblance in peracarid composition to the Antarctic shelf. At this slope depth Amphipoda dominate the samples and Cumacea also occur in high numbers, while Isopoda are less abundant. For example, station 133-2 at depth of about 1500 m in the Powell Basin, even though displaying a very distinctive pattern of abundance, has a relative peracarid composition similar to that of shelf samples. Whether the distributional patterns observed represent a clear break in the faunal composition between shelf and deep sea in the Southern Ocean or rather a gradual change cannot be answered from the data obtained so far. However, this change, abrupt or not, seems to take place in deeper waters than in other areas of the world oceans. A possible reason for this might be the fact that the Antarctic shelf is deeper than that of other continental shelf areas due to the weight of the ice shield on the continent. This special characteristic of
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the Antarctic continental shelf, combined with glacial periods of ice extensions and retreats, has led scientists to argue that the Antarctic organisms have developed a higher potential of eurybathy (Brey et al., 1996). The missing thermocline, as mentioned above, also may support the colonization of deeper waters by shelf fauna and vice versa, processes, which have been described as Antarctic submergence or emergence (see Brandt et al., 2007a, b). An continued analyses of all peracarid individuals to species level, as exemplified by Brandt et al. (2007a, b) for the isopods, will be necessary before community patterns of the Peracarida can be investigated on the background of depth, latitude and longitude as well as environmental parameters. Such an analysis based on these first patterns will broaden our knowledge on the zoogeography and bathymetry of all peracarid taxa considerably.
Acknowledgments We thank Prof. Dr. Dieter Fu¨tterer, chief scientist on Polarstern cruise ANT XIX/3-4, and Dr. Eberhard Fahrbach, chief scientist on Polarstern cruise ANT XXII/3, and the crew of R.V. Polarstern. Many thanks to all pickers and sorters, namely Dr. Marina Malyutina, Alexandra Toletti, Anna Marquart, Britta Wulf, David Hopff, Laura Wu¨rzberg, Antje Fischer and Lydia Kramer. We are grateful to Dr. Brigitte Ebbe for correcting the English language. The German Science Foundation kindly provided financial support (Br 1121/20-1/3 & Br 1121/26-1). This is ANDEEP publication # 74.
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Brandt, A., 1999. On the origin and evolution of Antarctic Peracarida (Crustacea, Malacostraca). Scientia Marina 63 (1), 261–274. Brandt, A., 2000. Hypotheses on Southern Ocean peracarid evolution and radiation (Crustacea, Malacostraca). Antarctic Science 12 (3), 269–275. Brandt, A., 2001. Great differences in peracarid crustacean density between the Arctic and Antarctic deep sea. Polar Biology 24, 785–789. Brandt, A., 2005. Evolution of Antarctic biodiversity in the context of the past: the importance of the Southern Ocean deep sea. Antarctic Science 17 (4), 509–552. Brandt, A., Barthel, D., 1995. An improved supra- and epibenthic sledge for catching Peracarida (Crustacea, Malacostraca). Ophelia 43 (1), 15–23. Brandt, A., Mu¨hlenhardt-Siegel, U., Siegel, V., 1998. An account of the Mysidacea (Crustacea, Malacostraca) of the Southern Ocean. Antarctic Science 10 (1), 3–11. Brandt, A., Bro¨keland, W., Brix, S., Malyutina, M., 2004a. Diversity of Southern Ocean deep-sea Isopoda (Crustacea, Malacostraca)—a comparison with shelf data. Deep-Sea Research II 51, 1753–1768. Brandt, A., De Broyer, C., Gooday, A.J., Hilbig, B., Thompson, M.R.A., 2004b. Introduction to ANDEEP (Antarctic benthic deep-sea biodiversity: colonization history and recent community patterns)—a tribute to Howard L. Sanders. Deep-Sea Research II 51, 1457–1465. Brandt, A., Brenke, N., Andres, H.