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State of the Marine Environment Report for Australia: The Marine Environment - Technical Annex: 1

Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland

Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995

ISBN 0 642 17399 0

Australia's marine ecosystems: the continental shelf and slope

Gary C. B. Poore

Department of Crustacea
Museum of Victoria
71 Victoria Cresent
Abbotsford, Vic. 3067

The long coastline of the Australian continent is surrounded by a gradually sloping submarine shelf of 2.5 million km2. About half of this area is less than 50 m deep. Beyond a depth of 150-200 m the gradient increases quickly away from the shore and the sea floor becomes the continental slope, another 1.5 million km2 in area.

Knowledge of the environment and biology of the continental shelf and slope can best be described as patchy. Much of the coast is remote from human habitation and its large size and climatic range make generalisations difficult. The geology of surface sediments, which is an important determinant of the biological communities, has been studied superficially and the literature reviewed briefly by Bunt (1987). His contribution is summarised here with some additions.

The biology of the benthos of the continental shelf and slope has not been systematically studied. Only three areas have been investigated intensively: the North West Shelf and slope, the Great Barrier Reef lagoon and Coral Sea and Bass Strait and the south-eastern Australian slope. The methods and objectives of these studies are very different and I treat them as case studies almost impossible to compare. Almost nothing is known of other areas and, surprisingly, there is little information on the biology of the continental shelf off Sydney, Australia's largest city, apart from a superficial report edited by Jones (1977).

Information on off-shore commercial fisheries is reviewed elsewhere in this volume.

Geology

The continental shelf is a continuous feature ranging from 15 km wide off the south-eastern coast to 400 km wide in the Timor Sea. It joins Australia to New Guinea in the Arafura Sea and Torres Strait. The shelf has a gradual slope to its greatest depth at 150-200 m and its generally featureless surface is broken by occasional reefs, dunes and ridges. Its topography is more complex in the north-east (where the Great Barrier Reef occupies the outer shelf) and north-west.

The Australian slope is continuous with that of Indonesia and New Guinea. In parts it is much steeper than continental slopes elsewhere in the world. Gradients of up to 40o are common and extend to the abyssal plain at about 4000 m depth. The slope is intersected by canyon systems, especially in eastern and western Bass Strait, where very steep gradients and rocky outcropping occur. It is further interrupted by several terraces, major plateaus and troughs, best developed on north-eastern and western margins of the mainland and on southern Tasmanian coasts. Each plateau has its shallowest depth at between 900 m and 2000 m. Minor terraces also occur in the Great Australian Bight. The south-eastern slope is steeper, yet without major troughs and terraces (Figure 1).

Surface sediments, which play an important part in the nature of infaunal and epifaunal communities, vary geographically and with depth. Bunt (1987) reviewed current knowledge and referred to the major recent works.

The sediments of the Arafura Shelf and most of the west coast are mostly coarse and calcareous with finer sediments further offshore. Bryozoans are predominant but foraminiferans and calcareous algae contribute at greater depth. There is little sedimentation from terrigenous sources except in estuaries.

Terrigenous muds and quartz sands dominate the inner shelf sediments of the Great Barrier Reef lagoon. Carbonates, with a fair contribution from corals, increase in concentration towards the outer lagoon and are predominant on the outer continental shelf. In the Gulf of Carpentaria sediments are much finer than further south and are rich in marine faunal remains.

The shelf of New South Wales has mostly relict sandy sediments - terrigenous close to the coast and carbonates with increasing depth below 60 m. Muds are uncommon. It is thought that sandy sediments are transported north and feed the shelf of southern Queensland.

Figure 1: Major features of benthic environments of the Australian shelf and slope (from Bunt 1987)

Figure 1

The southern continental shelf is covered with course calcareous and mainly relict sands, the remains of bryozoans, molluscs and foraminifera. This predominantly carbonate province which extends from south-eastern to south-western Australia is the largest of its kind in mid-latitudes and differs from that in the tropics where coral is the major contributor (James et al. 1992). Terrigenous sediment input is negligible except near the only major river, the Murray. The dominance of carbonates prevails in Bass Strait, the only anomaly being the presence of quartz sands in varying percentages off eastern Victoria.

