Benthic invertebrate communities of the eastern Great Australian Bight

Legal description

The Commonwealth marine area is a matter of national environmental significance under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). As such, actions that have, will have, or are likely to have, a significant impact on it require approval.

In February 2014, the National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) became the sole designated assessor of petroleum and greenhouse gas activities in Commonwealth waters in accordance with the Minister for the Environment’s endorsement of NOPSEMA’s environmental authorisation process under Part 10, section 146 of the EPBC Act. Under the streamlined arrangements, impacts on the Commonwealth marine area by petroleum and greenhouse gas activities will be assessed solely through NOPSEMA.

Key ecological features are not matters of national environmental significance and have no legal status in their own right. However, they may be considered as components of the Commonwealth marine area.

This KEF was identified in the South-west Marine Bioregional Plan. Under section 176 of the EPBC Act, once a bioregional plan has been made the minister responsible for the environment must have regard to it when making any decision under the Act to which the plan is relevant.

Values

The temperate carbonate platform that forms the shelf of the Great Australian Bight (James et al. 2001) is an iconic feature, both on the Australian margin and in the world. This region has been recognised for its global significance for cool-water carbonate habitats, and has been used as a type-case example for shallow cool-water environments seen in the geological rock record (Richardson et al. 2005).

Soft-sediment benthic invertebrate communities of the eastern Great Australian Bight shelf form some of the world’s most diverse soft-sediment ecosystems. A 2002 survey of benthic marine life sampled 798 species, including 360 species of sponge, 138 ascidians and 93 bryozoans, many of which were new to science (Ward et al. 2006). The shelf in this area of the region is part of the world’s largest cool-water carbonate province. Invertebrate skeletons and shells make up more than 80 % of the shelf sediments. The high levels of biodiversity have been attributed to the unusual width of the continental shelf, the high degree of geographic isolation from similar habitats, and the opportunities for incursions by tropical species in the Leeuwin Current.

National and/or regional importance

The Benthic invertebrate communities of the eastern Great Australian Bight are defined as a key ecological feature as they are a community that is nationally important to biodiversity.

Location

Situated off southern, central Australia, the shelf of the Great Australian Bight constitutes one of the world’s largest temperate carbonate platforms, covering approximately 260 000 km² (James et al. 2001).

The spatial boundary of this KEF has not yet been defined.

Description

By global standards, the marine environment of the South-west Marine Region has high biodiversity and large numbers of species native to the region (known as endemism). Particular hotspots for biodiversity are the Houtman Abrolhos Islands, the overlap between tropical and temperate fauna along the west coast, the Recherche Archipelago and the soft sediment ecosystems in the Great Australian Bight.

Benthic invertebrate communities of the eastern Great Australian Bight shelf are highly biodiverse, soft sediment ecosystems. Surface sediments are dominated by heterozoan carbonate fragments comprising bryozoans, molluscs, porifera, rhodoliths and other invertebrates (Richardson et al. 2005). Unimpeded south-westerly waves and swells create a high-energy environment that can create wave abrasion at depths to 60 m. In deeper environments, sediments are moved intermittently during winter storms, transporting fine-grained sediments off-shelf. This is the major physical process down to approximately 120 m (James et al. 2001). Epifaunal assemblages of macroinvertebrates are dominated by filter feeders (primarily porifera, but also ascidians and bryozoans), which provide habitat and resources for a diverse community of crustaceans and molluscs (Ward et al. 2006). The relative abundance of these filter-feeding communities is largely determined by the availability of deepwater nutrients, which is controlled by processes of upwelling and downwelling across the shelf (James et al. 2001). There is a significant positive relationship between species richness and biomass, both of which decline with increasing depth and increasing percentage of fines (mud) in sediment (Ward et al. 2006).

The benthic invertebrate communities of the eastern Great Australian Bight includes the Great Australian Bight Marine Park, which has a Benthic Protection Zone that is within a region that experiences year-round downwelling and arrested carbonate production (James et al. 2001). Ward et al. (2006) and Currie et al. (2008, 2009) found that the Benthic Protection Zone of the park effectively represents the regional biodiversity, epi- and infaunal assemblages of the eastern Great Australian Bight.

Model information

The department and CSIRO conducted collaborative research on KEFs to identify indicators for monitoring marine ecosystem health in the Commonwealth marine area. As part of this process, qualitative models were developed for 27 KEFs, including the Benthic invertebrate communities of the eastern Great Australian Bight. Further information can be found at https://publications.csiro.au/publications/publication/PIcsiro:EP132926.

