Cape Mentelle upwelling

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

The Cape Mentelle upwelling is caused by prevailing southerly winds in the region, that counteract the Leeuwin Current’s driving force, drawing relatively nutrient-rich water from beneath the Leeuwin Current (where nutrient levels are higher), up the continental slope and onto the inner continental shelf (at depths of less than 50 m) (Pattiaratchi 2007). The increase in nutrients support phytoplankton blooms at the surface that provide the basis of an extended food chain characterised by feeding aggregations of small pelagic fish, larger predatory fish, seabirds, dolphins and sharks (Pattiaratchi 2007). The Cape Mentelle upwelling is adding to the local biodiversity, through increased production.

National and/or regional importance

The Cape Mentelle upwelling is defined as a key ecological feature for its relatively high productivity and aggregations of marine life.

Location

The Cape Mentelle upwelling occurs during summer months between Cape Leeuwin and Cape Naturaliste in the south-west corner of Australia; it is the most intense upwelling contributing to the Capes Current (Pattiaratchi 2007 and references therein).

The spatial boundary of this KEF, as defined on the Conservation Values Atlas, includes waters between 100m and 450m depth between Cape Naturaliste and Cape Leeuwin. As this KEF is a seasonal oceanographic event, with variations in time, space and intensity, the spatial boundary should be used as a guide only.

Description

The biological productivity of the South-west Marine Region is low by global standards and in comparison with other Australian marine regions. This is due to 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, including the Cape Mentelle upwelling, have a disproportionate influence on the region’s ecosystems, which is the reason that they are identified as key ecological features.

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 Cape Mentelle upwelling are changes in sea temperature and change in oceanography as a result of climate change.

Pressures assessed as of potential concern are ocean acidification as a result of climate change, bycatch and oil pollution. 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).

Bycatch
There is a general lack of evidence to demonstrate the performance of bycatch mitigation measures (Bensley et al. 2010). In relation to bycatch of non-protected species, Phillips et al. (2010) indicate that current levels of independent observers preclude a cumulative assessment of the catch of non-target species, but recommend that such assessment is important to understand more broadly the environmental effects of fishing and to underpin an ecosystem approach to fisheries management (Phillips et al. 2010). In the absence of an integrated assessment of the cumulative ecosystem effects of current catch rates for a number of non-target species, this pressure is of potential concern.

Oil pollution
A number of the key ecological features, including the Cape Mentelle upwelling, have characteristics that make their ecosystems and communities vulnerable to the effects of an oil spill, for example, features that include localised areas of high productivity, which attract large aggregations of marine life.

The intensity and distribution of activities implicated in oil spills, such as oil production and transport, are likely to increase in the region.

Australia has a strong system for regulating industry activity that is the potential source of oil spills and this system has been strengthened further in response to the Montara oil spill. While oil spills are unpredictable events and their likelihood is low based on past experience, their consequences, especially for threatened species at important areas, could be severe.