The information needed for an appraisal of the relationships between fauna and grain size, sorting and composition is only at a gross level (Bunt 1987). Nevertheless, it can be said that the Australian shelf environment is peculiar, especially in the south, in the dominance of coarse particles and the virtual absence of terrigenous material. Combined, these two attributes contribute to complex microhabitats which may be responsible for high macrofaunal densities and species diversity.

Biological communities

The physical environment of the continental shelf and slope is almost entirely soft sediments and is inhabited by infauna (burrowing into the top few centimetres), epifauna (attached to or walking on the sediment) and demersal species (swimming over the bottom). Offshore reefs of hard rocky substrate are a small part of this environment and are little known except in shallow water where diving makes sampling possible. Exceptions are where the biology of abalone on reefs has been investigated. Information on this is covered elsewhere in this report.

North-western Australia

In 1982 CSIRO began a multidisciplinary study of the demersal fish and invertebrate communities of the North West Shelf with the objectives of investigating and managing finfish and crustacean fisheries. The epibenthos of this region is composed of sponges, gorgonians, soft corals and sea-pens and it is scattered over areas of rippled bare sand. The demersal crustaceans taken by Ward and Rainer (1988) from 47 trawl and sled samples at 40 m and 80 m depth numbered 308 species. Most were crabs but penaeid prawns and carid shrimps were also diverse. Depth was the only environmental parameter correlated with the distribution of these species. The number of taxa from the North West Shelf is higher than that caught in more intense surveys in the western Atlantic. The infauna of the North West Shelf has an extremely rich species complement and is dominated by polychaete worms (Rainer 1991). Rainer suggested that the high diversity is consistent with the combined effects of disturbances from tidal and storm-generated water movement, predation by fishes and crustaceans and low productivity in an oligotrophic environment. Small individual body size, another peculiarity of this North West Shelf fauna, may enable rapid turnover in an area of low nutrient levels.

Great Barrier Reef shelf and slope

Information on the lagoon of the Great Barrier Reef comes from several studies with very different objectives.

Cannon, Goeden and Campbell (1987) recorded about 700 species of fishes and macro-invertebrates from trawls on the northern and southern continental shelf and slope. Their data were uneven and they were able to detect only slight depth-related distributional patterns. Rainer and Munro (1982) and Rainer (1984), working in the nearby but very different Gulf of Carpentaria, found that depth played a major part in explaining the distribution of fishes and cephalopods.

Near Townsville, Birtles and Arnold (1983, 1988) found that the most diverse epifaunal taxa (echinoderms with 103 species and molluscs with 196 species) were divided into an inshore community on muddy sediments at 22 m depth and an offshore community on sandy sediments at greater depths. Calcareous rubble acts as a point of attachment for solitary and colonial animals and these 'multispecies isolates' are important in contributing to the diversity of shelf epifauna. The relative importance of different feeding strategies (filter-feeding, browsing, carnivory and deposit-feeding) varied haphazardly across the shelf. Tietjen (1991) reported high diversity of nematode assemblages across the Great Barrier Reef lagoon resulting from sedimentary heterogeneity (caused by cyclones and trawling), a rich bacterial flora, continuous warm temperatures and low macrofaunal abundance.

Alongi and colleagues have examined shelf communities from the point of view of structure and function without the taxonomic resolution of other studies (Alongi 1989, Alongi & Christoffersen 1992). They found that benthic standing crops and processes on the central shelf appear to be regulated by low and intermittent inputs of detritus, warm temperatures and physical disturbance, both natural and anthropogenic. These factors and oligotrophic conditions perpetuate the dominance of a pioneering regime.

Cruises on the north-eastern slope led to the discovery of new macrofaunal taxa but the published work on communities is devoted to microbial and meiofaunal composition (Alongi 1987, 1992; Alongi & Pichon 1988). Alongi and Pichon suspected that densities of metazoan meiobenthos in the western Coral Sea was lower than could be expected due either to low rates of detrital input or to rapid transport of turbidites. The slope in this region seems more oligotrophic than in other parts of the world.

Bass Strait and the slope off south-eastern Australia

Several scientific exploratory expeditions have passed through Bass Strait since a French expedition in 1802. Yet none has resulted in more than very local reports on the biological communities or taxonomic publications (Poore 1979). The Museum of Victoria's Bass Strait survey (1979-1984) aimed to provide material for systematic study - specimens on which to base descriptions of new species and from there to work towards an understanding of the area's evolutionary history. In the survey, about 300 samples were taken from a wide area of the shelf using a variety of grabs, sleds and trawls. Dozens of papers describing new taxa in many taxonomic groups of animals have resulted from this survey and many more will appear over the next decades. For the first time, there may be sufficient distributional information to present a synthesis of the Bass Strait fauna - at least for some taxa.

