Carbonate bank and terrace system of the Sahul 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-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 Sahul banks are the single most extensive region of banks and shoals in the Australian exclusive economic zone forming a nearly continuous chain of complex submerged algal banks on the middle and outer shelf (Heap & Harris 2008). A survey conducted in 2012 indicates there are even more banks than previously thought with an extra 41 banks and pinnacles covering 152 km² identified (Nichol et al. 2013).

The Carbonate banks and terrace system of the Sahul Shelf are regionally important because of their role in enhancing biodiversity and local productivity relative to their surrounds. The carbonate banks and terraces provide areas of hard substrate in an otherwise soft sediment environment which are important for sessile species. Elevated hard substrates provide ideal habitats as they provide substrate to which organisms can adhere and expose filter-feeders to the maximum amount of passing nutrients.

Rising steeply from depths of about 80 m some banks emerge to within 30 m of the water surface, allowing light dependent organisms to thrive. Banks that rise to at least 45 m water depth support more biodiversity. Communities of sessile benthic invertebrates including hard and soft corals, sponges, whips, fans and bryozoans (Nichol et al. 2013, NERP MBH 2014). The banks are also recognised as a biodiversity hotspot for sponges as they are home to more sponge species and different communities than the surrounding seafloor. The banks and terraces are biodiversity hotspots for sponges with more species and different communities than the surrounding seafloor (NERP MBH 2014).

National and/or regional importance

The Carbonate banks and terrace system of the Sahul Shelf is defined as a key ecological feature for its role in enhancing biodiversity and local productivity relative to its surrounds as it is a unique seafloor feature supporting relatively high species diversity, making it regionally significant.

Location

The Carbonate banks and terrace system of the Sahul Shelf are located in the western Joseph Bonaparte Gulf and to the north of Cape Bougainville and Cape Londonderry. The carbonate banks and terraces are part of a larger complex of banks and terraces that occurs on the Van Diemen Rise in the adjacent North Marine Region. The banks consist of a hard substrate and flat tops at depths of 150–300 metres. Each bank occupies an area generally less than 10 square kilometres and is separated from the next bank by narrow sinuous channels with depths up to 150 metres (Brewer et al. 2007).  

The spatial boundary of this KEF, as defined in the Conservation Values Atlas, includes the banks/shoals and terraces as defined in the Geoscience Australia geomorphic features dataset which fall between 125° E and the Northwest Marine Region Boundary around 129° E. The banks/shoals to the south of 13° S and west of 127° E have been excluded.

Description

More than 90 % of carbonate banks in the North-west Marine Region are in the Northwest Shelf Transition Bioregion and the North-west Marine Region contains up to 60 % of banks and shoals in the entire Australian exclusive economic zone. The carbonate banks and terraces of the outer Sahul shelf were built by repeated episodes of reef growth during high sea level phases of the last two million years. These features were then shaped by erosion and weathering during the low sea level of a following ice age (NERP MBH 2014).

Warm water from the Indonesian Throughflow is thought to drive nutrients from deepwater to shallower water within the euphotic zone up to 100 metres in depth. The heads of channels are likely to be areas of localised high primary productivity which in turn supports secondary productivity, due to the convergence of ocean currents, tidal flows and upwellings of cold nutrient-rich water (Brewer et al. 2007). Such productivity may also be influenced by seasonal weather patterns such as monsoonal storms and winds (Brewer et al. 2007). Cyclones and north-westerly winds during the north-west monsoon (approximately November–March) and the strong offshore (south-easterly) winds of the south-east monsoon (approximately April–September) facilitate the upwelling of nutrients from deepwater to shallow-water environments (Brewer et al. 2007).

Tidal currents play an important role in regulating turbidity levels and in redistributing sediment around banks and terraces, with some smaller banks partly buried by sediment. Demersal fish communities respond to these spatial patterns, occurring in larger and more diverse populations on the shallower less turbid banks (Nichol et al. 2013, NERP MBH 2014).

