Habitats and Benthos of a Deep-Sea Marginal Plateau, Lord Howe Rise, Australia

57 Habitats and Benthos of a

Deep-Sea Marginal Plateau, Lord Howe Rise, Australia Peter T. Harris, Scott L. Nichol, Tara J. Anderson Andrew D. Heap Marine and Coastal Environment Group, Geoscience Australia, Canberra, ACT, Australia

Abstract Lord Howe Rise is a deep-sea marginal plateau located in the Coral Sea and Tasman Sea, 125,000 km2 in area and 750–1,200 m in water depth. An area of the western flank of northern Lord Howe Rise covering 25,500 km2 was mapped and sampled by Geoscience Australia in 2007 to characterize the deep-sea environments and benthic habitats. Geomorphic features in the survey area include ridges, valleys, plateaus, and basins. Smaller superimposed features include peaks, moats, holes, polygonal furrows, scarps, and aprons. The physical structure and biological composition of the seabed were characterized using towed video and sampling of epifaunal and infaunal organisms. These deep-sea environments are dominated by thick, depositional, soft sediments (sandy mud), with local outcrops of volcanic rock and mixed gravel–boulders. Ridge, valley, and plateau environments were moderately bioturbated, but few organisms were directly observed or collected. Volcanic peaks were bathymetrically complex hard-rock structures that supported sparse distributions of suspension feeders (e.g., cold-water corals and glass sponges) and associated epifauna (e.g., crinoids and brittle stars). Isolated outcrops along the sloping edge of one ridge also supported similar assemblages, some with high localized densities of coral-dominated assemblages. Key Words: Deep sea, plateau, epibenthic, cold-water corals, multibeam sonar, Coral Sea, Tasman Sea

Introduction Lord Howe Rise is a marginal plateau located in the Coral Sea and Tasman Sea, composed mainly of continental fragments that detached from the eastern margin of Seafloor Geomorphology as Benthic Habitat. DOI: 10.1016/B978-0-12-385140-6.00057-8 Copyright © 2012 Crown Copyright. All rights reserved.

778

Seafloor Geomorphology as Benthic Habitat 150° E

20° S

160° E

Lord

Coral Sea

170° E

2

0

km

500 Study area

Bathymetry 0m 1

Howe

30° S

8,700 m

se

Ri

40° S

Tasman Sea

Outer limit of EEZ Extended continental shelf

Figure 57.1  Location map of study area on the Lord Howe Rise, eastern Australian margin. The outer limit of the exclusive economic zone (EEZ) and extended continental shelf of Australia: 1—Lord Howe Island; 2—dotted line represents Lord Howe seamount chain.

continental Australia during the late Jurassic and Cretaceous [1]. Lord Howe Rise is an extensive feature of the South Pacific Ocean, spanning 2,800 km in latitude (19°S to 43°S) and 450–650 km wide. The crest of the Lord Howe Rise lies around 750–1,200 m below sea level and it is surmounted by small volcanic islands and seamounts (i.e., Lord Howe Island and Lord Howe seamount chain), while the 2,000-m isobath outlines the base of the plateau (Figure 57.1). The oceanic environment is part of the lower-bathyal biome having intermediate surface primary production, low dissolved oxygen in bottom waters, and mean bottom water temperatures of between 1.7°C and 2.3°C [2]. The region is influenced by eddies shed from the East Australia current [3].

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The Lord Howe Rise benthic environment is in a near-pristine condition with “low” human impact according to Halpern et al. [4]. The region is affected by shipping, limited fishing, and laying of telecommunications cables but is otherwise considered to be near pristine. In October–December 2007, part of the western flank of northern Lord Howe Rise was mapped and sampled by Geoscience Australia using the New Zealand research vessel Tangaroa [5]. Bathymetric data were collected

Table 57.1  Data Acquired at Sampling Stations during Geoscience Australia Survey TAN0713 [5] Geomorphic Feature/Unit

Station

Video

Still Images

Ridge Volcanic peak Ridge Valley Ridge Valley Ridge Plateau Ridge Ridge Ridge Ridge Ridge Volcanic peak Ridge Volcanic peak Ridge Ridge Ridge Ridge Ridge Ridge Ridge Ridge Plateau Ridge Volcanic peak Volcanic peak Volcanic peak Plateau Plateau Plateau Plateau Plateau

