Submerged coral reefs of the Gulf of Carpentaria

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 submerged coral reefs of the Gulf of Carpentaria are characterised by submerged patch, platform and barrier reefs that form a broken margin around the perimeter of the Gulf of Carpentaria basin. Twenty per cent of the reefs found in the North Marine Region are situated within this key ecological feature and it is likely that more reefs than have been identified occur there (Harris et al. 2007).

Relict reef structures support typical northern Australian coral reef fauna including octocorals, sponges, ascidians, gorgonians and reef fish. The submerged reefs also provide breeding and aggregation areas for many fish species including mackerel (Scomberomorus spp.) and large commercially fished snapper (Lutjanid spp.). They offer refuges for sea snakes and apex predators such as sharks (DEWHA 2007), and they sustain invertebrates such as crustaceans and polychaete worms, invertivorous fish and turtles (Marshall & Schuttenberg 2006). Coral trout species that inhabit Gulf reefs are smaller than those found in the Great Barrier Reef and may prove to be an endemic subspecies (DEWHA 2007).

National and/or regional importance

The submerged coral reefs of the Gulf of Carpentaria are defined as a key ecological feature in recognition of their high aggregations of marine life, biodiversity and endemism.

Location

The submerged coral reefs of the Gulf of Carpentaria are located in the Northern Shelf provincial bioregion and have only recently been described (Harris et al. 2004).

The spatial boundary of this KEF, as defined in the Conservation Values Atlas, includes known submerged coral reefs in the southern part of the Gulf of Carpentaria as identified by Geoscience Australia. Spatial data was created by interpreting locations and extents from hand drawn maps and should be treated as indicative only.

Description

The coral reefs exhibit flat-topped path reef morphology and rise from sea-floor depths of 30–50 metres. Reef platforms occur at depths of around 18–30 metres and reef tops or crests lie at depths of up to 20 metres (Harris et al. 2008).

Relict reef structures support a thin patchy cover of slow growing (one metre per 1000 years) live corals that includes large plate corals (Turbinaria spp.), abundant hard corals and a large proportion of soft corals (Harris et al. 2007). Corals found in the submerged reefs in the area have zooxanthellae that are bleach resistant (A Baker, pers. comm., 8 November 2009). The relatively closed circulation of the Gulf of Carpentaria may restrict recolonisation of reef species from the Indo–Pacific and limit dispersal within the Gulf (Condie et al. 2005).

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 submerged coral reefs of the Gulf of Carpentaria. 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 are assessed as of concern for the Submerged coral reefs of the Gulf of Carpentaria.

Pressures assessed as of potential concern are changes in sea temperature and ocean acidification as a result of climate change; marine debris 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 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, 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). Coastal fringing coral reef communities are expected to be affected (Hoegh-Guldberg 2011); however, any likely impacts on corals within the submerged reefs of the Gulf of Carpentaria, and the plateaux and saddle north-west of the Wellesley Islands are unknown.

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 plateaux and saddle north-west of the Wellesley Islands and the submerged coral reefs of the Gulf of Carpentaria, a decrease in coral abundance could lead to changes in ecosystem structure, processes and connectivity between the plateaux and reef waters. Nutrients and organic matter sourced from dynamic reef complexes could also be disrupted.

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.

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).

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).