Pressure

During the development of marine bioregional plans, a regional pressure analysis was conducted to assess present and emerging pressures affecting conservation values in the Marine Regions and the effectiveness of mitigation and management arrangements that are currently in place to address these pressures. The analysis enabled pressures to be categorised in terms of their relative importance or concern and has informed the identification of regional conservation priorities. For the purpose of the plans, pressures are defined broadly as human-driven processes and events that do or can detrimentally affect the region's conservation values.

No pressures were assessed as of concern for the Diamantina Fracture Zone.

Pressures assessed as of potential concern are changes in sea temperature, change in oceanography and ocean acidification as a result of climate change, and physical habitat modification. These pressures are described in more detail below.

Changes in sea temperature (climate change)
Sea temperatures have warmed by 0.7 °C between 1910–1929 and 1989–2008, and current projections estimate ocean temperatures will be 1 °C warmer by 2030 (Lough 2009). The implications of changes to the region’s sea temperatures and oceanography for deeper ocean environments are uncertain, although sea-water warming at depth is expected to have significant implications for benthic and demersal fish.

Change in oceanography (climate change)
The strength of the Leeuwin Current has decreased slightly since the 1970s. This weakening is expected to continue, although this prediction currently has low confidence (Feng et al. 2009). The Leeuwin Current is the basis of much of the region’s biological productivity, and its strength and seasonal/climatic variability are a primary driver for the intensity, timing and locations of productivity events in the region, from the waters off Geraldton to the Great Australian Bight (Pattiaratchi 2007). Numerous water bodies and currents converge at the Naturaliste Plateau, including the subtropical convergence front. These complex water dynamics are likely to experience considerable changes if there are significant changes to ocean circulation in the region, with flow-on effects for the entire ecosystem.

Ocean acidification (climate change)
Driven by increasing levels of atmospheric CO₂ and subsequent chemical changes in the ocean, acidification is already underway and detectible. Since pre-industrial times, acidification has lowered ocean pH by 0.1 units (Howard et al. 2009). Furthermore, climate models predict this trend will continue, with a further 0.2–0.3 unit decline by 2100 (Howard et al. 2009). Projected changes in Australian waters by 2070 include a decline in pH of 0.2 units (Lawrence et al. 2007). There is a high level of uncertainty about the effects of ocean acidification on marine life. While some organisms might be able to adapt (Orr et al. 2009), anticipated changes to phytoplankton and zooplankton have the potential to detrimentally affect ecosystem processes and the structure of ecological communities. Research on the impact of ocean acidification on Antarctic krill has found that increased levels of CO₂ kill their embryos (Kawaguchi et al. 2011). Krill are an important part of the food chain because they feed on phytoplankton and zooplankton, and are a key food source for many species that occur in Australian waters. Consequently, acidification impacts have the potential to affect species further up the food chain. The potential effects of increased acidity on the region’s biodiversity also include changes to growth and population dynamics of some shell-forming organisms, impacts on the reproductive and metabolic functions of a number of fish and invertebrate species, and sensitivity of some early-life stages to acidification (Orr et al. 2009).

Physical habitat modification
In the region, disturbance and physical changes to benthic habitats result from bottom trawling, although the extent of trawling varies broadly among the fisheries in the region. Sponges, bryozoans and sea squirts are particularly vulnerable to trawling impacts (Currie et al. 2009). Bottom trawling occurs on the west-coast shelf edge and slope, and loss or modification of the benthic community is likely (20 out of 48 habitat types occurring within the fishery area are at risk; Wayte et al. 2007). Similarly, in the Great Australian Bight, trawling covers limited ground relative to the fishery area but is concentrated on the diverse habitats of the shelf edge and upper slope. Effects of benthic habitat loss on demersal communities and associated ecosystems are not certain. The effects of demersal gillnet deployment over the species-rich benthic communities of this area have not been investigated. The habitats associated with the ancient coastline of the Great Australian Bight at depths of 80–100 m, although generally not suited to trawling, might also be subject to habitat loss and modification through the impacts of fishing gear. Other sources of benthic habitat loss or modification at more localised scales are dredging, construction of infrastructure, and laying of underwater pipelines and cables. The implications of benthic habitat loss on the region’s species and ecosystems are not adequately understood.