Marine Ecology. ISSN 0173-9565
ORIGINAL ARTICLE
A test of the seamount oasis hypothesis: seamounts support higher epibenthic megafaunal biomass than adjacent slopes Ashley A. Rowden1, Thomas A. Schlacher2, Alan Williams3, Malcolm R. Clark1, Robert Stewart1, Franziska Althaus3, David A. Bowden1, Mireille Consalvey1, Wayne Robinson2 & Joanne Dowdney3 1 National Institute of Water & Atmospheric Research (NIWA), Wellington, New Zealand 2 Faculty of Science, Health & Education, University of the Sunshine Coast, Maroochydore DC, Queensland, Australia 3 Commonwealth Scientific and Industrial Research Organisation (CSIRO), Wealth from Oceans Flagship, Marine Laboratories, Hobart, Tasmania, Australia
Keywords Biomass; epibenthic; megafauna; oasis hypothesis; seamount; slope. Correspondence Ashley A. Rowden, NIWA, Private Bag 14-901, Wellington, New Zealand. E-mail:
[email protected] Accepted: 8 March 2010 doi:10.1111/j.1439-0485.2010.00369.x
Abstract Seamounts have often been viewed as specialized habitats that support unique communities; this notion has given rise to several hypotheses about how seamount ecosystems are structured. One, the ‘seamount oasis hypothesis’, predicts that invertebrates are more abundant, speciose and attain higher standing stocks on seamounts compared to other deep-sea habitats. Because this hypothesis has remained untested for biomass, we ask two questions: (i) Do seamounts support a higher benthic biomass than nearby slopes at corresponding depths? (ii) If they do, which particular taxa and trophic groups drive observed difference in biomass? Analysis of more than 5000 sea-floor images reveals that the mean biomass of epibenthic megafauna on 20 southwest Pacific seamounts was nearly four times greater than on the adjacent continental slope at comparable depths. This difference is largely attributable to the scleractinian coral Solenosmilia variabilis, whose mean biomass was 29 times higher on seamounts. In terms of trophic guilds, filter-feeders and filter-feeders ⁄ predators made up a significantly greater proportion of biomass on seamounts, whereas deposit feeders and those with mixed feeding modes dominated at slope habitats. Notwithstanding support for the seamount oasis hypothesis provided by this study, the hypothesis needs to be critically tested for seamounts in less productive regions, for seamounts with a greater proportion of soft substratum, and in other parts of the oceans where scleractinian corals are not prevalent. In this context, testing of seamount paradigms should be embedded in a broader ecological context that includes other margin habitats (e.g. canyons) and community metrics (e.g. diversity and body size).
Introduction Seamounts are topographically distinct sea-floor features, up to a million of which may exist globally (Pitcher 2007). Yet, despite their vast number and widespread distribution, the biota of very few seamounts have been well sampled (Stocks 2009). Nonetheless, scientific study has Marine Ecology 31 (Suppl. 1) (2010) 95–106 ª 2010 Blackwell Verlag GmbH
generated a range of hypotheses and paradigms about the ecological structure and function of seamounts, mostly related to the potential for seamounts to have an insular or island character and ⁄ or a morphology that generates particular environmental conditions. One hypothesis is that seamounts are biologically highly productive. This hypothesis originated from observations of higher abun95
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dance and biomass of fish associated with seamounts compared to elsewhere in the ocean (see review by Morato & Clark 2007). However, what originated as a paradigm for seamount fish has since been extended to include invertebrates, although without any direct evidence (McClain 2007). With the resurgence in seamount research in the late 1990s and early 2000s, the question of whether benthic invertebrates are relatively more abundant on seamounts was examined more critically. Samadi et al. (2006) speculated that, like vent and seep habitats, seamounts could support populations of organisms with high abundances. They reasoned that seamounts were ‘places where a high trophic input allows an abundance of species and high population density [because] interactions between prominent topographic features and water masses increases turbulence and mixing, and enhances local biomass production by moving up nutrients in the euphotic zone’. This hypothesis, related to ones posed previously (e.g. Boehlert & Genin 1987), and an extension of a hypothesis posed and examined for chemosynthetic ecosystems in the deep sea (see Carney 1994), was formalized by Samadi et al. (2006) as the ‘seamounts as oases of productivity’ hypothesis (hereafter ‘seamount oasis hypothesis’). To date, this hypothesis has only been tested for larger invertebrates, using species richness as a proxy for abundance. Samadi et al. (2006) examined species richness data for squat lobsters (Galatheoidea) from Norfolk Ridge seamounts and an area of adjacent slope off New Caledonia (Southwest Pacific), and reported that ‘each individual seamount appeared to be richer than the restricted explored area on the slope, although a similar area to that of one seamount was sampled’. In another study, O’Hara (2007) found that brittle star (Ophiuroidea) species richness on seamounts was not elevated when compared to non-seamount areas. This author found that whilst seamounts can ‘exhibit high overall species richness for low number of samples … this did not increase with additional sampling at the rates found in non-seamount areas’ (O’Hara 2007). However, as noted by McClain (2007), testing the seamount oasis hypothesis by examining species richness is not ideal because diversity on seamounts could be controlled by competing explanations. McClain (2007) therefore suggested that future examinations of the seamount oasis hypothesis be restricted to tests for increased biomass. The generalization that invertebrates are more productive, more numerous or attain higher standing stocks on seamounts has persisted largely because of observational data showing dense concentrations of filter-feeding organisms, such as corals, on the peaks of seamounts (see references in review by Rogers et al. 2007). Although McClain et al. (2009) did demonstrate that the frequency of occur96