-G., Brix, S., GuerreroKommritz, J., Mu¨hlenhardt-Siegel, U., Wa¨gele, J.-W., 2005. Diversity of peracarid crustaceans (Malacostraca) from the abyssal plain of the Angola basin. Organisms, Diversity and Evolution 5, 105–112. Brandt, A., Brix, S., Bro¨keland, W., Choudhury, M., Kaiser, S., Malyutina, M., 2007a. Deep-sea isopod biodiversity, abundance and endemism in the Atlantic sector of the Southern Ocean—results from the ANDEEP I–III expeditions. DeepSea Research II, this issue [doi:10.1016/j.dsr2.2007.07.015]. Brandt, A., Gooday, A.J., Brix, S.B., Bro¨keland, W., Cedhagen, T., Choudhury, M., Cornelius, N., Danis, B., De Mesel, I., Diaz, R.J., Gillan, D.C., Ebbe, B., Howe, J., Janussen, D., Kaiser, S., Linse, K., Malyutina, M., Brandao, S., Pawlowski, J., Raupach, M., 2007b. The Southern Ocean deep sea: first insights into biodiversity and biogeography. Nature 447, 307–311. Brenke, N., 2005. An epibenthic sledge for operations on marine soft bottom and bedrock. Marine Technology Society Journal 39 (2), 13–24. Brey, T., Dahm, C., Gorny, M., Klages, M., Stiller, M., Arntz, W.E., 1996. Do Antarctic benthic invertebrates show an extended level of eurybathy? Antarctic Science 8 (1), 3–6. Clarke, A., Aronson, R.B., Crame, A., Gili, J.-M., Blake, D.B., 2004. Evolution and diversity of the benthic fauna of the Southern Ocean continental shelf. Antarctic Science 16 (4), 559–568. Dahl, E., 1954. The distribution of deep-sea Crustacea. In: on the distribution and origin of the deep sea bottom fauna. International Union of Biological Science (B) 16, 43–46. De Broyer, C., Jazdzewski, K., 1996. Biodiversity of the Southern Ocean: towards a new synthesis for the Amhipoda (Crustacea). Bollettino del Museo Civico di Storia Naturale di Verona 20, 547–568. De Broyer, C., Jazdewsky, K., Dauby, P., 2003. Biodiversity patterns in the Southern Ocean: lessons from Crustacea. In:
ARTICLE IN PRESS W. Bro¨keland et al. / Deep-Sea Research II 54 (2007) 1752–1759 Huiskes, A.H.L., Gieskes, J.R., Schorno, R.M.L., van der Vies, S.M., Wolff, W.J. (Eds.), Antarctic Biology in a Global Context. Leiden, The Netherlands. Fahrbach, E., 2006. The Expedition ANTARKTIS-XXII/3 of the Research Vessel ‘‘Polarstern’’ in 2005, vol. 533. Berichte fu¨r Polar- und Meeresforschung, pp. 1–246. Gage, J.D., Tyler, P.A., 1991. Deep-Sea Biology: A Natural History of Organisms at the Deep-Sea Floor. Cambridge University Press, pp. 1–504. Hessler, R.R., 1974. The structure of deep benthic communities from central oceanic waters. In: Miller, C. (Ed.), The Biology of the Oceanic Pacific, pp. 79–93. Hessler, R.R., Jumars, P.A., 1974. Abyssal community analysis from replicate box cores in the central North Pacific. DeepSea Research 21, 185–209. Hessler, R.R., Sanders, H.L., 1967. Faunal diversity in the deep sea. Deep-Sea Research 14, 65–78. Holme, N.A., 1962. Benthos in Antarctic waters. In: Carrick, R.M., Holdgate, M., Prevost, J. (Eds.), Biologie Antarctique. Premier Symposium Organise` par le S.C.A.R., Hermann, Paris, pp. 319–322. Holme, N.A., McIntyre, A.D. (Eds.), 1971. Methods for the Study of Marine Benthos. International Biological Programme-IBP Handbook No. 16. Blackwell Scientific Publications, Oxford/Edinburgh, p. 334. Howe, J., 2006. Recent sedimentation and geochemistry across the Northern Weddell Sea and adjacent deep-water regions, Antarctica. In: Fahrbach, E. (Ed.), The Expedition ANTARKTIS-XXII/3 of the Research Vessel ‘‘Polarstern’’ in 2005, vol. 533. Berichte fu¨r Polar- und Meeresforschung, pp. 196–208. Howe, J.A., Wilson, C.R., Shimmield, T.M., Diaz, R.J., Carpenter, L.W., 2007. Recent deep-water sedimentation, trace metal and radioisotope geochemistry across the Southern Ocean and Northern Weddell Sea, Antarctica. Deep-Sea Research II, this issue [doi:10.1016/j.dsr2.2007.07.007].
