Gulf of Carpentaria coastal zone

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

National and/or regional importance

Location

The Gulf of Carpentaria coastal zone stretches from west Cape York Peninsula to Limmen Bight in the south-west Gulf of Carpentaria.

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, is the coastal area in the Gulf of Carpentaria, which extends from the Queensland and Northern Territory jurisdictional boundary (generally 3 nm from the coast) to the 20 m depth contour, derived from the 2009 bathymetry grid of Australia produced by Geoscience Australia.

Description

Sediments in the feature are relatively fine muddy sands and sandy muds (Somers 1987; Somers & Long 1994). Waters in the feature are well mixed throughout the year but are more heavily influenced by freshwater flows during the monsoon (Burford & Rothlisberg 1999). Mixing of freshwater flows and ocean currents tend to trap nutrients within the coastal zone (Wolanski & Ridd 1990), leading to high productivity and diverse and abundant marine life in this area (NOO 2003). A large proportion of the coastal waters of the Gulf of Carpentaria lie within the jurisdictions of Queensland and the Northern Territory. These inshore waters support mangroves, seagrasses and coral reefs, which help to drive primary production and diversity in contiguous offshore Commonwealth waters (Poiner et al. 1987; Wightman et al. 2004). They are also the source of organic matter found in Commonwealth waters that is transported through ontogenetic migration of fish and crustaceans (Brewer et al. 1991; Kenyon et al. 2004; Salini et al. 1990). 

Species found in this key ecological feature include marine turtles (olive ridley, green, hawksbill and loggerhead) (Poiner et al. 1990), 16 species of sea snake (Fry et al. 2001), colonial and solitary seabirds (e.g. terns, frigatebirds, white bellied sea eagles, osprey, brown boobies) (Chatto et al. 2004a), dugongs (Marsh et al. 2008), and aggregations of fish and sharks (DEWHA 2007). Small whales (false pilot whales) and bottlenose dolphins are numerous (Freeland & Bayliss 1989), and sawfishes (freshwater and green), syngnathids, rare rays and other elasmobranchs are also present (Griffiths et al. 2006; Smith et al. 2006; Stobutzki et al. 2000).

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 coastal zone.

Pressures assessed as of potential concern are sea level rise, changes in sea temperature and ocean acidification as a result of climate change; physical habitat modification from storm events and offshore construction; extraction of living resources; and changes in hydrological regimes. 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) and in northern Australia have been documented to entangle sharks, cetaceans, large piscivorous fishes and turtles (Kiessling 2003). In the Arafura Sea, reports suggest high numbers of marine species are being harmed and killed by debris while at sea, or as a result of injuries on shore (Chatto 1995 in Kiessling 2003). A number of different plastic types are recorded as impacting seabirds, with derelict fishing nets dominating entanglement records (Ceccarelli 2009). Smaller plastic items tend to be found wrapped around seabirds’ bills or legs (Ceccarelli 2009). Shearwaters, terns and petrels are known to ingest plastics. Some seabirds ingest plastic that they mistake for food. This can eventually cause mortality of individuals (Baker et al. 2002).

Marine debris affects a range of species although the extent of impacts is currently unquantified. For example, marine debris may disrupt breeding cycles of individual animals and compromise foraging habitats. Turtle mortality associated with derelict nets in the Gulf of Carpentaria is uncertain but is likely to amount to many hundreds of turtles per year (Limpus 2009). For example, in the six weeks after Cyclone Abigail in February 2001, it is estimated that more than 4000 nets were washed ashore containing around 400 (order of magnitude) turtles. Similar strandings and entanglements have been recorded for subsequent cyclonic events (Limpus 2009).

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).