rence of some species on Davidson Seamount was higher than in nearby Monterey Canyon, seamount data are very sparse, and thus the oasis hypothesis, as related to biomass, remains quantitatively untested. Consequently, the two questions addressed here are: (i) Do seamounts support a higher benthic biomass than nearby slopes at corresponding depths? (ii) If so, which particular taxa and trophic groups drive the observed differences? We examine both questions using a large dataset of sea-floor images from seamounts and adjacent continental margins in the Southwest Pacific.
Methods Study regions
The two study regions are located in the southwest Pacific Ocean, one east of New Zealand on the Chatham Rise, and the other southeast of Australia off Tasmania (Fig. 1a). The Australian study area included the ‘Tasmanian seamounts’, where 12 features ranging in elevation from 125 to 565 m, and the adjacent slope off southern Tasmania were sampled (Fig. 1b). The New Zealand study region included the ‘Graveyard seamount complex’, where eight features ranging in elevation from 100 to 350 m and the adjacent slope on the Chatham Rise were sampled (Fig. 1c). The presence of natural fish aggregations on these seamounts (Koslow et al. 2001; Clark & O’Driscoll 2003) suggests that they likely provide an elevated food supply compared to adjacent slope areas. Seamounts in the Australian and New Zealand study regions have been subjected to bottom trawling since the early 1980s and mid-1990s, respectively. Fishing on the Tasmanian seamounts ceased in 1999, and on three of the Graveyard seamounts in 2001, when these seamounts received protected status (Koslow et al. 2001; Brodie & Clark 2003). Further details about the seamounts and slope habitats, and fishing effort are provided by Althaus et al. (2009) and Clark & Rowden (2009). Data sources
Australian data were sourced from two CSIRO companion voyages in 2006 (SS200611) and 2007 (SS200702). A towed stereo camera system (Shortis et al. 2008) and an epibenthic sled (1.2 · 0.6 m mouth opening and 25-mm net mesh size; Lewis 1999) were used to sample the sea floor. Camera transects were distributed in regular radial pattern on the seamounts (running from summit to base), and randomly on the slope. Slope transect sites for the present study were selected to be unfished and adjacent, and at similar depths, to the seamounts. The mean Marine Ecology 31 (Suppl. 1) (2010) 95–106 ª 2010 Blackwell Verlag GmbH
Rowden, Schlacher, Williams, Clark, Stewart, Althaus, Bowden, Consalvey, Robinson & Dowdney
length of camera transects was 3500 ± 1300 m (SE). Oblique images were taken automatically at 10–15-s intervals at a target height of 2–4 m above the seabed. Sled tows were co-located with camera transects on seamounts (22 stations), and the slope (13 stations) to the extent possible, or otherwise randomly located on closely adjacent areas on seamounts and slope. New Zealand data were sourced from an NIWA Seamounts Programme survey of the Graveyard seamounts in 2006 (TAN0604) and a New Zealand Ocean Survey 20 ⁄ 20 survey of the Chatham Rise in 2007 (TAN0705). A towed camera system (the Deep-Towed Imaging System, DTIS, Hill 2009) and an epibenthic sled (1 · 0.4 m mouth opening and 30-mm stretched net mesh size) similar to that used to sample Tasmanian seamounts and slope were deployed on both voyages. Camera transects were distributed in a regular radial pattern on the seamounts (running from summit to base), and randomly on the slope, within strata designed for broad-scale habitat mapping purposes. Slope transect sites were selected
A test of the seamount oasis hypothesis
to be unfished (one exception) and adjacent, and at similar depths, to the seamounts. The mean lengths of camera transects on seamounts and slopes were 824 ± 48 m (SE) and 1270 ± 102 m (SE), respectively. Vertical images were taken automatically at 20-s intervals on seamounts and at 15-s intervals on the slope at a target height of 2–3 m above the seabed. Sled stows were distributed randomly on seamounts (41 stations), and were run along the line of the camera transects at slope sites (six stations). Table 1 summarizes the seabed image sampling effort and distribution among seamount and slope habitat, and study regions (see also Fig. 1). Seabed images provided the primary data for the present study, whilst fauna recovered by benthic sleds were used to construct biomass conversion factors. Image analysis
Fauna that were distinctly identifiable in the images (>1 cm, but mainly >3 cm) are hereafter termed ‘epi-
a
b
c
Fig. 1. (a) Map of study regions in the southwest Pacific, and sampling locations on seamount and slope habitats off (b) Australia, south of the island of Tasmania, and (c) New Zealand, on the northern flank of Chatham Rise. Triangles = seamount location, circles = start position of slope transect, bathymetric contour lines are for 500 m, 1000 m, and 2000 m (in order of increasing darkness of blue).