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Kussakin, O.G., 1967. Fauna of Isopoda and Tanaidacea in the coastal zones of the Antarctic and Subantarctic waters. [Translation from Russian by the Israel Program for Scientific Translations, Jerusalem, 1968.]. Biological Reports of the Soviet Antarctic Expedition (1955–1958) 3, 220–389. Larsen, K., 2005. Deep-sea Tanaidacea (Peracarida) from the Gulf of Mexico. Crustaceana Monographs 5, 380. Linse, K., Brandt, A., Hilbig, B., Wegener, G., 2002. Composition and distribution of suprabenthic fauna in the south eastern Weddell Sea and at King George Island. Antarctic Science 14 (1), 3–10. Rehm, P., Thatje, S., Arntz, W.E., Brandt, A., Heilmayer, O., 2006. Distribution and composition of macrozoobenthic communities along a Victoria-Land Transect (Ross Sea, Antarctica). Polar Biology 29, 782–790. Rehm, P., Thatje, S., Mu¨hlenhardt-Siegel, U., Brandt, A., 2007. Composition and distribution of the peracarid crustacean fauna along a latitudinal transect of Victoria Land (Ross Sea, Antarctica) with special emphasis on the Cumacea. Polar Biology 30, 871–881. Rex, M.A., Stuart, C.T., Hessler, R.R., Allen, J.A., Sanders, H.L., Wilson, G.D.F., 1993. Global-scale latitudinal patterns of species diversity in the deep-sea benthos. Nature 365, 636–639. Rothlisberg, P.C., Pearcy, W.G., 1977. An epibenthic sampler to study the ontogeny of vertical migration of Pandalus jordani (Decapoda Caridea). Fisheries Bulletin 74, 994–997. Tattersall, W.M., Tattersall, O.S., 1951. The British Mysidacea. Ray Society, London. Thomson, M.R.A., 2006. Lone-stones. Reports on Polar and Marine Research 533, 209–212. Zimmer, C., 1941. Cumacea. In: Bronns, H.G., Klassen und Ordnungen des Tierreichs, Bd V, Abt. 1, Buch 4, Teil 5, 1–222pp.
Deep-Sea Research II 54 (2007) 1752–1759 www.elsevier.com/locate/dsr2
Composition, abundance and distribution of Peracarida from the Southern Ocean deep sea Wiebke Bro¨kelanda,, Madhumita Choudhuryb, Angelika Brandtb a
Forschungsinstitut Senckenberg, DZMB, Su¨dstrand 44, 26382 Wilhelmshaven, Germany Biozentrum Grindel: Zoologisches Museum, Universita¨t Hamburg, Martin-Luther-King-Platz 3, D-20146 Hamburg, Germany
b
Accepted 4 July 2007 Available online 3 August 2007
Abstract Peracarida are an important component of both the deep-sea fauna and the fauna known from the Antarctic shelf. The peracarid composition from epibenthic-sledge samples obtained during the expeditions ANDEEP I–III is shown and compared to that of other deep-sea basins and the Antarctic shelf. The Amphipoda were the most abundant taxon in the samples, with 43% of all peracarid individuals, followed by the Isopoda (35%), Cumacea (16%), Tanaidacea (5%) and Mysidacea (1%). Highest abundances were found at the shallower stations, but a clear trend towards decreasing abundances with depth could not be confirmed. Although the decrease of amphipod numbers and the increase of isopod numbers with depth in general correspond to other areas, e.g., the Angola Basin, the Southern Ocean deep sea seems to be special by harboring more amphipod specimens at medium depths, thus reflecting the situation on the Antarctic shelf. Outstanding by their number of specimens were the samples from station 133-2 at about 1500 m depth in the Powell Basin, which contained the highest number of specimens, and from station 152-6 in the Bransfield Strait, where the number of specimens was strikingly low despite the moderate depth of about 2000 m. r 2007 Elsevier Ltd. All rights reserved. Keywords: Peracarida; Composition; Abundance; Distribution; Southern Ocean; Deep sea
1. Introduction Peracarida are known to be an important component of Antarctic zoobenthos (De Broyer et al., 2003; De Broyer and Jazdzewski, 1996). On the Antarctic shelf, where benthic decapods are almost absent (Arntz et al., 1994; Clarke et al., 2004), Peracarida have undergone an extensive
Corresponding author.
E-mail addresses: [email protected] (W. Bro¨keland), [email protected] (M. Choudhury), [email protected] (A. Brandt). 0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.07.014
radiation (Brandt, 1999, 2000, 2005; De Broyer et al., 2003), especially the Amphipoda, which are thought to be a cold adapted group (Barnard and Barnard, 1983), and Isopoda, which thrive on the Antarctic shelf (Holme, 1962). Both taxa have developed a broad variety of lifestyles, ranging from predators and scavengers to filter feeders and deposit feeders, and include benthic, pelagic and bentho-pelagic forms. The other peracarid taxa, Cumacea, Tanaidacea and Mysidacea, display a smaller range of lifestyles and are less abundant and speciose. Cumacea are mostly benthic, spending their lives predominantly buried into the substrate, but with occasional pelagic phases (Zimmer, 1941).
