Gulf of Carpentaria basin

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 Gulf of Carpentaria is believed to be one of the few remaining near-pristine marine environments in the world (Bustamante et al. 2010; Coles et al. 2004; Wightman et al. 2004). Primary productivity in the Gulf of Carpentaria basin is mainly driven by cyanobacteria that fix nitrogen (Burford et al. 2009), but is also strongly influenced by seasonal processes. The soft sediments of the basin are characterised by moderately abundant and diverse communities of infauna and mobile epifauna dominated by polychaetes, crustaceans, molluscs and echinoderms.

The Gulf of Carpentaria basin also supports assemblages of pelagic fish species including planktivorous and schooling fish, and top predators such as shark, snapper, tuna and mackerel (Smith et al. 2006). The Gulf is also an important migratory route for seabirds, shore birds and marine turtles.

National and/or regional importance

Location

The Gulf of Carpentaria basin is located in the Northern Shelf Province provincial bioregion and is characterised by gently sloping soft sediments and water varying in depth from around 45–80 metres. Sediment types differ across the basin—shelf sandy muds (less than 50 % sand) are found on the western side, shelf muddy sands (50–80 % sand) on the eastern side and relict sands and muddy sands dominate the sea floor of the southern basin (DEWHA 2007; Somers & Long 1994). The waters in the Gulf of Carpentaria mix little with waters of the Arafura and Coral seas (Condie & Dunn 2006; Forbes 1984), so that they form a distinct semi-enclosed system with limited inputs from either oceanographic or terrestrial sources.

The spatial boundary of this KEF is based on the Geoscience Australia geomorphic features dataset.

Description

During the monsoon, Gulf waters become stratified, resulting in the development of high concentrations of chlorophyll at depths of around 40 metres. In the dry season (April–October), strong south-east trade winds mix Gulf waters and resuspend nutrients generated from benthic microbial processes high in the euphotic zone. This results in primary productivity throughout the water column (Burford & Rothlisberg 1999). Higher-order species including cetaceans and large pelagic fish prey on pelagic species that benefit from this productivity (Hosack & Dambacher 2011).

Nitrogen is also derived from benthic pathways created by detrital rain that is degraded by microbes (Burford et al. 2009). This source of nutrients is consumed by epifauna and invertebrate infauna (Hosack & Dambacher 2011). Snapper and other detritus-feeding fish consume the epifauna, which links higher-order predators such as sharks to both the benthic and pelagic food webs (Griffiths et al. n.d. in Hosack & Dambacher 2011).

The soft sediments of the Gulf of Carpentaria basin are characterised by benthic invertebrates including echinoids (e.g. heart urchins and sand dollars), sponges, solitary corals, molluscs, decapods, bryozoans, sea cucumbers and sessile tunicates (Long et al. 1995; Haywood et al. 2005). Deposit-feeding epifauna in the soft sediments are more abundant than suspension-feeding epifauna (Long et al. 1995).

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 Gulf of Carpentaria basin. 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.

Marine debris is a pressure assessed as of concern for the Gulf of Carpentaria basin.

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.

Marine debris
Marine debris such as derelict fishing nets are an increasing global threat to marine life (MacFadyen et al. 2009). In northern Australia, debris have entangled sharks, cetaceans, large piscivorous fishes and turtles (Kiessling 2003). Much of the marine debris found along the northern Australian coastline, including derelict fishing nets, is believed to be of foreign origin (Roeger et al. 2005). Northern Australia is especially vulnerable to marine debris given the proximity of intensive legal and illegal fishing operations, difficulties in surveillance and enforcement of existing management arrangements, and ocean circulation patterns that are likely to concentrate floating debris before dumping it on coastlines and beaches (Kiessling 2003).

Reports suggest a high number of marine species are being harmed and killed by debris while at sea, or as a result of injuries onshore (Chatto 1995 in Kiessling 2003). For example, turtle mortality associated with ghost nets in the Gulf of Carpentaria is unquantified, but is likely to amount to many hundreds of turtles per year (Limpus 2009). Marine debris may disrupt the breeding cycles of individual animals and compromise foraging habitats. Monofilament is highly persistent in the marine environment and has been documented as a major source of coral mortality in heavily fished localities (Smith et al. 2006). Marine species such as seabirds are also vulnerable to injury and mortality from marine debris through ingestion of lost or discarded plastics and other rubbish. In addition, marine debris such as derelict fishing gear can impact the Gulf of Carpentaria basin through physically scouring or damaging the sea floor. A threat abatement plan was prepared in 2009 (DEWHA 2009t).

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, 2004).

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).

Growth of mangroves and seagrasses may increase with elevated levels of atmospheric and dissolved CO₂, but any rises in sea level could at the same time compromise mangrove and seagrass habitats that support a diversity of marine life (Hill et al. 2002 cited in Hobday et al. 2006). Any decline in the ecological health of coral reef, mangrove and seagrass habitat within the coastal waters of the Gulf of Carpentaria will have a direct impact on ecosystem processes and biodiversity within adjacent offshore Commonwealth waters (Blaber 2009). This is because nutrients and organic matter exported from dynamic inshore complex habitats to offshore environments, as well as ontogenetic (lifecycle) movements of fish and crustacea between inshore and reef systems and offshore waters, are likely to be disrupted (Haywood & Kenyon 2009). 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).