04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 29 30 31 32 33 42 43 44 45 46

✓ ✓

✓

Surface Sediments

Infauna

✓

✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

Epifauna

✓

✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓

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Seafloor Geomorphology as Benthic Habitat

using a 30-kHz Simrad multibeam sonar system over a survey area of 25,500 km2 and gridded to a spatial resolution of 50 m. Seabed habitats and associated assemblages were observed using underwater towed video at 32 stations, while benthic organisms (epifauna and infauna) were collected from 14 and 12 stations, respectively (Table 57.1). Surface sediment samples were collected by grab, boxcore, and piston core at 28 representative stations across the survey area (Table 57.1; Figure 57.2).

Figure 57.2  Multibeam sonar bathymetric map of northern Lord Howe Rise based on a 50-m grid, showing station locations from Geoscience Australia survey TAN0713.

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Geomorphic Features and Habitats Bathymetry across the western flank of the Lord Howe Rise plateau is characterized by a trend of increasing water depth toward the southwest, with a gentle regional gradient of 0.3° (Figure 57.2). Within this area, large-scale geomorphic features are broadly oriented north–south, tending to a northwest–southeast alignment across the western part of the survey area. Overall, the relief of the area is on the scale of tens of meters. However, local bathymetric anomalies exist where peaks, valleys, and scarps introduce hundreds of meters of relief to the seafloor [6]. The classification of seabed geomorphic features used here is based on feature names and definitions from the International Hydrographic Organization [7], with additional terms for other small-scale features taken from the literature [8] (Table 57.2). Geomorphic features were identified using bathymetric profiles drawn Table 57.2  Definitions for Geomorphic Features, Units, and Elements Mapped within the Lord Howe Rise Survey Area Geomorphic Feature

Definition

Plateau

A flat or nearly flat elevation of considerable areal extent, dropping off abruptly on one or more sides.

Geomorphic unit Basin Ridge Valley

A depression in the seabed, more or less equidimensional in plan and of variable extent. An elongated narrow elevation of varying complexity having steep sides. A relatively shallow, wide depression, the bottom of which usually has a continuous gradient.

Geomorphic element Apron Escarpment (scarp)

Hole Moat

Peak Polygonal furrows

A gently dipping surface, underlain primarily by sediment, at the base of any steeper slope. An elongated, characteristically linear, steep slope separating horizontal or gently sloping sectors of the seabed in nonshelf areas. A small local depression, often steep sided, in the seabed. An annular depression that may not be continuous, located at the base of many seamounts, oceanic islands, and other isolated elevations. A prominent elevation either pointed or of a very limited extent across the summit. A network of cracks in the seabed forming a polygonal pattern. Cracks can be straight to curved, with patterns, lengths, depth, and width showing great variation.

Note: All definitions are from the International Hydrographic Organization standardized list of undersea features [6], except polygonal furrows taken from Goudie [8].

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Seafloor Geomorphology as Benthic Habitat

in IVS 3D Fledermaus software (version 6). Boundary contours were then mapped as polygons in ArcGIS, from which geomorphic features, units, and elements were interpreted (Figure 57.3). In this study, the Lord Howe Rise plateau is the primary geomorphic feature upon which geomorphic units are mapped; these include ridges, valleys, and basins. Geomorphic elements are superimposed on these units and include peaks, moats, holes, polygonal furrows, scarps, and aprons [6]. Ridges: These are the most extensive geomorphic units in the survey area, covering 12,700 km2 (Figure 57.3). The eastern sector of the mapped area is occupied by

Figure 57.3  Geomorphic features, units, and volcanic peaks of northern Lord Howe Rise, with surface areas and water depth ranges indicated. Texture of surface sediments at sample stations is also shown.

