Meso-scale eddies (several locations)

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 South-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

Meso-scale eddies are important food sources, particularly for mesozooplankton, given the broader region’s nutrient-poor conditions, and they become prey hotspots for a complex range of higher trophic-level species. Meso-scale eddies and seasonal upwellings have a significant impact on the regional production patterns (Waite et al. 2007).

The meso-scale eddies of this region are important transporters of nutrients and plankton communities, taking them far offshore into the Indian Ocean, where they are consumed by oceanic communities. They are likely to attract a range of organisms from the higher trophic levels, such as marine mammals, seabirds, tuna and billfish. The eddies play a critical role in determining species distribution, as they influence the southerly range boundaries of tropical and subtropical species, the transport of coastal phytoplankton communities offshore and recruitment to fisheries.

National and/or regional importance

Meso-scale eddies (several locations) are defined as pelagic key ecological features for their high productivity and aggregations of marine life.

Location

Eddies and eddy fields form at predictable locations off the western and south-western shelf break - south-west of Shark Bay; offshore of the Houtman Abrolhos Islands; south-west of Jurien Bay; Perth Canyon; south-west of Cape Leeuwin; and south of Albany, Esperance and the Eyre Peninsula (Pattiaratchi 2007).

Driven by interactions between currents and bathymetry, persistent meso-scale eddies form regularly (three to nine eddies per year) within the meanders of the Leeuwin Current. These features range between 50–200 km in diameter and typically last more than five months.

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, is available as indicative point data only.

Description

The biological productivity of the South-west Marine Region is low by global standards and in comparison with other Australian marine regions because of the low-nutrient tropical waters carried south by the Leeuwin Current and its effect in suppressing upwelling of nutrients from deeper cold waters and the absence of significant rivers contributing nutrients into the marine environment through run-off. Small seasonal upwellings occur regularly at known locations and, because of the overall nutrient-poor nature of the region’s waters, these hotspots of productivity have a disproportionate influence on the region’s ecosystems, which is the reason that they are identified as key ecological features. The meso-scale eddies are one of the main areas of relatively higher seasonal productivity in the region.

The eddies are important transporters of nutrients and plankton communities, locally (within the eddy) enhancing primary productivity but also potentially diluting plankton concentrations and removing larval fish from the continental shelf (Waite et al. 2007).

Meso-scale eddies form within unstable meanders of the Leeuwin Current as it flows south along the west Australian shelf break, and may be triggered by interactions between the north-flowing Leeuwin Undercurrent, local bathymetric features (e.g. the Perth Canyon) and the Leeuwin Current (Rennie et al. 2007; Waite et al. 2007).

Meso-scale eddies are observed in surface ocean currents as bodies of water that typically occur in counter-rotating pairs or triplets. Eddies are identified through satellite altimeter data. Cyclonic (clockwise-rotating) eddies and anticyclonic (anticlockwise-rotating) eddies have cold and warm core surface signatures respectively (Feng et al. 2007). Typically, cyclonic cold-core eddies are associated with increased production of plankton communities, while anticyclonic warm-core eddies are unproductive; off Australia’s west coast however, this pattern is reversed (Feng et al. 2007). The anticyclonic eddies are thought to support production at their centre through entrainment of productive shelf waters during eddy formation (Feng et al. 2007). They are consistently associated with high phytoplankton biomass; transport coastal phytoplankton communities offshore; and support much larger communities of phytoplankton than the surrounding waters (Moore et al. 2007; Thompson, et al. 2007). They therefore provide an important food source for mesozooplankton in otherwise oligotrophic waters (Waite et al. 2007). It is highly probable that these features attract a range of organisms from the higher trophic levels, such as marine mammals, seabirds, tuna and billfish (DEWHA 2008a), although further research is required to confirm this. On the other hand, the meso-scale eddies may contribute to the low productivity of finfishes on continental shelf in the region (Caputi et al. 1996; Gaughan 2007) through the removal of larval fish (through offshore entrainment) and through dilution of plankton concentrations near the coast (Waite et al. 2007).

In summary, these eddy systems may have a profound effect on pelagic production in the region, driving offshore production by transporting nutrients and entire pelagic communities offshore, and also generating upwellings of nutrient-rich deeper water. However, these processes have not yet been studied in detail. A major challenge to understanding their importance in the region is the complexity and variability of eddy systems.

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.

Pressures assessed as of concern for the meso-scale eddies are changes in sea temperature and change in oceanography as a result of climate change.

Ocean acidification as a result of climate change is a pressure assessed as of potential concern. 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). The south-west of Western Australia in particular is one of three hotspots in the Indian Ocean where rising temperature trends exceed the Indian Ocean basin average (Feng et al. 2009). All key ecological features considered important for the region’s primary productivity and biological production are at risk from climate change. Changes in sea temperature and oceanographic processes have been implicated in the region in shifts in distribution of marine species, changes to prey variability (including positive changes for some species, such as bridled tern) and effects on reproductive time and success (Dunlop 2009; Gaughan et al. 2002; Surman & Nicholson 2009). The overall implications for ecosystem processes in the region are not known.

Change in oceanography (climate change)
The strength of the Leeuwin Current has decreased slightly since the 1970s. This weakening is expected to continue, although this prediction currently has low confidence (Feng et al. 2009). The Leeuwin Current is the basis of much of the region’s biological productivity, and its strength and seasonal/climatic variability are a primary driver for the intensity, timing and locations of productivity events in the region, from the waters off Geraldton to the Great Australian Bight (Pattiaratchi 2007). The long-term implications for the region’s ecosystems and its key ecological features are uncertain. This pressure is considered of concern for those key ecological features important for the region’s productivity and for which the Leeuwin Current is considered a primary ecological driver.

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). Projected changes in Australian waters by 2070 include a decline in pH of 0.2 units (Lawrence et al. 2007). There is a high level of uncertainty about the effects of ocean acidification on marine life. While some organisms might be able to adapt (Orr et al. 2009), anticipated changes to phytoplankton and zooplankton have the potential to detrimentally affect ecosystem processes and the structure of ecological communities. Research on the impact of ocean acidification on Antarctic krill has found that increased levels of CO₂ kill their embryos (Kawaguchi et al. 2011). Krill are an important part of the food chain because they feed on phytoplankton and zooplankton, and are a key food source for many species that occur in Australian waters. Consequently, acidification impacts have the potential to affect species further up the food chain. The potential effects of increased acidity on the region’s biodiversity also include changes to growth and population dynamics of some shell-forming organisms, impacts on the reproductive and metabolic functions of a number of fish and invertebrate species, and sensitivity of some early-life stages to acidification (Orr et al. 2009).