Polar Biol (2013) 36:445–451 DOI 10.1007/s00300-012-1269-1
SHORT NOTE
Macrofaunal crustaceans in the benthic boundary layer from the shelf break to abyssal depths in the Ross Sea (Antarctica) Anne-Nina Lo¨rz • Stefanie Kaiser • David Bowden
Received: 6 September 2012 / Revised: 18 November 2012 / Accepted: 20 November 2012 / Published online: 2 December 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract The New Zealand International Polar Year– Census of Antarctic Marine Life (NZ IPY-CAML) project added to previous benthic studies in the Ross Sea by extending sampling north from the continental shelf to previously unsampled areas of the shelf break, slope, abyssal plain and seamounts in the region. The aim of the current study is to give first insights into the deep-sea community structure of the Ross Sea focussing on a component of the benthic boundary layer that is macrofaunal crustaceans collected one metre above the seafloor. We assess changes in Ross Sea crustacean community composition from the shelf break (474 m) to the abyss (3,490 m) and compare the Ross Sea crustacean fauna to areas elsewhere in the Southern Ocean. Analyses reveal high relative abundances, suggesting an important role in the food web. Among the peracarid crustaceans, there was a decline in the proportion of amphipods with increasing depth. Three of 15 isopod families (Acanthaspidiidae, Nannoniscidae and Desmosomatidae) were identified to species level and about 72 % of the species were new to science. Isopod diversity in the Ross Sea abyss appears to be comparable to that in the highly speciose Weddell deep sea. Standardised sampling of these crustacean communities allows setting the biodiversity of the Ross Sea into a global context. Electronic supplementary material The online version of this article (doi:10.1007/s00300-012-1269-1) contains supplementary material, which is available to authorized users. A.-N. Lo¨rz (&) D. Bowden National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Wellington 6021, New Zealand e-mail:
[email protected] S. Kaiser Biozentrum Grindel and Zoological Museum, University of Hamburg, Martin-Luther-King-Platz 3, Hamburg, Germany
Keywords Southern Ocean Deep sea Peracarida Isopoda Ross Sea Weddell Sea
Introduction Benthic research in the Ross Sea to date has been mostly in coastal waters, particularly in the McMurdo Sound and Terra Nova Bay regions (e.g. Dayton et al. 1974; Pearse 1986; Chiantore et al. 2002) and along the Victoria Land coast as part of the Latitudinal Gradient Project (LGP, Cummings et al. 2006; Thrush et al. 2006; Cummings et al. 2010). In addition, oceanographic and benthic research programmes over several decades have given us at least an outline picture of the types of benthic assemblages present across much of the continental shelf and how these are linked to depth and substrate (e.g. Bullivant 1959; Bullivant and Dearborn 1967; Barry et al. 2003; Ducklow et al. 2006; Smith et al. 2007). As elsewhere in the Southern Ocean, deep benthic habitats of the continental slope and abyssal depths remain largely unsampled (Arntz et al. 1994; Clarke and Johnston 2003; Brandt et al. 2007). New Zealand has a long history of benthic research in the Ross Sea, beginning with the International Geophysical Year (IGY, 1958–59) (Bullivant 1959; Bullivant and Dearborn 1967). The New Zealand International Polar Year–Census of Antarctic Marine Life (NZ IPY-CAML) project added to previous benthic studies in the Ross Sea by extending sampling north from the continental shelf to previously unsampled shelf break, slope, abyssal plain and seamount habitats in the region. In the Southern Ocean, crustaceans are among the most diverse groups (Clarke and Johnston 2003) and are widely distributed across shelf and deep-sea benthic environments;
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particularly, peracarid crustaceans are a rich and abundant faunal component of the Southern Ocean shelf, slope and deep-sea benthos (Clarke and Johnston 2003; Brandt et al. 2007). Five peracarid orders occur in the Southern Ocean (amphipods, cumaceans, isopods, mysids and tanaids). Their remarkable ecological and evolutionary success in the Antarctic might be partly due the Cenozoic extinction of brachyuran decapods, and the subsequent occupation of their niches by peracarids (Brandt 1999). Yet, while amphipods are more prevalent at shelf depth, isopods have been revealed to be remarkably diverse in the deep sea (Brandt et al. 2007). Here, crustacean assemblages sampled during the IPY voyage of the RV Tangaroa to the Ross Sea (TAN0802) are examined across a depth gradient from the shelf break to abyssal depth. Benthic boundary layer (BBL) assemblages can consist of varying proportions of meroplanktonic organisms (e.g. crustacean larvae), holoplanktonic organisms (e.g. euphausiids) and motile suprabenthic organisms (Dauvin and Vallet 2006; Lo¨rz 2011). Samples were collected using a fine mesh epibenthic sledge (Brenke 2005) designed to sample macrofauna from the water column immediately above the seabed. This design of sledge has been successfully deployed sampling the BBL in several areas of the world, particularly at high southern latitudes including the Weddell- (Linse et al. 2002; Lo¨rz and Brandt 2003; Brandt et al. 2007), Scotia- and Amundsen seas (Kaiser et al. 2009), and around New Zealand (Lo¨rz 2011). The aim of the current study is to give first insights into the deep-sea community structure of the Ross Sea region and to address the following questions: 1. 2.