Sea level rise (climate change)
Global sea levels have risen by 20 cm between 1870 and 2004 and predictions estimate a further rise of 5–15 cm by 2030, relative to 1990 levels (Church et al. 2009). Longer term predictions estimate increases of 0.5 m to 1.0 m by 2100, relative to 2000 levels (Climate Commission 2011). Sea level rise will potentially reduce critical nursery habitats such as seagrass and mangroves, particularly in the southern Gulf of Carpentaria (Hill et al. 2002 cited in Hobday et al. 2006), for many marine species (Hobday et al. 2008). Declines in coastal seagrass and mangrove habitats could lead to changes in ecosystem structure, processes and connectivity between inshore and offshore habitats. Nutrients and organic matter sourced from productive coastal environments could be disrupted, as could ontogenetic (lifecycle) movements by fish and crustaceans between inshore habitats and the offshore Commonwealth marine environment (Haywood & Kenyon 2009).

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 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). Extended periods of elevated temperature in shallow estuarine and coastal waters within the Gulf of Carpentaria coastal zone are likely to impact the distribution of fish and prawn nursery habitats, such as estuarine and coastal seagrasses and mangroves (Hobday et al. 2008).

Coastal fringing coral reef communities are expected to be affected (Hoegh-Guldberg 2011). 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).

Habitat declines are likely to have implications for marine species aggregations and biodiversity within the Gulf of Carpentaria coastal zone through disruption of ecosystem processes and connectivity between coastal and offshore ecosystems (Blaber 2009; Haywood & Kenyon 2009). During an El Niño event in the 1990s, for example, the supply and survival of reef-fish larvae within coral reef habitats was impacted by elevated sea temperature, causing declines in reef fish communities (Lo-Yat et al. 2011).

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).

For the Gulf of Carpentaria coastal zone, changes in dissolved CO₂ levels represent a threat to calcifying organisms such as corals, pteropods and coccolithophores (Poloczanska 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.

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 the Gulf of Carpentaria coastal zone; and localised impacts on a broad range of benthic species.

There are mining tenements to the west and north-west of Groote Eylandt for submarine mining of manganese ore (Groote Resources Ltd 2010). Although any activities associated with these tenements will occur in Northern Territory waters, they have the potential to impact ontogenetic (lifecycle) movements of crustacean and fish species from inshore to offshore Commonwealth waters (Haywood & Kenyon 2009). There are also mining tenements in Commonwealth waters south of Groote Eylandt.

Physical habitat modification (storm events)
Storms and cyclones have heavily modified marine habitats in the North Marine Region, and their intensity is predicted to increase (Hyder Consulting 2008). Present indications are that modest to moderate (up to 20 %) increases in average and maximum cyclone intensities are expected by the end of the century in some regions (Walsh & Ryan 2000). Cyclone activity can destroy seagrass beds and impact mangrove habitats (Hobday et al. 2008). For example, in 1985, Cyclone Sandy removed approximately 20 % of the seagrass beds in the Gulf of Carpentaria, which took approximately 10 years to recover (Hill et al. 2002 cited in Hobday et al. 2006).

Shallow habitats including mangroves are most at-risk from severe weather. Intensive storms can cause very high levels of damage, and increased frequency of storms means that habitats and communities have less time to recover between storm events. Habitat loss will occur when the frequency and intensity of severe weather events exceed the habitat’s ability to recover from one event to the next (Chin & Kyne 2007). Impacts associated with storms and cyclones include declines in coral reef, mangrove and seagrass communities located in nearshore waters, which can lead to changes in ecosystem processes and connectivity within adjacent offshore Commonwealth waters (Blaber 2009). Nutrients and organic matter exported from dynamic inshore complex habitats may be disrupted. Ontogenetic (lifecycle) movements by fish and crustacea between inshore and offshore waters are also likely to be disrupted (Haywood & Kenyon 2009).

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).

Changes In hydrological regimes
Changes in flows and characteristics of fresh water entering the Gulf of Carpentaria coastal zone have the potential to impact higher trophic species and commercial fishery species due to a reduction in the transport of terrigenous nutrients (sediments derived from land erosion and carried out to sea by rivers) to coastal habitats, loss of food availability and changes to triggers, such as flooding, that are emigration cues for key marine species (Burford et al. 2010). Impacts on key ecological features in the North Marine Region related to changes in hydrological regimes may be of greater concern if there is significant growth in agricultural and water resource development in adjacent coastal areas (CSIRO 2009).