Shelf break and slope of the Arafura Shelf

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 (KEFs) 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 North 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 shelf break and slope of the Arafura Shelf is situated in a major biogeographic crossroad where biota is largely affiliated with the Timor–Indonesian–Malay region (Hooper & Ekins 2005). Ecosystem processes operating in this area are largely unknown, but oceanographic processes, possibly associated with the Indonesian Throughflow and surface wind–driven circulation resulting from the north-west monsoon, are thought to be of strong influence (DEWHA 2007). The Indonesian Throughflow transports warm waters from the western Pacific Ocean through the Indonesian archipelago into the Timor and Arafura seas. This is likely to influence pelagic dispersal of nutrients, species and biological productivity. Pelagic dispersal in turn drives long-term patterns of transport and dispersal of larvae, juvenile and migrating adult organisms across the area.

National and/or regional importance

Location

The shelf break and slope of the Arafura Shelf is located north of the Tiwi Islands, near the edge of Austraila’s Exclusive Economic Zone.

The spatial boundary of this KEF, as defined in the Conservation Values Atlas, includes the area of slope north of the Van Diemen Rise from the Geoscience Australia geomorphic features dataset. This KEF also includes an adjacent area of shelf, extending south to the terrace edge of the Van Diemen Rise in the western part and bounded by the 100m depth contour in the east. At the eastern end, the area of shelf included extends to approximately the eastern extent of the slope. The reefs and pinnacles within these slope and shelf areas are also included.

Description

The shelf break and slope of the Arafura Shelf is characterised by continental slope, patch reefs and hard substrate pinnacles (Harris et al. 2005). Seaward of the Van Diemen Rise, the shelf edge occurs at water depths of 120–180 metres (Jongsma 1974). On the outer shelf and upper shelf slope carbonate sediments are mixed with terrigenous clays from Indonesian rivers (Heap et al. 2004).

Localised seasonal influences of the Indonesian Throughflow are likely to occur between the north-west monsoon and south-east trade winds. During the monsoon period (December to March) surface currents are weak and have no distinct direction (Harris et al. 1991).

Phytoplankton and invertebrates have been sampled in the area (Hallegraeff & Jeffrey 1984; Wilson 2005), and primary production of phytoplankton is thought to form the basis for offshore food webs (DEWHA 2007). Fish communities that occur in this key ecological feature represent the break between the Timor Province provincial bioregion and the Timor Transition provincial bioregion (Last et al. 2005). Records show at least 284 demersal fish species are found in the area (Last et al. 2005), including commercially fished red snapper species (Lutjanus erythropterus). The area is also likely to support whale sharks, sharks and marine turtles (DEWHA 2007).

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 Shelf break and slope of the Arafura Shelf.

Pressures assessed as of potential concern are changes in sea temperature and ocean acidification as a result of climate change; extraction of living resources and oil pollution. These pressures are described in more detail below.

Changes in sea temperature (climate change)
Sea temperatures have warmed by 0.7 ºC between 1910–1929 and 1989–2008, and current projections estimate ocean temperatures will be 1 ºC warmer by 2030 (Lough 2009). This projected temperature change is expected to exceed the threshold for inducing coral bleaching on an annual basis (Hoegh-Guldberg 1999, 2005).

Increases in sea temperature is expected to impact on the key ecological features of the North Marine Region, particularly through changes to the distribution, abundance, physiology, morphology and behaviour of zooplankton and pelagic, benthic and demersal fishes (Hobday et al. 2006).

Coral reefs and their associated fauna are especially vulnerable to impacts connected to changes in sea temperature through coral bleaching and mortality. Any decreases in coral abundance could lead to changes in ecosystem structure, processes and connectivity between coral reefs and adjacent waters. Nutrients and organic matter sourced from dynamic reef complexes and ontogenetic (lifecycle) movements of fish and crustaceans may be disrupted (Haywood & Kenyon 2009). Epifauna such as sponges, algae and coralline algae may also be impacted by elevated seawater temperatures (Brooke et al. 2009).

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).

Ocean acidification is of particular concern for those key ecological features of the North Marine Region that have coral formations. Acidification of corals is likely to alter the distribution and abundance of corals generally (Hobday et al. 2006), and atmospheric CO₂ levels above 500 parts per million will severely compromise coral viability (Hobday et al. 2006). The acidification of the ocean will impair the ability of species with calcareous shells such as echinoderms, crustaceans and molluscs to maintain shell integrity, resulting in reductions in the abundance and biodiversity of these species (Lawrence et al. 2007).

Ocean acidification may also cause changes to the composition of ecological community structures dependent on hard substrate environments, which may in turn impact on food sources for higher trophic-level species.

Extraction of living resources (IUU fishing)
In recent years, foreign illegal, unregulated and unreported (IUU) fishing has been a considerable issue across northern Australian waters for the threat it poses to target and bycatch species, border security, quarantine concerns and conservation of the marine environment more generally (Vince 2007). For example, in 2005, 13 018 illegal foreign fishing vessels were observed in Australian waters, and of those, only 600 were apprehended by Australian officials (Vince 2007). The number of foreign IUU fishing vessels sighted in northern Australian waters has declined significantly since 2005, but there is some concern that fewer numbers of more powerful and sophisticated IUU fishing vessels may now be targeting Australian stocks (Lack & Sant 2008).

IUU fishing has the potential to adversely affect widespread populations of multiple species, possibly with long-term, permanent impacts. Illegal foreign fishers have tended to target sharks for the valuable fin market, although the full extent of IUU fishing for shark in northern Australia is largely unquantified. Selected shark stocks targeted by foreign IUU fishers have declined or are overfished (Heupel & McAuley 2007). IUU catch of sharks is estimated to be twice that of reported legal catch (Heupel & McAuley 2007).

Oil pollution (oil rigs, onshore and offshore mining operations)
Australia has a strong system for regulating industry activity that is the potential source of oil spills and this system has been strengthened further in response to the Montara oil spill. While oil spills are unpredictable events and their likelihood is low based on past experience, their consequences, especially for threatened species at important areas, could be severe. A number of the key ecological features of the region have characteristics that make their ecosystems and communities vulnerable to the effects of an oil spill; for example, features that include localised areas of high productivity, which attract large aggregations of marine life.

The intensity and distribution of activities implicated in oil spills—such as oil production and transport—are likely to increase in the region.