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the largest ridge, which extends 150 km north to south and is 30–50 km wide, with slightly convex slopes of up to 0.5°. Locally, holes and broad depressions introduce topographic variability of 50–100 m to the ridge crest across distances of 10–60 km. North of latitude 27.1°S and within the central to western part of the survey area, the seafloor is characterized by a relatively complex terrain of a multiple ridges and valleys. Here the ridges are associated with volcanic peaks that generally sit on the mid- to lower-ridge slopes. Valleys: These are formed across water depths ranging from 1,600 m in the east to 2,400 m in the west of the survey area and cover 2,900 km2. The deepest valley in the survey area is located in the far southwest, in 2,000–2,400 m water depth (Figure 57.3). This valley extends 30 km in an east–west direction, widening from 6 km at the headwall to 16 km at the mouth. The slope valley floor decreases from 20° at the headwall to 1.5° along the upper reaches and0.5° along the lower reaches and thalweg. Elsewhere, valleys are of similar dimensions (30–45 km long and 3.5–14 km wide), with gradients of 0.01–0.03°. Plateaus: Low-gradient plateaus cover 9,850 km2 of the survey area in water depths ranging from 1,700 to 2,200 m (Figure 57.3). The most extensive area of plateau occupies 8,365 km2 in the southwest sector of the mapped area. Here the seabed slopes gently to the southwest at 0.2° between 1,900 and 2,100 m water depth. The central-north sector is also occupied by a small low-relief plateau that covers 600 km2 and is bordered by ridges. Volcanic peaks: The 16 volcanic peaks mapped within the survey area cover 31 km2 and range in height from 65 to 450 m (Figure 57.3). The two largest peaks are located in the shallowest water depth (1,400 m) near the eastern margin of the survey area and rise to 950 and 1,020 m water depth, respectively. Of the 16 volcanic peaks, 13 are clustered into three groups on ridges located in the western, central, and eastern sectors of the mapped area. All peaks have conical shapes, with slope gradients of 10–30°, and a moat at their base that is up to 50 m deep (Figure 57.4). Basin: The southwest corner of the survey area captures a small section of steepening seafloor that extends from 2,400 to 2,600 m water depth. This is the edge of the Middleton Basin, which extends westward from this point [5]. Sediments: Grain size properties of the 28 sediment samples were determined by sieve separation of the gravel, sand, and mud fractions and by laser granulometry on the combined mud and sand fractions, using a Malvern Mastersizer 2000. Carbonate content of sediments was measured by acid digestion of a bulk subsample. Twenty-five of these samples were classified as sandy mud, and the other three as muddy sand (Figure 57.3). Mean grain size ranges from medium to very coarse silt (9–47 μm), and all samples are very poorly sorted. Bulk carbonate content ranges from 85% to 94%, incorporating forams and other nannofossils that have formed stiff dewatered deposits. Samples were collected from ridges, peaks, holes, the main plateau, and a valley, with slightly coarser grained sediments (muddy sands) occurring on peaks, small ridges, and holes. However, this is not a consistent pattern, as other peaks, ridges, and holes are characterized by sandy mud. Overall, there is no clear relationship between sediment type and geomorphic setting within the sampled area [6].

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Figure 57.4  A typical volcanic peak located on a ridge in the central-north sector of the mapped area. This peak is 250 m high with slopes of up to 30°. The distribution of key taxa across the peak and adjacent seabed are plotted for station 33 (depth range 1,360–1,610 m). “No occurrence” denotes the absence of these key taxa. Data are presented for each 15-second video frame along the video transect.

Biological Communities Assessments of benthos in this study are based primarily on underwater video, supplemented by higher-resolution still images and taxonomic identification of collected epifauna and infauna [9,10]. Seabed habitats and benthic assemblages were characterized in real-time using Characterization of the Benthos and Ecological Diversity (C-BED), a three-tiered characterization scheme that quantifies substratum cover, bedform/relief, and the presence of macroorganisms and percent cover of key taxa at 30-second intervals along each transect, whereby the seabed at each 30-second location is assessed for a period of 15 s (details provided in Refs. [9,10]). Over the entire survey area, substrata was dominated by homogeneous soft sediments that comprised 84% of the seabed, with 12% volcanic outcrops, and the remaining 4% comprising a range of mixed habitats with gravels or boulders. Seabed relief was generally flat, with rare occurrences of low- (9%) and moderate- (2%) relief habitats and sand waves (1%). Overall, these environments supported sparse biological assemblages. Bioturbation marks (e.g., burrows (53%), tracks (38%),