During the survey, the only quantitative study in Bass Strait was on a very small area in eastern Victoria. Here, grab samples from 1.2 m2 of benthos enabled 353 species of invertebrates to be discriminated, about half of them crustaceans, the rest polychaetes and molluscs (Parry, Campbell & Hobday 1990). Compared with data from similar areas in other parts of the world, species diversity for this eastern Victorian area was exceptionally high (Parry, Campbell & Hobday 1990). None of the four sites sampled had more than 45% of the total number of species collected, suggesting that greater effort would reveal more species. More recent sampling from 10 m2 in the same area turned up over 900 species (unpublished data). Interestingly, the fauna 100 km west at similar depth and sampled in the same manner is also rich but many species recorded in the eastern samples are replaced by others there.

Qualitative sampling on the continental slope of south-eastern Australia at depths between 200 m and 3000 m have also discovered a rich fauna. For example, the aplacophoran molluscs (a group of Tethyan origin) are concentrated on the slope and there are numerous undescribed species of them in this region (Scheltema 1990). Three species-pairs, with one of each pair occurring on the slope and the other on the shelf, indicate local speciation and a slope origin for some shelf species. Such a pattern is expected in other taxa.

The isopod crustaceans are the best studied taxon in the area so far, and 359 species belonging to 36 families are represented. Ten per cent can be identified to known species - a much smaller fraction than in comparable surveys conducted elsewhere on the slope. Poore, Just and Cohen (1994) found that depth is the main environmental factor correlated with species composition. The expected numbers of species calculated by Poore and Wilson (1993) varied between samples and matched the highest recorded in other oceans (Figure 2). The results were similar for ostracode crustaceans where more than 90 mostly new species were discovered (Kornicker & Poore in press).

Conclusions: unexplored seas

Vast areas of the continental shelf and all except two tiny areas of the slope have never been surveyed by biologists.

Figure 2: Expected number of species of benthic isopod crustaceans in individuals calculated from Hulbert rarefaction curves (E(S100)) versus latitude. Data labelled: A - South-eastern Australia; b - Angola Basin, c Argentine Basin, d Brazil Basin, e - Gay Head/Bermuda Transect, f - Mediterranean Sea, g - Guiana Basin, h - Scotia Basin, j - Weddell Sea, k - West European Basin; L Norwegian Basin, m - Greenland Basin3;* - off New Jersey and Delaware, USA, arrowed (from Poore & Wilson 1993).

Figure 2

The results of the three case studies discussed here reflect differences in study objectives. Hence it is not possible to compare them in terms of species diversity, community composition - environment relationships or functional ecology.

Latitudinal gradients in species diversity are hinted at and, as would be expected, these differ with taxonomic group. Benthic sediments are dominated by coarse calcareous biogenic particles and this may be responsible in part for a diverse fauna throughout all of Australia's continental seas, even in its colder waters. Fine muddy terrigenous sediments dominate only near the major rivers of the northern half of Australia and may be the only places where species diversity is depressed.

Recent research in north-eastern Australia has concentrated on benthic processes and there is now a fair taxonomic knowledge of the largest epifauna. However, taxonomy of the infauna is poorly studied. To some extent this emphasis reflects the alternative interest of biologists in the effects of large rivers on the Great Barrier Reef and its lagoon. In contrast to our knowledge of north-eastern Australian benthic processes, almost nothing is known of processes in south-eastern Australia where recent terrigenous inputs are negligible. However, the infauna in south-eastern Australia - especially of crustaceans, polychaetes and molluscs - is taxonomically better understood.

References

Alongi, D.M. 1987. The distribution and composition of deep-sea microbenthos in a bathyal region of the western Coral Sea. Deep-Sea Res., 34: 1245-1254.

Alongi, D.M. 1989. Benthic processes across mixed terrigenous-carbonate sedimentary facies on the central Great Barrier Reef continental shelf. Contin. Shelf Res., 9: 629-663.