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Tanaidacea are restricted to benthic life and are either epibenthic, epizoic or tube dwellers (Larsen, 2005). The appearance of Mysidacea, with their pelagic and hyperbenthic lifestyle (Tattersall and Tattersall, 1951; Brandt et al., 1998), in epibenthic sledge (EBS) samples is coincidental rather than representative of their actual distribution. The fact that they occur in swarms might explain their low species richness and patchy occurrence in the samples (Brandt, 1993). In the deep sea, like in the Antarctic, Peracarida are an important component of the benthic fauna. Especially, the Isopoda are known to be abundant and speciose in abyssal environments (Hessler and Sanders, 1967; Kussakin, 1967; Hessler, 1974; Gage and Tyler, 1991; Rex et al., 1993; Brandt et al., 2004a, 2005). The Southern Ocean deep-sea peracarid composition was hardly known prior to the ANDEEP expeditions (Brandt et al., 2004b). This paper gives an overview over the peracarid composition in the ANDEEP samples from the EBS, based on abundance data only as most peracarid orders are not yet sorted to species level. 2. Methods The data were collected during the ANDEEP I–III (ANT XIX/3-4, ANT XXII/3) expeditions with R.V. Polarstern to the Southern Ocean. Samples were taken by means of an EBS (after Rothlisberg and Pearcy, 1977; Brandt and Barthel, 1995; Brenke, 2005) in depths between 774 and 6348 m. Stations were located in the Drake Passage, off Elephant Island, near the South Shetland Islands, in the Weddell Sea, the Powell Basin, off the South Sandwich Islands, off Kapp Norvegia, and in the Cape Basin in the South Atlantic. In total, 40 stations were sampled with the EBS (Table 1) covering an area of 129,895 m2. The EBS has an epi-net and a supra-net with a mesh size of 500 mm. It was trawled in a standardized way across the seafloor (Brandt et al., 2004a). Nevertheless, the trawling distances varied between 711 and 6464 m due to the different depths of deployment; therefore the numbers of specimens were standardized to 1000-m hauls for the comparative analysis. Trawling distances were calculated on the basis of velocity of ship and winch during trawling and heaving until the sledge left the ground. The EBS collects mainly epibenthic, but also some endobenthic and suprabenthic organisms
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randomly. The samples from the cod ends were fixed immediately in 96% pre-cooled EtOH and kept in the cold storage for 48 h. The samples were sorted on board and later in the laboratory in the Zoological Museum of the University of Hamburg into the major taxonomic groups. Only the specimens from the cod ends were counted and used for this study. 3. Results In total, 38,010 peracarids were sorted from the EBS samples, comprising 16,359 Amphipoda (43% of all Peracarid specimens), 13,446 Isopoda (35%), 6060 Cumacea (16%), 1729 Tanaidacea (5%) and 416 Mysidacea (1%). The high number of Amphipoda is mainly due to station 133-2 in the Powell Basin, where 7602 specimens were collected. If this station is excluded, isopods become the most numerous group (39%; Amphipoda 32%). The number of specimens varies markedly between stations (Fig. 1; compare also figure of stations in the introduction of this volume). The deepest sample from 142-6 (6384 m; ANDEEP II) contained only five (1.18/1000 m2) specimens, while at station 133-2 (1584 m; ANDEEP III) 10,735 (9222.50/1000 m2) specimens were collected. There is no continuous decline in abundance with depth, but stations below 4000 m generally have abundances below 300 specimens, while abundances at stations between 2000 and 4000 vary between about 16 and 1350 specimens. Among the stations above 2000 m are those with highest abundances (74-6 and 133—2; ANDEEP III). Station 152-6 in the Bransfield Strait in about 2000 m was characterized by the strikingly low number of 12 specimens (uncorrected number). The three most abundant peracarid taxa were Amphipoda, Isopoda and Cumacea. In some of the seven geographical regions (Drake Passage, Elephant Island, South Shetland Island, Powell Basin, South Sandwich Islands, Weddell Sea and Cape Basin), the relative abundance of these taxa shows a correlation with depth (Fig. 2). In the Weddell Sea, the relative number of isopods increases with depth, whereas the relative number of Cumacea decreases; Amphipoda show only a slight decrease with depth. The increase of Isopoda with depth is also visible in the Powell Basin and the South Sandwich Islands, as is the decrease of Amphipoda, while the Cumacea show no clear trend. In contrast, no such trend can be observed in the region of the Drake Passage, Elephant Island and the South Shetland Islands.