How does crustacean abundance and composition change in the Ross Sea across depth? How does the Ross Sea BBL crustacean fauna compare to areas elsewhere in the Southern Ocean?
Methods Sampling Macrobenthic crustaceans were collected by means of a fine mesh epibenthic sledge (Brenke 2005) referred to here as the Brenke sledge. The sledge consists of a steel frame supporting two nets, one above the other, such that the lower (‘‘epibenthic’’) net collects samples from 0 to 0.60 m above the seabed and the upper (‘‘suprabenthic’’) net from 0.77 to 1.12 m above the seabed. Although the nets are positioned at different heights above the seabed, in practice, there is considerable overlap between them in terms of the fauna sampled (Brenke 2005). Here, we report results from the upper net only, which is likely to comprise a large
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proportion of the BBL present. The net has an opening of 100 cm width and a 500-lm mesh net with 300-lm rigid cod-end container. The opening has a door which remains closed in the water column and is opened via a mechanical lever when the sledge is in contact with the seabed. Macrofauna are defined here as all taxa retained on a 500-lm mesh and thus include taxa such as copepods and ostracods, which are sometimes classified as meiofauna. Seven Brenke sledge samples collected from the northern part of the Ross Sea region during the TAN0802 expedition were analysed: one from the shelf break, two from the continental slope, three on the abyssal plain and one on the summit plateau of Admiralty Seamount (Fig. 1, Table 1). At sea, the cod-end samples were elutriated with filtered seawater in 20 L bins to extract delicate fauna intact. Residual sediments were then sieved (300 lm) and all samples were preserved in 99 % ethanol. Seabed swept area was calculated from the times of arrival at and departure from the seabed, the vessel’s speed and the width of the net opening (1 m). Towed distance was calculated as the Great Circle distance between the start and end positions of the tow recorded by the ship’s Global Positioning System (GPS). Sample processing Crustaceans were only counted if the head was present. Samples were first sorted to higher taxonomic groups, then Isopoda were identified to family level and selected families (Acanthaspidiidae, Desmosomatidae and Nannoniscidae) to genus and, where possible, species level. Data analysis Abundance data were standardised to number of individuals in relative abundance per 1,000 m2 of seabed area prior to analysis. Variations in abundance of peracarid orders across the study area were visualised by means of bar graphs. Mean abundances per site were used to compare Brenke sledge collections made during the IPY Ross Sea expedition to other Antarctic and New Zealand samples taken with the same or similar gear.
