Deep Sea Research Part II: Topical Studies in Oceanography
Benthic environments of the Lord Howe Rise submarine plateau: Introduction to the special volume
Introduction
Submarine plateaus are conspicuous, albeit uncommon, geomorphic features of the ocean floor. GIS analysis of one global seafloor geomorphic features map published by Agapova et al. (1979) indicates that plateaus cover an area of the ocean floor equal to around 11.6 million km2 (about 2.6% of the global ocean floor; Table 1). Plateaus are defined by the International Hydrographic Office (IHO, 2001) as “areas of flat ocean floor that are raised above the level of surrounding seafloor, delimited by a steep slope”. The largest plateaus were formed from fragments of continental crust that became isolated from continents and subsided below sea level following the rifting apart of continental land masses. Other plateaus are produced from large volcanic eruptions or from tectonic uplift of ocean crust. Plateaus are normally draped by younger marine sediments. Since the tops of plateaus are cut-off from receiving any sediment from the continents because of their raised profile, the hemipelagic sediment source is restricted to that supplied from the overlying water column. Hence sediments draping plateaus accumulate slowly. In general, very few studies have been published on the benthic habitats or ecosystems associated with plateaus.
Of the known plateaus in the world ocean, about 1.6 million km2 are located within the Australian exclusive economic zone (EEZ). Heap and Harris (2008) mapped 61 plateaus in part of the Australian EEZ (excluding Heard Island and the Australian Antarctic Territory), the nine largest of which are shown in Fig. 1. These figures indicate that within this portion of Australia's marine jurisdiction there are some 13.8% of the world's known plateaus (Table 1). By comparison, the Australian EEZ covers only around 2.3% of the total area of the world ocean. This over-representation of plateaus in Australia's EEZ is part of that country's natural endowment, but these features have proven to have another value. The abundance of plateaus adjacent to its continental margin has provided the basis for Australia's 2.56 million km2 of confirmed extended continental shelf (ECS) lying beyond the 200 nautical mile EEZ around Australia and its external island territories (Fig. 1). Australia has the right to explore and exploit its mineral and other non-living resources, as well as living sedentary resources, of the seafloor and subsoil within its ECS. Understanding the ecology of plateaus and protecting and preserving their marine environments, particularly benthic habitats, are particularly important for Australia, given that this type of environment is unusually common within Australia's marine jurisdiction.
Among the largest plateaus in Australia's marine jurisdiction is the Lord Howe Rise, located in the Tasman Sea between Australia, New Zealand and New Caledonia. The plateau is a fragment of continental crust that separated from Australia between 50 to 80 million years ago (Stagg et al., 2002, Willcox and Sayers, 2002, Van de Beuque et al., 2003). It has subsided to an average depth of around 1500 m and because the overlying ocean is oligotrophic, pelagic sediments accumulate on its flat top at a sluggish rate of only a few centimetres per thousand years (Grant and Dickens, 2002). The boundaries of Lord Howe Rise extend beyond Australia's EEZ and ECS (Fig. 1) and its total surface area is estimated to be around 1.5 million km2, clearly outlined by the 2000 m isobath. The shallow crest of the Rise is generally at about 750 to 1200 m below sea level and is surmounted by a number of small volcanic edifices that, within Australia's jurisdiction, are associated with islands and reefs (i.e. Lord Howe Island, and Elizabeth and Middleton Reefs). The Lord Howe Rise benthic environment is in a near-pristine condition (“low” human impact according to Halpern et al., 2008) – the region is, however, affected by shipping, limited fishing and laying of telecommunications cable routes. Until recently, very few surveys of its seafloor environments had been carried out.
In 2003, a major biodiversity survey of fishes and benthic invertebrates of the Lord Howe Rise and adjacent Norfolk Ridge was initiated by an international team of scientists from France, Australia and New Zealand, the “NORFANZ” survey. Papers in this special issue by Williams et al. and by Zintzen et al. report on the demersal fishes and benthic invertebrates sampled during that survey.
In October-December, 2007, the northern portion of Lord Howe Rise was mapped and sampled by Geoscience Australia during survey TAN0713, using the New Zealand research vessel Tangaroa as part of the Australian government's Offshore Energy Security Program (Heap et al., 2009). This program collected essential baseline environmental data used to support government decision-making related to energy resource assessments and environmental regulation and conservation. It is an example of the integrated approach being adopted by Geoscience Australia in understanding the resource and environmental values of the remote, frontier parts of Australia's marine jurisdiction. The papers in this collection focus on the environmental aspects of the data collected and the energy resource aspects will be covered in later publications. Bathymetric data were acquired using a 30 kHz Simrad multibeam sonar system over a survey area of over 25,500 km2 (Fig. 1) and underwater video and sediment information were collected at representative sites throughout the survey area. The papers in this special issue by Anderson et al. and Przeslawski et al. provide information on the benthic ecology of the region. Papers by Dadd et al., Nichol et al., and Radke et al. report on the geology of dredged rock samples, regional geomorphology and geochemical properties of surficial sediments sampled.
This special issue brings together the results of geological and biological studies carried out during these two surveys (2003-NORFANZ and 2007-TAN0713 Tangaroa) on the Lord Howe Rise. In the following, the main findings are synthesised and the implications of the work for regional marine planning and conservation are considered.
Section snippets
Geomorphic features of Lord Howe Rise
Nichol et al. (2011) describe geomorphic features mapped within a 25,500 km2 section of Lord Howe Rise where multibeam sonar data were collected in 2007 (Fig. 1). Broad-scale features, having length and width dimensions of tens of kilometres, include plains, ridges, valleys and basins. Fine-scale features, with dimensions of hundreds of metres to kilometres, are superimposed upon broad-scale features. They include volcanic peaks (Fig. 2), moats, holes, (polygonal) cracks, scarps and (potential)
Demersal fish
The paper by Zintzen et al. (2011) reports that of 348 demersal fish species collected from 135 samples during the NORFANZ survey, 25% are potentially new species. Relatively high levels of regional endemicity and diversity are attributed to geological history, and the complexity of contemporary climatology, hydrography and habitat diversity of the region. These workers found that the fish fauna on the Lord Howe Rise is distinct from the other regions studied in the survey.
Regional patterns of benthic fauna diversity
Williams et al. (2011)
Implications of observations for marine zone management
The Lord Howe Rise benthic habitats and communities described in the papers of this special volume have implications for management and conservation at the national level, and are also notable with respect to international criteria for the identification of ecologically and biologically significant areas. At a national level, the Lord Howe Island Marine Park is the only listed heritage site within the region and, together with the Elizabeth and Middleton Reefs Marine National Nature Reserve,
Conclusions
The papers included in this special volume represent a major advance in knowledge about benthic habitats of parts of the Lord Howe Rise, but probably also about the ecology of the whole Rise as well as plateaus in general. The benthic habitats present on Lord Howe Rise include hard/rocky substrates covering a small area of volcanic peaks (around 31 km2 within the studied area) and parts of other larger seamounts (e.g., the Lord Howe Island seamount) which support rich and abundant epifaunal
Acknowledgments
Bathymetric data and seabed photographs presented in this paper were collected during survey TAN0713 as part of the Offshore Energy Security Program (2007-2011) funded by the Commonwealth Government of Australia. The paper was improved by reviews provided by Andrew Heap and Phil Symonds. This paper is published with permission of the Chief Executive Officer, Geoscience Australia.
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