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Table 1. Summary of image sampling effort and distribution on slope and seamount habitat off Australia and New Zealand (see Fig. 1 for geographic position of sampling locations). area
habitat
station ⁄ site name
no. transects
no. images
mean depth (m)
min. depth (m)
max. depth (m)
Australia
slope
Huon 1000 – stn 16 Huon 1000 – stn 45 Huon 1000 – stn 47 Huon 1000 – stn 53 Tasman 1000 – stn 73 Tasman 1200 – stn 72 Tasman 1200 – stn 45 Tasman canyon – stn 62 Dory Hill Hill B1 Hill K1 Hill U Mini Matt Mongrel Patience Pedra Sister 1 (south) Sister 2 (north) Z5 Z16 TAN0705 ⁄ 188 TAN0705 ⁄ 190 TAN0705 ⁄ 192 TAN0705 ⁄ 194 TAN0705 ⁄ 234 TAN0705 ⁄ 258 TAN0705 ⁄ 259 Diabolical Ghoul Gothic Graveyard Morgue Pyre Scroll Zombie
1 1 1 1 1 1 1 1 1 1 3 4 1 2 1 2 1 2 1 1 1 1 1 1 1 1 1 8 5 8 8 16 8 4 8
46 69 163 183 194 42 156 34 83 54 95 186 17 252 53 222 130 76 20 37 71 60 64 61 60 67 63 202 49 199 485 755 367 187 337
929 951 902 880 1149 1182 1166 831 1200 1259 1431 1240 1210 1073 998 1029 1073 1120 1194 1207 997 895 791 1200 1017 1214 1213 997 982 1108 950 1088 1116 982 994
867 809 686 620 967 1175 1078 738 1091 1139 1248 1099 1179 778 904 734 864 903 1176 1061 997 895 791 1200 1017 1214 1213 930 941 1016 761 914 1022 904 918
991 1046 1131 1112 1276 1188 1288 896 1418 1405 1682 1401 1237 1244 1097 1276 1280 1277 1250 1363 997 895 791 1200 1017 1214 1213 1049 1008 1159 1099 1199 1199 1079 1079
seamount
New Zealand
slope
seamount
benthic megafauna’ and were identified to the lowest practicable taxonomic level and counted with the aid of image analysis software packages (PHOTOSHOP, IMAGEJ). Data for both unitary and colonial taxa were recorded as numbers of individuals per image frame. The only taxa for which this was potentially ambiguous were matrix-forming scleractinian corals (primarily Solenosmilia variabilis), which often cover large areas of seabed but in which live polyps are concentrated in localized patches (i.e. much of the skeletal matrix is not living). Because live polyps are distinctive in colour (orange-pink), it was possible to count patches of living coral in the images to obtain a count-based abundance estimate. To combine the CSIRO and NIWA data, a common level of taxonomic grouping – based on commonalities in taxon 98
occurrence and identification – was decided upon (Table 2). Data were then checked for consistency of identifications between regions and, where necessary, images were re-analyzed prior to data analysis. The main feeding mode of each taxonomic grouping (Table 2) was assessed based on existing knowledge of the biology and ecology of the organisms. The area of each seabed image was determined either by calculation from calibrated stereo image pairs (CSIRO data, see Althaus et al. 2009 for detail) or from parallel-scaling lasers projected in the image (NIWA data). In images where the two lasers were not visible (6.5%), scale was estimated by reference to fauna of known size (e.g. orange roughy Hoplostethus atlanticus; mean standard length 35 cm). The average area per image was 4.8 ± 0.07 m2 (SE) for the Australian data, and Marine Ecology 31 (Suppl. 1) (2010) 95–106 ª 2010 Blackwell Verlag GmbH
Rowden, Schlacher, Williams, Clark, Stewart, Althaus, Bowden, Consalvey, Robinson & Dowdney
A test of the seamount oasis hypothesis
Table 2. Biomass of epibenthic megafauna encountered in deep-sea images on slope and seamount habitat off Australia and New Zealand.
