Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7
Aquatic ecosystems (continued)
Key pressures on aquatic ecosystems include:
- changes in natural flow regimes due to water extraction and supply
- direct modification or destruction of important habitats
- barriers to the movement of plants and animals upstream
- effects of poor water quality
- competition from introduced and exotic animal and plant species.
Many inland aquatic ecosystems are affected by multiple pressures. The following sections highlight the key pressures on Australia's aquatic ecosystems.
Many native plant and animal species have adapted to natural patterns of water flow (or flow regimes) to survive and reproduce. While altering the flow regime may be advantageous for some exotic species, changing the natural flow regime is generally detrimental to native plant and animal species.
Flow regimes are affected by urbanisation, land clearing, water use and instream storages such as dams and weirs. In many regulated river systems, there has been a reduction in the frequency, magnitude and duration of minor and moderate flood events that are essential for sustaining riparian vegetation, wetlands and floodplain ecosystems (Kingsford 2000). Higher water residence times and reduced 'flushing' have been shown to be a factor in the switch of clear and macrophyte-dominated ecosystems to turbid and algal-dominated ecosystems (Harris 2001). This switch can often be abrupt and is highly resistant to switching back (i.e. shows strong hysteresis) (Scheffer 1998). The switch to a turbid and algal-dominated ecosystem can also exponentially increase the degradation of an ecosystem.
The use of rivers as irrigation supply channels can also dramatically alter a river's flow regime. Water demand for irrigation is greatest in summer and river flows are often artificially increased to meet these demands. In winter, there is less demand for irrigation water so flows are reduced while water storages are refilled. This is the opposite to the natural seasonal variation in flow of many inland waters. Such a dramatic change to the flow regime can have significant impacts on plants and animals that have adapted to low flows in summer and higher flows in winter (see Figure 23).
Figure 23: Modelled median monthly flow volumes (GL) on the Murray River at Albury under natural and 1988 development conditions
Source: MDBC 2001.
Changes in flow regimes can have lasting ecological effects. For example, seven species of macroinvertebrates disappeared after Lake Pedder in Tasmania was created (McComb & Lake 1990). Native freshwater fish such as golden perch, silver perch and Murray cod have been particularly affected by reduced flooding of key habitats in the Murray-Darling Basin (Thorncraft & Harris 2000). Colonial waterbirds breed on only a few large floodplain wetlands in Australia, so changes to the flooding cycle of these wetlands may have long-term impacts on populations of these species (Kingsford 2000).
When developing environmental water allocations, the timing of the delivery of environmental flows (i.e. flow regime) as well as the volume of flow must be considered to make sure that it mimics natural flow regimes.
The most dramatic change in natural flow regime has occurred in the Murray-Darling Basin (see Figure 24). Before water extraction began in the Murray-Darling Basin, natural flows exceeded 100 GL/month 95% of the time. As a result of water extraction, the current flows exceed 100 GL/month only 50% of the time. The frequency of occasions when there is no flow at the River Murray mouth has increased from 1 year in 20 under natural conditions to 1 year in 2 under current conditions (Baker et al. 1998). A similar change in natural flow regimes is likely in many of the other river systems of the Murray-Darling Basin and regulated coastal river systems.
Case study 6: Loss of aquatic species in Chowilla floodplain
The Chowilla floodplain is a wetland of international importance in the Murray-Darling Basin. Pre-development flows in the Chowilla floodplain were on average 13 400 000 ML/yr (Kingsford 2000). Development of the Murray-Darling Basin has resulted in 9 801 000 ML/yr of water being used for irrigation, therefore reducing flows that would have naturally inundated floodplain wetlands. Floodwaters, which used to reach the floodplain about every 1.2 years, now reach the floodplain every 2.5 years (Kingsford 2000). Moderate flooding now only occurs every three years and lasts half as long. Large floods that previously inundated the floodplain every three years and lasted for three months now occur every 10 years and last for two months (Kingsford 2000).
The reduction in flood frequency has had considerable impact on native floodplain plant and animal species. Populations of 18 species of snails in the lower Murray River have declined in the last 50 years. Snails are important foods of native fish species and waterbirds and therefore their decline in population may also affect these species (Kingsford 2000). Reduced flows have allowed dense littoral plants, reeds (Phragmites australis) and cumbungi (Tyhpa spp.) to become established in weir pools, displacing other naturally occurring species (Kingsford 2000).