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Table 1 Station list of ANDEEP I–III including abundance data of the peracarid taxa Depth (m)
Latitude
Longitude
Haul length (m)
Isopoda
Amphipoda
Cumacea
Tanaidacea
Mysidacea
ANDEEP I 41-3 42-2 43-8 46-7 99-4 105-7 114-4 129-2
26.01.2002 27.01.2002 03.02.2002 30.01.2002 12.02.2002 12.02.2002 17.02.2002 22.02.2002
2370 3689 3962 3894 5191 2308 2921 3640
59122.240 S–59122.570 S 59140.300 S–59140.320 S 60127.130 S–60127.190 S 60138.330 S–60138.060 S 61106.400 S–61106.400 S 61124.160 S–61124.250 S 61143.540 S–61143.510 S 59152.210 S–59152.200 S
60104.060 W–60104.050 W 57135.420 W–57135.640 W 56105.120 W–56104.810 W 53157.380 W–53157.510 W 59116.570 W–59117.610 W 58151.560 W–58151.560 W 60144.210 W–60144.430 W 59158.750 W–59158.630 W
4928 4766 4782 5639 5336 2881 4482 4076
43.83 189.89 41.20 256.63 5.60 2.34 99.73 21.59
22.32 114.77 46.84 547.61 2.25 9.02 20.97 22.57
7.31 99.87 15.89 480.42 0.56 3.47 19.19 10.06
2.23 22.66 5.86 35.87 0.75 0.35 4.02 6.13
2.44 3.77 0.42 27.78 0 0.69 2.68 1.22
ANDEEP II 131-3 05.03.2002 132-2 06.03.2002 133-3 07.02.2002 134-3 09.03.2002 135-4 10.03.2002 136-4 12.03.2002 137-4 14.03.2002 138-6 17.03.2002 139-6 20.03.2002 140-8 21.03.2002 141-10 23.03.2002 142-6 24.03.2002 143-1 25.03.2002
3053 2086 1121 4069 4678 4747 4976 4542 3950 2970 2312 6348 774
65119.830 S–65119.990 S 65117.750 S–65117.620 S 65120.170 S–65120.080 S 65119.200 S–65119.050 S 65100.050 S–65159.970 S 64101.540 S–64101.510 S 63144.980 S–63144.740 S 62158.080 S–62157.990 S 58114.100 S–58114.150 S 58115.980 S–58116.280 S 58125.070 S–58124.630 S 58150.780 S–58150.440 S 58144.690 S–58144.450 S
51131.610 W–51131.230 W 53122.810 W–53122.860 W 54114.300 W–54114.340 W 48103.770 W–48102.920 W 43103.020 W–43100.820 W 39106.880 W–39106.880 W 38147.750 W–38148.230 W 27154.100 W–27154.280 W 24121.200 W–24121.210 W 24153.730 W–24154.090 W 24100.780 W–24100.740 W 23157.750 W–23157.590 W 25110.280 W–25110.660 W
3553 2523 1314 4553 2773 5306 4581 4147 6464 4183 3094 4221 1441
258.09 13.48 564.69 11.20 149.3 6.97 15.28 46.06 9.90 32.99 11.83 0.47 40.25
117.93 12.68 244.29 4.61 10.82 0.94 3.93 17.84 2.94 21.57 73.69 0.47 131.58
18.01 1.19 416.29 3.30 3.61 0.94 1.09 1.69 4.18 16.50 29.73 0.24 9.02
7.04 0 57.08 0.88 2.52 0.57 1.53 3.62 1.24 12.67 9.37 0 9.02
10.41 5.95 9.89 0 0.72 0 0 0.48 0.31 0.24 11.64 0 5.55
ANDEEP III 16-10 26.01.2005 21-7 29.01.2005 59-5 14.02.2005 74-6 20.02.2005 78-9 22.02.2005 80-9 23.02.2005 81-8 24.02.2005 88-8 27.02.2005 94-14 02.03.2005 102-3 06.03.2005 110-8 10.03.2005 121-11 14.03.2005 133-2 16.03.2005 142-5 18.03.2005 150-6 20.03.2005 151-7 21.03.2005 152-6 23.03.2005 153-7 29.03.2005 154-9 30.03.2005
4720 4577 4655 1032 2149 3100 4382 4931 4891 4801 4695 2659 1584 3405 1984 1183 1998 2096 3803
41107.550 S–41107.020 S 47139.870 S–47138.520 S 67130.750 S–67129.810 S 71118.420 S–71118.330 S 71109.520 S–71109.340 S 70138.450 S–70139.180 S 70131.080 S–70132.230 S 68103.840 S–68103.640 S 66139.080 S–66137.160 S 65133.180 S–65134.320 S 64159.200 S–64100.910 S 63138.270 S–63137.310 S 62146.730 S–62146.330 S 62111.360 S–62111.360 S 61149.130 S–61148.520 S 61145.670 S–61145.420 S 62120.640 S–62119.910 S 63119.820 S–63119.180 S 62132.520 S–62131.310 S