taxon Foraminifera Foraminifera Porifera Porifera Hexacorallia, Actiniaria Anemones Hexacorallia, Antipatharia Antipatharians Hexacorallia, Sceractinia Enallopsamia spp.a Madrepora occulataa Scelarctinia (colonial) Scleractinia (solitary) Solenosmilia variabilisb Hexacorallia, Zoanthidea Zooanthids Octocorallia, Alcyonacea Anthomastus spp. Chrysogorgia spp. Chrysogorgiidae gen. A sp. A Corallium spp.b Gorgonians Isididae Narella spp. Paragorgia spp.a Primnoidae Soft corals Thouarella spp. Octocorallia, Pennatulacea Pennatulacea Hydrozoa, Hydroidolina Hydroids Stylasterids Bryozoa Bryozoans Brachiopoda Brachiopods Mollusca Scaphopoda Gastropods Octopus Echiura Echiurans Annelida, Polychaeta Polychaetes Pycnogonida Pycnogonids Crustacea, Isopoda Isopods Crustacea, Decapoda Crabs Galatheoidea Lobsters Pagurids Prawns
feeding mode
mean individual weight (ww, g)
n
SE
Q75
Q100
F
1
0.33
F
105
42.49
6.52
5.00
443.00
P
19
24.17
10.64
2.00
160.00
FP
2
38.00
23.00
15.00
61.00
FP FP FP FP FP
7 4 3 12 5
31.86 111.85 6.33 18.73 769.74
7.91 54.74 0.33 3.79 522.47
11.70 21.00 6.00 6.70 39.70
72.50 248.00 7.00 40.00 2840.00
FP
3
95.33
53.01
35.00
201.00
F FP FP FP FP FP FP FP FP FP FP
5 4 1 8 16 7 2 7 7 1 9
8.97 8.50 1.00 50.61 93.63 28.15 35.25 120.87 12.93 5.00 9.26
5.76 2.62
2.50 2.50
32.00 14.00
16.25 64.55 5.68 33.25 36.04 4.10
3.00 5.00 6.36 2.00 31.00 3.25
113.60 1053.00 41.67 68.50 260.00 33.50
4.22
4.50
43.00
FP
2
39.50
24.50
15.00
64.00
F FP
14 9
10.07 5.44
4.57 1.28
2.00 2.00
64.18 12.00
F
1
5.00
F
8
4.16
0.49
2.40
6.75
P OP P
1 43 1
1.00 12.99 6.00
2.88
2.00 6.00
77.00 6.00
D
2
2.00
2.00
2.00
FD
19
6.77
2.20
29.33
2
5.00
5.00
5.00
OP
27
1.18
0.10
1.00
3.00
OP OP P OP OP
19 30 2 31 34
11.55 2.41 8.00 11.40 3.98
5.61 0.37 3.00 2.46 0.41
4.00 1.26 5.00 3.00 1.67
112.00 11.00 11.00 51.00 14.00
P
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1.47
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Table 2. (Continued).
taxon Echinodermata, Asteroidea Asteroids Brisingids Echinodermata, Echinoidea Dermechinus horridus Echinoids Echinodermata, Crioidea Crinoids (not stalked) Crinoid (stalked) Echinodermata, Holothurioidea Holothurians Echinodermata, Ophiuroidea Ophiuroids Chordata, Ascidiacea Ascidians
feeding mode
n
mean individual weight (ww, g)
SE
Q75
Q100
P F
39 5
47.26 218.50
17.57 61.90
4.00 56.00
650.00 375.00
F D
4 18
49.25 140.12
7.22 21.34
38.00 44.00
69.00 417.50
F F
25 2
10.48 43.17
2.38 8.83
2.00 34.33
59.00 52.00
DF
6
206.17
71.07
82.00
550.00
OPF
67
14.12
4.50
1.06
165.71
7
21.35
2.97
11.60
32.00
F
Tabulated values are wet weights (ww, g) derived mostly from on-board measurements of fresh sled tow samples. Because small (