Groundwater-dependent ecosystems rely on groundwater for their existence and health. Examples of groundwater-dependent ecosystems include:
- terrestrial vegetation, including dependent fauna that have seasonal or episodic dependence on groundwater
- wetlands, including aquatic communities and fringing vegetation dependent on groundwater fed lakes and wetlands
- aquifer and cave ecosystems
- river base flow systems, including aquatic and riparian ecosystems that exist in or adjacent to streams that are fed by groundwater
- terrestrial fauna
- estuarine and near-shore marine ecosystems, whose ecological function has some dependence on discharge of groundwater.
Some well-known examples of groundwater-dependent ecosystems include:
- Great Artesian Basin mound springs, which are vulnerable to changes in groundwater pressure and flux
- lakes of the Swan Coastal Plain which are vulnerable to pollution and changing surface water - groundwater interactions
- Exmouth Cape and its karstic groundwater ecosystems
- saline discharge lakes in Western Murray Basin which are vulnerable to changes in groundwater levels
- paperbark swamps in the tropical far north which are vulnerable to groundwater development.
There are thousands of groundwater-dependent ecosystems in Australia, and it is thought that many of the ecosystems associated with groundwater discharge form the basis of many of Australia's national parks, particularly in the arid and semi-arid zones (LWRDDC 1998a). The current challenge is to identify these groundwater-dependent ecosystems and their environmental water requirements. This will allow environmental water provisions to be put in place to protect those ecosystems which are at highest risk of degradation from groundwater use and extraction. Some groundwater aquifers are already experiencing over-extraction and many more are likely to be developed in the near future. The case study on the wetlands at Lake Jandabup illustrates the interrelated nature of groundwater on ecosystems.
Case study 7: Environmental water requirements for Lake Jandabup
Lake Jandabup is a shallow lake situated on the Swan Coastal Plain near Perth. It is of high significance in terms of its conservation value, and supports environmentally significant flora and fauna. The Swan Coastal Plain contributes significant volumes of water to the Perth water supply. The coastal plain needs to be managed to ensure the integrity of the area is maintained without reducing its conservation value.
The lake's high conservation value is based on:
- fringing rush/sedge vegetation surrounding the lake, including some rare species
- regionally significant wading bird habitat, including some listed under the Japan-Australia Migratory Bird Agreement (JAMBA)
- its importance as a breeding site for the little bittern (Ixobrychus minutus)
- the fact that it has the highest aquatic macroinvertebrate species richness of all wetlands sampled in the region.
Lake Jindabup is a conservation reserve. To ensure the preservation of its current conservation value, an investigation into groundwater impacts and levels was made to identify the environmental water requirements. This investigation found that to maintain the current conservation values, the lake levels needed to be maintained between 46.1 mAHD and 43.8 mAHD. These levels allow for periodic drying of the lake bed to remove introduced predators from the lake, high flows to ensure sustainability of the sedge which required saturation for periods of the year, and maintenance of the current wading areas and aquatic invertebrates and macrophytes.
The groundwater extraction regime, which is encompassed by the environmental water provisions for the area, will hence be limited by the environmental water requirements for Lake Jandabup. The environmental water provisions could be achieved by limitations to extraction volumes, seasonal extraction licences, and other provisions to ensure the sustainability of the flora and fauna at the lake, and to conserve its natural status.
Through recognition of the dependence of the flora and fauna in and around the lake on groundwater, the area can be conserved. This allows for groundwater to be used for the benefit of the community without detriment to the environments dependent on it.
Source: 'Groundwater & Land Use Planning', September 1996, Centre for Groundwater Studies. Based on Balla & Davis 1993, 'Managing Perth's Wetlands to Conserve the Aquatic Fauna - Volume 5 of Wetlands of the Swan Coastal Plain'. Water Authority of Western Australia and Environmental Protection Authority.
Intact riparian vegetation and zones are important for maintaining the health of inland aquatic ecosystems. Riparian zones influence habitat composition, stability and energy inputs, and act as filters for the movement of water, nutrients, pollutants and sediments between terrestrial and aquatic systems (State of the Environment Advisory Council 1996). They are also an important habitat for many plant and animal species and can act as wildlife corridors. The loss of riparian vegetation can multiply the impact of other pressures (such as eutrophication) on aquatic ecosystems (Tubman & Price 1999). Riparian zones can also be affected by other pressures, especially changes in natural flow regimes and poor water quality (e.g. high water salinity). The condition and extent of riparian zones is discussed in Condition of aquatic ecosystems.