09155.940 E–09154.850 E 04115.790 E–04114.940 E 00100.230 W–00101.940 E 13158.210 W–13157.650 W 14100.760 W–13158.850 W 14142.860 W–14143.430 W 14134.820 W–14134.900 W 20131.390 W–20127.490 W 27109.260 W–27110.130 W 36133.240 W–36131.050 W 43102.050 W–43102.100 W 50137.160 W–50138.040 W 53102.570 W–53104.140 W 49127.620 W–49129.570 W 47127.510 W–47128.160 W 47107.190 W–47108.070 W 57153.120 W–57153.680 W 64136.440 W–64137.530 W 64139.450 W–64138.660 W
3198 2923 2878 711 2376 1778 2935 3488 3476 3283 2904 1945 1164 2251 1567 1383 2113 1954 2525
70.36 23.61 37.53 1037.97 172.56 346.46 128.45 76.83 26.47 28.33 172.18 192.8 2441.58 41.31 199.11 78.81 2.84 110.03 10.30
27.52 5.47 14.94 1486.64 175.08 290.21 66.44 26.95 20.14 6.40 18.94 165.55 6530.93 43.98 172.3 284.89 1.42 136.13 43.17
18.45 4.11 3.82 1383.97 269.78 88.86 17.04 5.73 2.30 1.22 4.82 56.04 75.6 15.99 72.11 34.38 0 77.79 21.00
7.19 1.71 6.60 551.34 33.25 15.19 32.03 8.89 1.44 1.83 7.58 44.73 168.38 1.33 17.87 28.92 1.42 36.85 8.32
0.63 0 2.43 43.60 8.00 5.62 1.02 0 0.29 0.30 1.03 4.11 6.01 0 14.68 12.29 0 3.07 0.40
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depth (m)
2000
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1400
1200
1000
800
600
400
200
0
number of specimens
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depth
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AII-143/774 m AIII-74/1032 m AII-133/1121 m AIII-151/1183 m AIII-133/1584 m AIII-150/1984 m AIII-152/1998 m AII-132/2086 m AIII-153/2096 AIII-78/2149 m AI-105/2308 m AII-141/2312 m AI-41/2370 m AIII-121/2659 m AI-114/2921 m AII-140/2970 m AII-131/3053 m AIII-80/3100 m AIII-142/3405 m AI-129/3640 AI-42/3689 m AIII-154/3803 m AI-46/3894 m AII-139/3950 m AI-43/3962 m AII-134/4069 m AIII-81/4382 m AII-138/4542 m AIII-21/4577 m AIII-59/4655 m AII-135/4678 m AIII-110/4695 m AIII-16/4720 m AII-136/4747 m AIII-102/4801 m AIII-94/4891 m AIII-88/4931 m AII-137/4976 m AI-99/5191 m AII-142/6348 m
7000
6000
5000
4000
3000
2000
1000
0
stations
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Fig. 1. Number of peracarid specimens obtained at ANDEEP stations and station depth. Station 74-6 and 133-2 are deleted to enhance the resolution of the remaining stations.
Cape Basin
AIII-21/4577 m AIII-16/4720 m
100%
AIII-74/1032 m AII-133/1121 m AII-132/2086 m AIII-78/2149 m AIII-121/2659 m AII-131/3053 m AIII-80/3100 m AII-134/4069 m AIII-81/4382 m AII-138/4542 m AIII-59/4655 m AII-135/4678 m AIII-110/4695 m AIII-102/4801 m AIII-94/4891 m AIII-88/4931 m AII-137/4976 m
80%
AII-143/774 m AII-141/2312 m AII-140/2970 m AII-139/3950 m
60%
AIII-151/1183 m AIII-133/1584 m AIII-150/1984 m AIII-142/3405 m
40%
AIII-153/2096 AI-105/2308 m AI-41/2370 m AI-114/2921 m AI-129/3640 AI-42/3689 m AIII-154/3803 m AI-46/3894 m AI-43/3962 m
20%
0%
Isopoda
Weddell Sea
Amphipoda
Drake Passage Powell South Elephant Island Basin Sandwich South Shetland Islands Islands stations
Cumacea
Fig. 2. Relative abundance of Amphipoda, Isopoda and Cumacea from ANDEEP I–III. Stations sorted according to geographical region and depth. Stations with less than 10 individuals (99-4, 136-4, 142-6, 152-6) are omitted.