Results and discussion How does crustacean abundance and composition change in the Ross Sea across depth? Abundance patterns A total of 16,224 individuals of macrofauna were collected, the crustaceans belonging to 11 taxonomic units of order or higher level. Crustacean abundance was highest at the shelf
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Fig. 1 Sampling locations (top) and depth profile (bottom) of sampling sites in the western Ross Sea. The 1,000 m isobath is shown in bold; the 500 m isobath is shown in light grey
break (C15, 470 m) and on the summit of Admiralty seamount (C24, 503 m), with more than 2,000 ind. 1,000 m-2, and lowest at deeper sites on the slope and the abyssal plain (Table 2). Euphausiacea and leptostracans were overall poorly represented and only abundant at the seamount station (C24). Cirripedia were collected at one station (C16) as epibionts of the pycnogonid Nymphon australe Hodgson, 1902, the barnacles of the genus
Litoscallpellum are common epibionts. Amphipoda dominated shallow samples above 1,000 m depth (C15, C16, C24), while Copepoda were dominant at two abyssal sites, C33 and C35. Copepoda are known to be frequent inhabitants of the BBL (Bradford-Grieve 2004). Across the transition from the shelf break to the base of the slope (sites C15, C16, C17), Isopoda constituted an increasing proportion of total Crustacea with increasing depth, matched
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Table 1 Brenke sledge station sampled via RV Tangaroa in the western Ross Sea 2008 from the shelf break down to slope and the abyssal plain, their location can be seen on Fig. 1 Cruise
Station
Site
Area
Date
Start lat.
Start long.
Finish lat.
Finish long.
Depth (maximum) [m]
Tow distance (m)
TAN0802
116
C15
Shelf break
20/02/08
72°36.22S
175°19.91E
72°36.33S
175°20.16E
474
247
TAN0802
127
C16
Mid slope
21/02/08
72°19.01S
175°28.43E
72°19.00S
175°28.94E
979
289
TAN0802 TAN0802
135 188
C17 C30
Lower slope Abyss
22/02/08 01/03/08
72°04.38S 68°33.13S
175°35.13E 178°22.32W
72°04.77S 68°33.64S
175°34.99E 178°21.19W
1,645 3,212
730 1,229
TAN0802
231
C33
Abyss
06/03/08
67°37.15S
178°54.71W
67°37.12S
178°56.04W
3,490
944
TAN0802
288
C35
Abyss
12/03/08
66°45.51S
171°09.14E
66°45.88S
171°09.75E
3,380
821
TAN0802
267
C24
Seamount
10/03/08
66°58.69S
170°49.79E
66°58.74S
170°50.50E
503
525
Table 2 Crustacean assemblage composition in the benthic boundary layer collected at seven stations in the Ross Sea; abundance values were standardised to 1,000 m2 Taxon
Site C15
C16
C17
C30
C33
C35
C24
Sum
Ostracoda
93
3
123
14
203
4
67
Cirripedia
0
31
0
0
0
0
0
507 31
Copepoda
16
0
144
6
562
613
291
1,632
Leptostraca
0
0
0
0
0
7
21
28
Amphipoda
2,187
575
396
102
132
87
724
4,203
Tanaidacea
24
35
78
7
40
18
23
225
Cumacea
20
0
25
13
101
26
2
187
Isopoda
53
79
384
134
85
23
703
1,461
Mysidacea
0
7
1
13
5
7
0
33
Euphausiacea
0
0
0
0
1
0
192
193
Reptantia
0
0
0
0
3
0
0
3 Fig. 2 Relative abundance (%) of peracarid orders at each sampling site
by a decline in the proportion of amphipods (Fig. 2). In two of the three abyssal stations, Tanaidacea were the most abundant taxon. The remaining peracarid taxa (Cumacea and Mysidacea) did not show any obvious patterns with depth. In all samples, asellote isopods were the most abundant isopod group, representing more than 93 % of all individuals across 15 families: Desmosomatidae, Nannoniscidae, Munnopsidae, Munnidae, Paramunnidae, Dendrotionidae, Haploniscidae, Macrostylidae, Ischnomesidae, Janiridae, Acanthaspidiidae, Mesosignidae, Stenetriidae, Haplomunnidae and Thambematidae (Online Resource 1). Of these, Munnopsidae was the most abundant family representing 56 % of total Isopoda, followed by Desmosomatidae (12 %) and Nannoniscidae (5 %), while Acanthaspidiidae constituted 2 %. On the summit of Admiralty Seamount (site C24), Munnopsidae and Janiridae were the most dominant taxa, while Munnopsidae, Nannoniscidae, Paramunnidae and Mesosignidae were most numerous at the base of the continental slope (site