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mounds (32%)) were the most common signs of life (61% of all basin locations had bioturbation marks), while sessile organisms, such as cold-water corals, sponges, crinoids, and brittle stars, occurred sparsely on rocky substrata (Figure 57.5). Motile species including shrimp and prawns (15%), fishes (8%), and jellyfish (5%) were also regularly recorded but were sparsely distributed and never abundant. The relationships between the physical structure of the seabed and biological assemblages are examined in detail in Anderson et al. [10]. Three benthic environments (peaks, ridges, and plateaus) were identified from these analyses as important predictors of biological patterns and are summarized below. Volcanic peaks (rocky outcrops): These are dominated by homogeneous hard substrata (71% of all locations within this geomorphic class), with a mixture of rock and soft sediments and homogeneous soft sediments (14 and 15%, respectively) occurring mostly at the base of peaks. Although volcanic outcrops are bathymetrically distinct in the multibeam images (Figure 57.2), at fine scales, these peaks were characterized by low or flat (82% combined) relief habitats (Figure 57.5A and B). The rocky substrata of peaks had surprisingly few attached or associated organisms, and almost no dense habitat-forming taxa. Suspension feeders, such as cold-water corals and sponges, were commonly seen (55 and 18% of all locations, respectively) (Figure 57.5), but were present only in low densities (mean 4.5 and 1%, respectively). Cold-water corals and sponges provided structure for other species, such as brittle stars (e.g., Ophiocreas oedipus, Ophiophycis john, Asteroschema tubiferum) and crinoids (Figure 57.5A–D). Dead coral rubble (mostly Enallopsammia) was also common on the upper slopes of peaks (mean 5.3%, range 0–55%). Motile species, including shrimp/prawns, fish, and jellyfish, were only sporadically recorded (10, 2, and 1%, respectively). Ridges (sediment covered): They are composed of flat soft sediments (92% of all ridge locations) with small (3–70 m) isolated low-relief rock outcrops (8% of ridge locations) that occurred along the flanks where the slope was 10°. Soft sediments were bioturbated (79%) by combinations of burrows (60%), tracks (42%), mounds (41%), and characteristic rosettes and crater rings (8.2 and 4.8%, respectively; e.g., Figure 57.5G). Very few epifaunal organisms were directly observed or collected in these soft-sediment environments. The isolated rock ridges supported patchy but diverse cold-water coral and sponge assemblages (mean 7%, range 0–80%; Figure 57.5C), characterized by bamboo (Keratoisis) and golden corals, the latter entwined with brittle stars (Asteroschema tubiferum). Dead coral rubble was also patchy (mean 7%, range 0–50%). Motile species, such as shrimp and prawns (18%), fishes (10%), and jellyfish (7%), were occasionally recorded. Infaunal diversity and abundance were also low in ridge environments, with 3–25 species/taxa per station (Figure 57.6). Standardized species richness values indicate that Station 31 (1,518 m), which is immediately adjacent to a peak, had the highest number of infaunal species. Although infaunal sample sizes are low, there appears to be no direct relationship between species richness and either depth or distance to rocky outcrop (Figure 57.6). Plateaus (soft sediments): They are composed of flat soft sediments (all plateau locations), bioturbated by burrows (86%), tracks (95%), and mounds (62%), with

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Seafloor Geomorphology as Benthic Habitat

Figure 57.5  Photographs taken on hard and soft substrata of the Lord Howe Rise, deep-sea plateau. Filter feeders on hard substrata: (A) glass sponge on volcanic peak with brittle stars and crinoids (station 33: 1,351 m); (B) spiraled gorgonian (Chrysogorgiidae: Iridogorgia), family on volcanic peak (station 19: 1,395 m); (C) cold-water corals with brittle stars and crinoids on rocky ridge (station 30: 1,566 m); (D) brittle star (Asteroschematidae) entwined in a Golden coral (Callogorgia) (station 32: 1,594 m); (E) sea star (Circeaster arandae) and bamboo corals (station 6: 1,532 m); (F) collection of soft-sediment associated brittle stars (station 11: 1,572 m); (G) ridge sediments with “crater-ring” marks (station 13: 1,437 m); (H) plateau sediments with acorn worm (Hemichordata) (station 29: 1,939 m). Scale bars are 20 cm unless otherwise defined.