relative number of specimens
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80 67.8
70 relative abundance of peracarid taxa %
Isopoda Amphipoda Cumacea Tanaidacea Mysidacea
60 54.7
50 43.0
40 35.5
34.4
35.4
37.1
32.2
30
27.1 24.3
20
22.8 19.2
15.9 12.5
10
7.3 1.1
0 total
5.8
5.0
4.5
1.8
0.5
1000-2000 m
2000-3000 m
3.1
1.5
3000-4000 m
6.5 0.6
4000-5000 m
depth interval Fig. 3. Relative abundance of peracarid taxa at ANDEEP stations from different depth intervals and at all ANDEEP stations.
The increase of Isopoda with depth is also obvious when depth ranges of 1000 m are considered (Fig. 3). Between 1000 and 2000 m, they comprise 27% of the four major peracarid taxa, increasing to 68% at depth between 4000 and 5000 m. The amphipods display a different pattern, their proportion decreasing from 55% between 1000 and 2000 m to 19% between 4000 and 5000 m. Cumacea have highest abundance (25%) at medium depth ranges (2000–3000 m), while Tanaidacea and Mysidacea show no distinct depth-related pattern. 4. Discussion For deep-sea sampling of the small macrofaunal organisms, like Peracarida, the EBS proved to be a very successful gear, as it yields many more organisms for different studies than other samplers, for example, the box corer. The mesh size of 500 mm with a 300-mm cod end to retain the specimens is well suited to catch this fraction of the benthic fauna. Besides, the specimens are mostly in a very good condition and therefore suitable for taxonomic studies. Despite these advantages, the EBS is not the best gear for quantitative sampling due to the varying
trawling distances and other sampling errors (Brenke, 2005). Nevertheless, the patchy distribution of deep-sea organisms makes a comprehensive assessment by means of quantitative gears, e.g., boxcorer or multicorer, difficult because of the small area sampled by these gears (Brandt, 1995), and because of the bow-wave effect, which also disturbes ‘‘quantitative’’ box-corer samples (Holme and McIntyre, 1971). Therefore, EBS samples may be used for the evaluation of the biodiversity only if the flaws of the gear in quantitative measurements are considered with due care (Brenke, 2005). The general rule that abundance decreases with depth due to decreasing food availability (Hessler, 1974; Gage and Tyler, 1991) is not fully confirmed here. Overall the peracarid abundances at stations below 3000 m tend to be lower than at shallower stations, but there is no regular decline in abundance with depth, indicating that food availability is not dependant on depth alone. Besides, the abovementioned citations refer to deep-sea areas in nonpolar regions and our results were obtained in the Weddell Sea, an area of intense deep-water production, a fact that might explain the patterns described above. Due to the deep-water formation, food can be deposited quickly even at abyssal depths (Brandt,
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1997; Linse et al., 2002) and has been frequently reported on underwater images of the SO deep-sea stations (Brandt et al., 2007a, b; Howe et al., 2007). This assumption is further supported by the fact that stations 133-2 and 152-6 (ANDEEP III) differ very much in abundance and species richness, although their depth difference is only 500 m. Station 133-2 in the Powell Basin, with its strikingly high abundance (10,735; uncorrected number), is probably located in an area of deep-water formation, where cold, saline water cascades down the continental slope into the deep sea, a source of Antarctic Bottom Water. The down-welling water mass influencing station 133-2 might serve as a perfect conduit for the passive transportation of shelf organisms downslopes, as well as for plankton or ice algae being produced in surface waters, possibly allowing the high numbers of specimens and species to coexist at this station (Brandt et al., 2007a, b). Station 152-6 in the Bransfield Strait, in contrast, is more isolated, not shaped by deep-water production (Fahrbach, 2006), and shows the second lowest abundance observed. Despite the moderate depth of about 2000 m, only 12 peracarid specimens were found at station 152-6. The sediment at this station is poorly sorted dark greenish mud and contains abundant diatoms and radiolarians. Monosulphide spots were common in the sediment cores and bottom currents are not visible at all (Howe, 2006). Among the deeper stations (below 2000 m) station 46 is conspicuous by high numbers of Amphipoda (2168; uncorrected number), followed by Cumacea (1902) and Isopoda (1016). This station was located north east of Elephant Island in the Scotia Sea. The high number of Amphipoda resembles the shelf fauna. Among the Isopoda the families Paramunnidae, Munnidae, Serolidae and Anthuridae, taxa with a distribution center on the shelf, were unusually abundant (see Brandt et al., 2007a, b), suggesting a strong faunal connection with the Antarctic shelf. While in other deep-sea areas, e.g., the Angola Basin (Brandt et al., 2005), Amhipoda are usually less abundant than Isopoda, they were the most abundant peracarid order in the ANDEEP samples, comprising 43% of all peracarid individuals. The depth-related pattern of peracarid composition (Fig. 3) generally corresponds to that in other ocean basins, with isopod numbers increasing with depth and amphipod numbers decreasing (Hessler, 1974; Hessler and Jumars, 1974). However, the relatively