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C17). Abyssal sites were dominated by Munnopsidae and Desmosomatidae. The Brenke sledge proved to be very effective for collecting mobile macrofauna in a size range poorly sampled by many conventional benthic gear types (such as multi- and box corers). Our results reveal that peracarid crustaceans constituted a large proportion of the total crustaceans, that is, 6,109 of the actual 8,503 individual crustaceans counted. Peracarid abundances were particularly high on Admiralty Seamount (C24). This might be related to enhanced productivity due to upwelling (Genin 2004; Clark et al. 2010) despite relatively oligotrophic conditions in surface waters overlying Admiralty Seamount (Bowden et al. 2011). The upper net of the Brenke sledge was specifically designed to sample macrofauna in the near-bottom water layers (Brenke 2005). However, many of the supranet samples presented in this study also contained fully benthic taxa, such as tanaids, cumaceans, and many amphipod and
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isopod taxa. This is likely to be caused by substratum disturbance by the towing cable or resuspension of sediments in the pressure wave ahead of the sledge. In contrast, substantial numbers of suprabenthic taxa, such as mysids and euphausiids as seen here, usually do not occur in the lower net (authors pers. observation), indicating that the nets may indeed sample subtly different faunas. Richness patterns The number of isopod families was highest on the lower slope (site C17, 14 families), followed by Admiralty Seamount (C24, 10 families), and the abyssal sites (C30 and C35, 9 families). Lowest values were obtained at the shallower stations (C15 and C16). Acanthaspidiidae, Desmosomatidae and Nannoniscidae were further identified to genus and species level (see Online Resource 2 for a complete species list). Twelve acanthaspidiid specimens could be assigned to five species in two genera (Ianthopsis and Acanthaspidia). Among these, one species was putatively undescribed (Ianthopsis sp. 16), while the remaining four belonged to already known species (Ianthopsis multispinosa Vanho¨ffen, 1914, I. nodosa Vanho¨ffen, 1914, I. nasicornis Vanho¨ffen, 1914, and Acanthaspidia drygalskii Vanho¨ffen, 1914). Thirty-nine specimens of Nannoniscidae belonged to ten species in five genera (Austroniscus, Exiliniscus, Nannoniscus, Regabellator and Nannoniscidae gen. nov.) of which nine species were potentially species new to science (described species include A. ovalis Vanho¨ffen, 1914). Thirty-seven specimens of Desmosomatidae belonged to ten species in five genera (Chelator, Disparella, Eugerdella, Mirabilicoxa and Prochelator). Eight species were putatively new to science, while two (Eugerdella serrata Brix, 2006 and Disparella maiuscula Kaiser and Brix, 2005) have been already described. Although species-level identification is so far limited to three isopod families, these preliminary analyses revealed diversity levels at one abyssal site (C30) to be remarkably high, and within the same range recorded from the specious Weddell Sea abyss for these families (Brandt et al. 2007). How does the Ross Sea benthic boundary layer crustacean fauna compare to areas elsewhere in the Southern Ocean? Many datasets collected with the Brenke sledge exist from several areas of the world, such as off Greenland (Brandt and Berge 2007), the Weddell- and Scotia deep sea (Bro¨keland et al. 2007) or off the South Sandwich islands (Kaiser et al. 2008). However, in these studies, data of supra- and epinet were merged, which does not allow a direct comparison to the IPY Ross Sea supranet data presented here. The only Brenke sledge supranet data
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published are from the shelf and slope of the Weddell Sea (Linse et al. 2002; Lo¨rz and Brandt 2003) and New Zealand (Lo¨rz 2011). Our data show peracarid abundance on the Ross Sea shelf break to be exceptionally high compared to data from the Weddell and Scotia Seas (Linse et al. 2002; Lo¨rz and Brandt 2003); that is, abundance levels of Ross Sea peracarids were 1 and 3 orders of magnitude higher than those found in the Weddell and Scotia Sea, respectively. This may reflect overall high surface productivity of the Ross sea region and the shelf in particular (Arrigo et al. 1998), although variation in abundance levels may be also caused by patchy distribution of fauna or seasonal effects (Rehm et al. 2006). Peracarid abundances of the mid-slope samples (1,000–2,000 m) presented here resemble values from the Weddell Sea (Linse et al. 2002). So