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Species richness (per 100 ml elutriate)

8 Ridge Plateau

7

31

23

6 5 13

4

21

24

16

18

3

27 2

30

26

4 1

29

5

3

93 1,

8

57 1,

8

51 1,

2

50 1,

49

5

1,

7

45 1,

4

44 1,

5

43 1,

41

8

1,

0

38 1,

36 1,

1,

36

0

0

Depth (m)

Figure 57.6  Species richness of infauna by station in relation to water depth and ridges and plateaus across the study area. Species richness is calculated as a standardized measure of species number per 10 ml of elutriate. Station numbers are indicated above each histogram bar.

characteristic acorn worms (phylum Hemichordata, class Enteropneusta) and their spiral and meandering trails (Figure 57.5H) [10,11]. Except for the sporadic occurrences of gorgonian whips, seapens, and hydroids (6% of locations), soft-sediment brittle stars (Figure 57.5F), motile species (shrimp/ prawns, fishes, and jellyfish—20, 5, and 1%, respectively), and very few epifaunal organisms were directly observed or collected from the plateau environment. Of the six plateau stations, station 29 (1,940 m) was the only one sampled for infauna, due to the difficulties in successfully collecting boxcore samples at depths 2,000 m. This station supported the lowest number of infaunal species (Figure 57.6), although it is unclear if this reflects any geomorphic differences.

Concluding Remarks In some deep-sea environments, rocky substrates support dense assemblages of suspension feeders, such as corals and sponges [12]. In this study, however, organisms were only sparsely recorded on rocky outcrops, with large areas of rock uncolonized. Epifaunal and infaunal diversity and abundances were low in soft-sediment habitats. While the array of cold-water corals and sponges recorded on rocky outcrops of both ridge and peak environments was diverse, epifaunal abundance was surprisingly low. Factors known to influence faunal diversity and abundance include nutrients, oxygen, organic content, and trace element levels [13]. However, it remains unclear

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Seafloor Geomorphology as Benthic Habitat

which of these factor(s) may be limiting epifaunal abundances and infaunal abundance and diversity across this region of Lord Howe Rise.

Acknowledgments This study was undertaken as part of the Offshore Energy Security Programme (2007–2011) funded by the Commonwealth Government of Australia. We thank the crew of RV Tangaroa for their technical support during survey TAN0713; Melissa Fellows (Geoscience Australia) who helped with production of the geomorphic map; and the taxonomic experts who identified biological specimens from this collection, with special thanks to Tim O’Hara (asteroids and ophiuroids), Rachel Przeslawski (infauna), Phil Allderslade, and Ron Thresher (cold-water corals). Reviews from Lene Buhl-Mortensen and M. Sayago-Gil are gratefully acknowledged. This chapter was produced with the support of funding from the Australian Government’s Commonwealth Environment Research Facilities (CERF) program and is a contribution of the CERF Marine Biodiversity Hub. This chapter is published with permission of the Chief Executive Officer, Geoscience Australia.

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[10] T.J. Anderson, S.L. Nichol, C. Syms, R. Przeslawski, P.T. Harris, Deep sea bio-physical variables as surrogates for biological assemblages, an example from the Lord Howe Rise, Deep Sea Res. Part II, 2011, 58: 970–978, doi:10.1016/j.dsr2.2010.10.xxx. [11] T.J. Anderson, R. Przeslawski, M. Tran, Distribution, abundance and trail characteristics of acorn worms at Australian continental margins, Deep Sea Res. Part II, 2011, 58: 979–991, doi:10.1016/j.dsr2.2010.10.xxx. [12] A.D. Rogers, The biology of seamounts, Adv. Mar. Biol. 30 (1994) 305–350. [13] L.A. Levin, R.J. Etter, M.A. Rex, A.J. Gooday, C.R. Smith, J. Pineda, et al., Environmental influences on regional deep-sea species diversity, Annu. Rev. Ecol. Syst. 32 (2001) 51–93.