Alongi, D.M. 1992. Bathymetric patterns of deep-sea benthic communities from bathyal to abyssal depths in the western South Pacific (Solomon and Coral Seas). Deep-Sea Res., 39: 549-565.

Alongi, D.M. & Christoffersen, P. 1992. Benthic infauna and organism-sediment reations in a shallow, tropical coastal area: influence of outwelled mangrove detritus and physical disturbance. Mar. Ecol. Progr. Seri., 81: 229-245.

Alongi, D.M. & Pichon, M. 1988. Bathyal meiobenthos of the western Coral Sea: distribution and abundance in relation to microbial standing stocks and environmental factors. Deep-Sea Res., 35: 491-503.

Birtles, A. & Arnold, P. 1983. Between the reefs: some patterns of soft substrate epibenthos on the central Great Barrier Reef. pp. 159-163, in J.T. Baker, R.M. Carter, P.W. Sammarco & K.P. Stark (eds), Proceedings: inaugural Great Barrier Reef conference, Townsville, Aug 28 - Sep 2. Townsville: JCU Press.

Birtles, A. & Arnold, P. 1988. Distribution of trophic groups of epifaunal echinoderms and molluscs in the soft sediment areas of the central Great Barrier Reef'. Proc. 6th Int. Coral Reef Symp., Aust., 3: 325-332.

Bunt, J.S. 1987. The Australian marine environment. pp. 17-42, in G.W. Dyne (ed.), Fauna of Australia. Vol. 1A. General articles. Canberra: Australian Government Publishing Service.

Cannon, L.R.G., Goeden, G.B. & Campbell, P. 1987. Community patterns revealed by trawling in the inter-reef regions of the Great Barrier Reef. Mem. Queensl. Mus., 25: 45-70.

James, N.P., Bone, Y., von der Borch, C.C. & Gostin, V.A. 1992. Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf; Lacepede, southern Australia. Sedimentology, 39: 877-904.

Jones, A.R. 1977 (ed.). An ecological survey of nearshore waters east of Sydney, N.S.W. 1973-75. Sydney: Australian Museum. 320 pp.

Kornicker, L.S. & Poore, G.C.B. (in press). Ostracoda (Myodocopina) of the SE Australian continental slope. Part 3. Smithson. Contrib. Zool.

Parry, G.D., Campbell, S.J. & Hobday, D.K. 1990. Marine resources off east Gippsland, southeastern Australia. Mar. Sci. Lab. Tech. Rept, 72. 166 pp.

Poore, G.C.B. 1979. A survey of marine invertebrate data for the Bass Strait region. Vic. Inst. Mar. Sci., Working Pap., 3. 22 pp.

Poore, G.C.B., Just, J. & Cohen, B.F. 1994. Composition and diversity of Crustacea Isopoda of the southeastern Australian continental slope. Deep-Sea Res., 41: 677-693.

Poore, G.C.B. & Wilson, G.D.F. 1993. Marine species richness (with reply by R.M. May). Nature, 361: 597-598.

Rainer, S.F. 1984. Temporal changes in a demersal fish and cephalopod community of an unexploited coastal environment in northern Australia. Aust. J. Mar. Freshwat. Res., 35: 747-768.

Rainer, S.F. 1991. High species diversity in demersal polychaetes of the North West Shelf of Australia. Ophelia Suppl., 5: 497-505.

Rainer, S.F. & Munro, I.S.R. 1982. Demersal fish and cephalopod communities of an unexploited coastal environment in northern Australia. Aust. J. Mar. Freshwat. Res., 33: 1039-1055.

Scheltema, A.H. 1990. Aplacophora as a Tethyan slope taxon: evidence from the Pacific. Bull. Mar. Sci., 47: 50-61.

Tietjen, J.H. 1991. Ecology of free-living nematodes from the continental shelf of the central Great Barrier Reef province. Est., Coast. Shelf Sci., 32: 421-438.

Ward, T.J. & Rainer, S.F. 1988. Decapod crustaceans of the North West Shelf, a tropical continental shelf of north-western Australia. Aust. J. Mar. Freshwat. Res., 39: 751-765.

The technical paper by Dr Poore was reviewed by Dr. G. Ross, Australian Biological Resources Study, Canberra; and Dr. D. Brunckhorst, Australian Nature Conservation Agency, Canberra.

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