Ecosystem characterisation of Australia's North West Shelf

Brief description

Habitats serve a variety of functions on the North West Shelf (NWS). They support the life history stages of a diverse suite of tropical species including commercially harvested ones. In addition to natural disturbance regimes, habitats are altered in response to the sectoral uses, which in turn affects the distribution and life histories of species. Habitats thus serve as the nexus linking species with uses and natural disturbance, and different habitats serve different purposes at various stages of the life history of a species. A detailed understanding of habitats, at least at the structural level, is thus a prerequisite for a more comprehensive understanding of ecological structure and functions on the North West Shelf.

This component of the North West Shelf Joint Environmental Management Study (NWSJEMS) aimed to collate and integrate data on habitats for the region of the North West Shelf extending from North West Cape to Port Hedland and from the coast to the 200 m isobath. The three main activities of the study were: Development of an integrated collection of information on habitats of the North West Shelf, including expert information; Application of the CSIRO Habitat Classification Framework to the data to determine the spatial nesting and structuring of habitat units on the North West Shelf; and Provision of the habitat structure classification for input into other models developed within NWSJEMS.

This record describes data of key benthic marine ecosystems and habitats. These maps and descriptions of their component attributes were designed to assist the process modelling of the ecosystem and impacts of uses, as well as directly supporting planning and management by Western Australian agencies and industries.

Lineage

The maps were produced primarily from existing information, but survey fieldwork was also undertaken to fill in critical gaps in spatial coverage and missing components. Disparate sources of information were integrated into a composite regional map describing the habitats of the North West Shelf at a variety of spatial scales. This strategy was developed around the application of a hierarchical habitat classification framework. This framework allowed information of different types (physical, biological, geological) to be recorded into the relevant spatial level so that an inventory of information describing habitats of various scales could progressively be built up. The first phase of the approach involved a comprehensive collation of information to support the application of the CSIRO Hierarchical Habitat Classification Framework. Various information sources including published documents; digital and paper maps; imagery; statistical analysis, and expert information have been used to inform the process. The application of the framework has allowed for the development of mapping units for the three levels of the classification comprising:

Provinces: the largest spatial scale of habitat structuring reflecting paleo-historic evolutionary processes; Biomes: represents habitat structures responding to the role of the largest environmental gradients in this case reflected by depth as a primary surrogate for a variety of biophysical processes. Substructure at this level reflect changes that are primarily orthogonal to the depth structures; and Biogeomorphological units: are habitat structures represented by fields of features or large geomorphic structures such as gulfs, bays or plateaus. Substructures within this level are morphologically related differences in the distribution of habitats. In areas where the data was adequate, it has also been integrated for an initial assessment of a fourth level referred to as primary biotopes. However, considerable additional field sampling would be required to map to this level for the whole study region.

From the integration and analysis of available data, mapping layers containing relevant information for the various levels of the hierarchical classification were generated. The study region lies within the Level 1 North Western Province of the IMCRA (1988) classification, which extends from North West Cape, to Cape Leveque. At this scale, the North West Shelf is a unique benthic regional environment on the continental shelf of Australia. As such it is a bioregion of national significance. The Level 2 biomic structure of the region contains three sub-units:

• Level 2A
units consist of demersal shelf and coastal zone;
• Level 2B
units identified are:
- coastal, consisting of estuaries, lagoons and embayments at less than 10 m depth;
- sub-tidal nearshore, covering the depth range 10 to 20 m;
- further offshore are the inner shelf (20 to 70 m), mid shelf (70 to 120 m) and outer shelf biomes (120 to 200 m) (the precise boundaries are still subject to analysis); and
• Level 2C
units along the coast consist of broad alongshore categorisation based on distinct basement structural features and their corresponding collection of biological attributes. The most detailed level of classification obtained for the region was to Level 3. Data availability allowed mapping to three levels for the coastal zone, and one level for the offshore areas. Data used for the offshore analysis consisted of research trawl records for fish species. Existing geomorphic and topographic mapping, combined with aerial photography and imagery was used for the inshore mapping. Expert information also provided assistance for determining mapping units for Levels 2 and 3 of the classification