Carbonate bank and terrace system of the Van Diemen Rise

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 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

Lying on the north-eastern side of the Joseph Bonaparte Gulf, the carbonate banks and valleys of the Van Diemen Rise provide more habitat divesity than in the central gulf, resulting in a higher diversity of epifauna (Przeslawski et al. 2011). The region has been identified as a sponge biodiversity hotspot (Przeslawski et al. 2014).

The banks, ridges and terraces of the Van Diemen rise are raised geomorphic features with relatively high proportions of hard substrate which support sponge and octocoral gardens. These, in turn, provide habitat to other epifauna, by providing structure in an otherwise flat environment (Przeslawski et al. 2011). As they are further from the coast, these raised features are influenced less by tides and the associated increased turbidity compared to the central Joseph Bonaparte Gulf. The variability in water depth and substrate composition may contribute to the presence of unique ecosystems in the channels.This may also contribute to the richness of epifauna found here.

Rich sponge gardens and octocorals have been identified on the eastern Joseph Bonaparte Gulf along the banks, ridges and some terraces (Heap et al. 2010, Przeslawski et al. 2014). Plains and deep holes/valleys are characterised by scattered epifauna and infauna that include polychaetes and ascidians. Epibenthic communities such as the sponges found in the channels support first and second-order consumers. Biophysical maps associated with clustering analysis (Ellis & Pitcher 2009) show greater clustering in this area, which indicates greater environmental variability compared with the rest of the North Marine Region.

National and/or regional importance

The Carbonate bank and terrace system of the Van Diemen rise is defined as a key ecological feature considered important for its role in enhancing biodiversity and local productivity relative to its surrounds and for supporting relatively high species diversity.

Location

The Carbonate bank and terrace system of the Van Diemen rise lies on the north-eastern side of the Joseph Bonaparte gulf, adjacent to the WA/NT border. This feature is part of a larger system associated with the Sahul banks to the north and Londonderry rise to the west. It is characterised by terrace, banks, channels and valleys. The carbonate banks and shoals found within the Van Diemen rise make up 80 % of the banks and shoals, 79 % of the channels and valleys, and 63 % of the terrace found across the North Marine Region.

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, includes the majority of terrace, bank/shoals and deep hole/valley areas from the Geoscience Australia geomorphic features dataset which lie northwest of Melville Island. The Malita shelf valley was spatially defined based on expert advice from Geoscience Australia. It comprises most of the valley area which lies below 140 m depth.

Description

The carbonate banks that form the Van Diemen rise are thought to be directly related to hydrocarbon seepage from the Bonaparte basin (O’Brien et al. 2002). The existing banks are thought to have formed through erosion of the exposed shelf during periods of low sea level approximately 12 000 years ago (Przeslawski et al. 2011).

Whilst reef-forming corals are rare throughout the Joseph Bonaparte gulf and Timor sea region, some locally dense hard corals were found on the banks of the Van Diemen rise during surveys in 2009 and 2010 (Przeslawski et al. 2011). Samples included species listed as near threatened, vulnerable and endangered on the IUCN Red List and suggest that coral communities on the Van Diemen rise are distinct from those elsewhere in northern Australia (Przeslawski et al. 2011).

Pelagic fish such as mackerel, red snapper and a distinct gene pool of gold band snapper are found in the Van Diemen rise (Blaber et al. 2005; Salini et al. 2006). Olive ridley turtles and sea snakes, including the olive seasnake and turtle headed seasnake, occur in the area (M Guinea, pers. comm., 2009). Sharks are also found in the area, although not as frequently as they once were (Blaber et al. 2009). The reason for this decline in shark numbers is unknown.

Channel systems between the banks range from approximately 60–150 metres to 10–40 metres in depth (Anderson et al. 2011). The variability in water depth and substrate composition across the feature may also contribute to the presence of unique ecosystems in the channels (Heap et al. 2010). Recent surveys suggest sediment found in some deeper channels may be from deposition in a brackish (marginal marine) or lacustrine environment, but the age and origin of these channel fill sediments is not well understood (Heap et al. 2010).

The Indonesian Throughflow transports warmer oligotrophic waters of lower salinity into the area from the tropical western Pacific Ocean. The extent to which this supports ecological functioning and biodiversity in the area is largely unknown.

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 Van Diemen rise. 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 Van Diemen rise.

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). This projected temperature change is expected to exceed the threshold for inducing coral bleaching on an annual basis (Hoegh-Guldberg 1999, 2005 cited in Hobday et al. 2006).

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).

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.