Tributary canyons of the Arafura Depression

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

Almost all canyons in the North Marine Region are located within this key ecological feature and endemic benthic species are believed to occur there (Wilson 2005). Primary productivity in this feature is likely to be associated with movements of water through the canyons and surface water circulation driven by seasonal north-west monsoon winds.

Studies have shown that biological diversity and ecosystem processes in the area are influenced by the heterogeneity of canyon habitats. The steep topography of the canyons, their diverse current regimes, nutrient enrichment and entrapment, detritus funnelling and diverse substrate types form widely divergent ecosystems (McClain & Barry 2010; Vetter 1994; Vinogradova 1959) which, in combination with the regional setting and geological origins of the area, strongly influence species biodiversity (Kloser et al. 2010; RJ Kloser, pers. comm., May 2011).

A survey conducted in the area has identified at least 245 macroscopic species, including a diverse variety of invertebrates (e.g. sponges, corals, sea anemones, tunicates, worms, crustaceans, brittle stars, feather stars) and six small fish species (Wilson 2005). It is estimated that a further 500 species could be identified from post-survey analysis of grab and dredge samples (Wilson 2005). Marine turtles (most likely olive ridleys) have been reported to feed in the vicinity of the canyons (Whiting et al. 2007).

National and/or regional importance

The Tributary canyons of the Arafura Depression are defined as a key ecological feature for their high productivity and high levels of biodiversity and endemism. These values apply to both the benthic and pelagic habitats within the feature.

Location

The tributary canyons of the Arafura Depression are located in the Timor Transition provincial bioregion. The canyons are approximately 80–100 m deep and 20 km wide. The largest of the canyons extend some 400 km from Cape Wessel into the Arafura Depression, and are the remnants of a drowned river system that existed during the Pleistocene era (Heap et al. 2004).

The spatial boundary of this KEF, as defined in the Conservation Values Atlas, includes all canyons in the Geoscience Australia geomorphic features dataset in the northernmost part of the region between approximately 132° 40' E and 136° 20' E.

Description

As a result of the history and geography of the Arafura Sea, sea-floor sediments are calcium carbonate rich with substantial but varying fractions of carbonate sand and sub-fossil shell fragments (Wilson 2005). Sampling in the area has revealed calcareous sand and gravel or sandy mud. The deeper channel sediments are composed of calcareous mud and some areas of rocky substrate (Logan et al. 2006).

Although some water flows through the key ecological feature may be associated with the Indonesian Throughflow, the strongest influence on water flows at the sea floor is thought to be tidal ranges that can exceed 5 m vertically (Wilson 2005). Sampling in the area has identified relatively high seafloor currents, particularly across ridges on Pillar Bank and hard grounds. These parts of the feature were also found to support diverse populations of large sessile filter-feeding biota such as sponges, octocorals and comatulacean crinoids (Wilson 2005). Primary productivity in this key ecological feature is believed to be associated with movements of water through the canyons and surface water circulation driven by seasonal north-west monsoon winds.

Detailed seabed mapping in the vicinity of Pillar Bank has shown a correlation between habitats and the biodiversity of various benthic fauna. Areas of high biodiversity and abundance generally correlate with harder substrates where sessile benthos such as sea whips and fans, soft corals, hydroids, crinoids and octocorals—some up to 50 cm in height—have been frequently found. In comparison, soft substrates tend to be associated with low-relief benthos that covers less than 5 % of the surface area (Logan et al. 2006).

Water temperatures recorded in parts of the tributary canyons are generally higher (14–16 °C at approximate depths of 230 m) than typical deep-sea environments (usually less than 8 °C) (Wilson 2005). However, despite these warmer temperatures, deepwater fauna such as stalked crinoids, hexactinellid sponges and deepwater pedunculate barnacles are known to occur (Wilson 2005).

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 Tributary canyons of the Arafura Depression.

Pressures assessed as of potential concern are changes in sea temperature and ocean acidification as a result of climate change; physical habitat modification; 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 °C between 1910–1929 and 1989–2008, and current projections estimate ocean temperatures will be 1 °C 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 ppm 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.

Physical habitat modification (offshore construction)
Although there is currently no extraction of oil or gas occurring within the North Marine Region, nine exploration wells have been drilled in the Arafura Basin adjacent to the tributary canyons of the Arafura Depression (DRET 2011) and two exploration permits cover much of the area. In addition, there are a number of offshore basins that are considered highly prospective for economically viable extraction of oil and gas deposits (Cadman & Temple 2003; Earl et al. 2006). The potential for pressures associated with oil and gas exploration and extraction in the Arafura Basin is therefore high. Physical habitat modification caused by offshore construction associated with these developments has the potential to cause temporary ecological impacts on the tributary canyons, and localised impacts on a broad range of benthic 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.