The area supports a wide range of pelagic animals with 32 species observed during a 2012 survey, including 11 shark species, black marlin, barracuda, olive ridley turtle, sea snakes and orca (Nichol et al. 2013, NERP MBH 2014). The banks are known to be foraging areas for loggerhead, olive ridley and flatback turtles (Donovan et al. 2008). Humpback whales, and green and freshwater sawfish are likely to occur in the area (Donovan et al. 2008).

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 30 KEFs, including the Carbonate bank and terrace system of the Sahul Shelf. 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 Carbonate bank and terrace system of the Sahul Shelf.

Pressures assessed as of potential concern are changes in sea temperature and ocean acidification as a result of climate change; and extraction of living resources. 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). KEFs supporting coral reef communities are vulnerable to bleaching and mortality from elevated sea temperatures. Projected temperature changes in Australian seas exceed the threshold for inducing bleaching events on an annual basis (Hobday et al. 2006). A decrease in coral abundance would lead to changes in ecosystem structure, processes, and connectivity between reefs and adjacent waters. Nutrients and organic matter sourced from dynamic reef complexes may be disrupted.

While the effects of increased sea temperatures are likely to vary greatly across communities and ecosystems, there is a high level of agreement from different datasets that warming is affecting distributional ranges and larval phase of tropical marine fishes (Munday et al. 2009). Changes in sea temperature may also result in changes to phytoplankton and zooplankton communities, with implications for trophic dynamics (Richardson et al. 2009) and fish larval supply and survival (Lo-Yat et al. 2011).

Climate change modelling predicts that by 2070 ocean water temperatures at a depth of 500 m will warm by 0.5–1 °C, which could adversely impact larval fish development and survival (Hobday et al. 2006). Benthic and demersal fish species may shift south and some populations may decline where ranges are bounded to the south (Hobday et al. 2006). Structurally complex epifauna (i.e. sponges, algae and coralline algae) may also suffer mortality from elevated water temperatures (Lawrence et al. 2007).

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 will compromise carbon accretion and, together with increasing ocean temperatures, may result in loss of ecosystems based on geologic features formed from coral or coralline algae (Hoegh-Guldberg 2011; Hoegh-Guldberg et al. 2007; Kleypas & Yates 2009; Kuffner et al. 2008). Increasing acidity impairs the ability of species with calcareous shells, such as echinoderms, crustaceans and molluscs, to maintain shell integrity resulting in reductions of the overall abundance and biodiversity of these species (see review by Kleypas & Yates 2009). A decrease in the abundance of fauna with carbonate-based skeletons and coral and coralline algal abundance, and the complex structural habitats they create, could lead to changes in ecosystem structures, processes, and connectivity between the reef complex and the adjacent deeper waters.

Extraction of living resources (IUU fishing)
Illegal, unreported and unregulated (IUU) fishing has in the past been a significant issue in the North-west Marine Region and posed a major threat to target species and the conservation of the broader marine environment. In 2005, 13 018 illegal fishing vessels were sighted in Australian waters and of those, 600 were apprehended by Australian officials (Vince 2007). While the number of illegal vessels sighted per day has declined since 2005, there is concern that more powerful vessels with more sophisticated equipment may now be being used (Lack & Sant 2008). The Australian Government is also concerned with the issues of border security and quarantine that coincide with IUU fishing activities (Vince 2007).

IUU fishers predominantly target shark species for the valuable fin market. IUU catch of sharks is estimated to be twice that of reported legal catch (Heupel & McAuley 2007). The full extent of IUU shark fishing in northern Australia is largely unquantified; however, shark stocks targeted by IUU fishers have declined or are overfished (Heupel & McAuley 2007).

The effect on the marine environment of these key ecological features following the removal of sharks is unknown. Due to the life history characteristics of sharks (long life, slow to mature and small numbers of offspring) it may take some time before the effects of overfishing of sharks in the region is reversed, despite drops in the level of IUU activity in the region.