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high proportion of Amphipoda in samples below 2000 m seems to be a peculiarity of the Southern Ocean deep sea. Even at the deeper stations (4001–5000 m), they are more abundant (19%) than in the Angola Basin. This is probably a result of the tremendous success of Amphipoda on the Antarctic shelf (Arntz et al., 1994; De Broyer and Jazdzewski, 1996), the lack of a thermocline, and the deep shelf, encouraging the submergence of amphipod species. Studies on the Antarctic shelf have shown Amphipoda to be the most abundant peracarid taxon in most cases (Brandt, 2001; Rehm et al., 2006, 2007) In addition, the coarse, glacial-marine sediments of the slope stations (Howe et al., 2007) might favour the occurrence of Amphipoda in the Southern Ocean in comparison to other areas at similar depths. Contrary to the mainly soft-bottomed deepsea areas of other world oceans, the deep-sea floor of the Southern Ocean is characterized by a relatively high proportion of hard-bottom created by drop stones (Thomson, 2006). Cumacea do not show strong differences in abundance with depth and seem to be well adapted to different sediment types. The peracarid composition at stations below 2000 m shows distinct differences to the peracarid composition on the Antarctic shelf. The Isopoda are the most abundant taxon at these stations (for increasing with depth; Dahl, 1954), although Amphipoda still comprise a relatively high number of individuals (Table 1). The relatively shallow stations (above 2000 m depth) show a higher resemblance in peracarid composition to the Antarctic shelf. At this slope depth Amphipoda dominate the samples and Cumacea also occur in high numbers, while Isopoda are less abundant. For example, station 133-2 at depth of about 1500 m in the Powell Basin, even though displaying a very distinctive pattern of abundance, has a relative peracarid composition similar to that of shelf samples. Whether the distributional patterns observed represent a clear break in the faunal composition between shelf and deep sea in the Southern Ocean or rather a gradual change cannot be answered from the data obtained so far. However, this change, abrupt or not, seems to take place in deeper waters than in other areas of the world oceans. A possible reason for this might be the fact that the Antarctic shelf is deeper than that of other continental shelf areas due to the weight of the ice shield on the continent. This special characteristic of
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the Antarctic continental shelf, combined with glacial periods of ice extensions and retreats, has led scientists to argue that the Antarctic organisms have developed a higher potential of eurybathy (Brey et al., 1996). The missing thermocline, as mentioned above, also may support the colonization of deeper waters by shelf fauna and vice versa, processes, which have been described as Antarctic submergence or emergence (see Brandt et al., 2007a, b). An continued analyses of all peracarid individuals to species level, as exemplified by Brandt et al. (2007a, b) for the isopods, will be necessary before community patterns of the Peracarida can be investigated on the background of depth, latitude and longitude as well as environmental parameters. Such an analysis based on these first patterns will broaden our knowledge on the zoogeography and bathymetry of all peracarid taxa considerably.
Acknowledgments We thank Prof. Dr. Dieter Fu¨tterer, chief scientist on Polarstern cruise ANT XIX/3-4, and Dr. Eberhard Fahrbach, chief scientist on Polarstern cruise ANT XXII/3, and the crew of R.V. Polarstern. Many thanks to all pickers and sorters, namely Dr. Marina Malyutina, Alexandra Toletti, Anna Marquart, Britta Wulf, David Hopff, Laura Wu¨rzberg, Antje Fischer and Lydia Kramer. We are grateful to Dr. Brigitte Ebbe for correcting the English language. The German Science Foundation kindly provided financial support (Br 1121/20-1/3 & Br 1121/26-1). This is ANDEEP publication # 74.
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