far, no Southern Ocean data exist to compare lower slope or abyssal samples taken by the supranet. Lo¨rz (2011) investigated crustacean BBL assemblages of the New Zealand shelf and slope; these samples were also collected by the upper net of the Brenke sledge. Comparing the peracarid abundance between New Zealand and Ross Sea waters reveals that peracarid abundance in the BBL of the Southern Ocean is comparable to that in the South-west Pacific. Very high peracarid abundances were sampled on the western Ross Sea shelf break and slope compared to previous surveys of these regions sampled with different gear types (Lo¨rz 2009). Exceptions include studies by Rehm et al. (2006), and Choudhury and Brandt (2009), which also revealed high abundances of (shelf) peracarid crustaceans. Species-level identifications of three isopod families allowed initial investigations of faunal affinities and richness of the Ross Sea slope and abyss compared to other areas of the Southern Ocean. Most genera are cosmopolitan and have already been recorded from Antarctic waters (cf. Brandt et al. 2007). Within Nannoniscidae, though, some species could not be assigned to any existing genus. Similar species have been recorded from the Weddell deep sea during ANDEEP expeditions (Brandt et al. 2007; S. Kaiser pers. comm.). Some species have been recorded from elsewhere in the Southern Ocean (i.e. Ianthopsis nodosa from the East Antarctic; Austroniscus ovalis, Weddell Sea and East Antarctic; Disparella maiuscula, Weddell Sea; Eugerdella serrata, Scotia arc, Weddell Sea, Ross Sea); most species (*72 %), however, are putatively new to science. Furthermore, some species, genera and families have been recorded from the Ross Sea for the first time (e.g. species: Ianthopsis nodosa; genera: Acanthaspidia, Disparella, Exiliniscus; families: Macrostylidae, Mesosignidae, Thambematidae, cf. Choudhury and Brandt 2009). Levels of novelty resemble values from the Weddell Sea (Brandt et al. 2007) and Amundsen Sea (Kaiser et al. 2009) probably
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reflecting lack of knowledge for some groups (e.g. smallsized asellotes) and some regions (Southern Ocean deep sea, Amundsen Sea, Ross Sea, see also Choudhury and Brandt 2009). Three species, Acanthaspidia drygalskii, Ianthopsis nasicornis and Eugerdella serrata, show a wide, circumAntarctic distribution (Brandt 1991; Choudhury and Brandt 2009; Kaiser et al. 2009). Raupach and Wa¨gele (2006) found A. drygalskii to represent a species complex, thus, it is very likely that future molecular studies will reveal Weddell Sea and Ross Sea lineages to be genetically distinct species. The identification of remaining isopod taxa, as well as analyses of other Ross Sea stations, are required to assess how Ross Sea isopod diversity varies between stations and with increasing depths. The high abundance of peracarid crustaceans in the Ross Sea as well as in the Weddell Sea and at the Antarctic Peninsula indicates their importance in the marine trophic/ food web in Antarctica. This kind of data is crucial to enable better representation of BBL communities in ecosystem food web models (Pinkerton et al. 2010). Acknowledgments This research was funded by the New Zealand Government under the New Zealand International Polar Year–Census of Antarctic Marine Life Project (IPY2007-01). We gratefully acknowledge project governance provided by the Ministry of Fisheries Science Team and the Ocean Survey 20/20 CAML Advisory Group (Land Information New Zealand, Ministry of Fisheries, Antarctica New Zealand, Ministry of Foreign Affairs and Trade, and National Institute of Water and Atmosphere Ltd). Part-funding was provided by the Ministry of Science and Innovation project C01X1001 (Protecting Ross Sea Ecosystems) and project COBR1302 (Biodiversity & Biosecurity). The NIWA Marine Invertebrate Collection Team, especially Sadie Mills, coordinated the sorting and registration processes. Marlene Timm (University of Hamburg) is thanked for her help identifying the Acanthaspidiidae. S. Kaiser acknowledges a grant provided by the University of Hamburg and the German Academic Exchange Service (DAAD). Two anonymous reviewers are thanked for their constructive critique to an earlier version of this paper.
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