Large woody debris has an important role in maintaining stream stability and ecosystem diversity in some inland waters (Treadwell et al. 1999). Large woody debris is also a significant source of carbon for inland waters. Research in Australia and overseas has shown that large woody debris surfaces support a wide diversity of aquatic invertebrates, including species that only live on large woody debris (Treadwell 2000).
However, desnagging of river systems (i.e. removal of large woody debris) has been widespread since European settlement. The aim of desnagging is flood mitigation and to avoid bank erosion, although data demonstrating the effectiveness of desnagging are not available (Wager & Jackson 1993).
Channelisation includes activities such as bank clearing, bank concreting and channel straightening, all of which can reduce water quality and destroy riparian and aquatic vegetation. Channelisation can also increase the speed and energy of flow in a river, leading to increased bank erosion. Channelised river systems have a significantly lower species richness compared with unmodified systems (Wager & Jackson 1993).
The lower Latrobe River between Lake Narracan and the Thomspon River confluence is one of the most disturbed river systems in Victoria due to riverbed desnagging, clearing of riparian vegetation, artificial cut-offs and channel widening. A total of 66 artificial neck cut-offs have been installed since 1870 along the lower Latrobe River, reducing the overall river length by 25%. The geomorphic and hydraulic changes in the river system are a product of these extensive modifications since European settlement (Reinfelds et al. 1995). All these works were designed to improve drainage but, although they are successful in reducing the frequency of minor flooding, they have little effect on larger floods (Reinfelds et al. 1995).
Artificial instream barriers have proved to have a significant impact on native fish populations. Common barriers include dams, weirs, regulators, farm dams, floodgates, causeways, culverts, pipes, channelised streams, bridge footings, and erosion control works (Thorncraft & Harris 2000). Major barriers isolate fish communities, restrict passage and result in changes in the fish community structures (Harris & Gehrke 1997) as many native fish need to migrate up and down river systems to breed, disperse and travel to spawning grounds. Of the 55 species of native freshwater fish in New South Wales, 32 are known to be migratory and require free passage to sustain populations (Thorncraft & Harris 2000).
In Victoria, 2438 existing barriers to fish movement and migration have been identified. High priority sites for fishway construction in the future have also been identified (McGuckin & Bennett 1999).
There are over 1700 barriers to fish movements in New South Wales rivers systems of the Murray-Darling Basin, with three rivers having over 300 separate barriers to fish movement (see Table 31).
|Catchment name||Weir or dam||Gated weir or regulator||Other||Total no. of barriers|
|Murray River (Riverina)||109||26||15||150|
Source: Thorncraft & Harris 2000.
Floodplain isolation occurs when the natural drainage of a floodplain is modified for flood mitigation, and through changes in flow regimes where overbank flows are either removed, or become less prevalent. Levees are often built to contain minor and moderate flooding events, reducing the floodplain area that receives regular inundation by flood waters. Many floodplain aquatic ecosystem and habitats (e.g. wetlands) require regular inundation and therefore their isolation from natural floodplain drainage patterns by levees can cause their disappearance from an area over a short period of time. Floodplain isolation impacts on river ecosystem health, as well as floodplain ecosystem health.
Floodplain isolation is one of the major causes of the loss of wetland area in the Rasmar-listed Macquarie Marshes in New South Wales (Kingsford 2000) and it is likely to be a major pressure in all highly developed floodplain areas.
The Australian River and Catchment Condition Database (ARCCD) includes data on the number of streams throughout Australia with levee banks (see Table 32). The data are biased towards New South Wales, as it is the only state to have up-to-date information. Better pressure indicators are the length of streams with levees or the extent of floodplain isolated. However, this information is not available. The data available indicate where levee banks are common, namely in the Murray-Darling Basin and South-east Coast drainage divisions.
|Drainage basin||Number of streams||Number of streams with levees||Percentage of streams with levees|
|Gulf of Carpentaria||263 742||5||0.002|
|Indian Ocean||127 948||0||0|
|Lake Eyre||366 242||14||0.004|
|North-east Coast||159 054||32||0.02|
|South Australian Gulf||23 334||18||0.08|
|South-east Coast||97 735||153||0.16|
|South-west Coast||36 527||0||0|
|Timor Sea||201 967||0||0|
|Western Plateau||177 482||0||0|
Source: Australia's Rivers and Catchment Condition Database, ERIN, Environment Australia.
The water quality of many of Australia's inland waters has declined as a result of human activities (see Water quality and pollutant sources). Poor water quality can be a significant pressure on the health of aquatic ecosystems. The key water-quality problems affecting aquatic ecosystems in Australia are:
- increasing instream water salinity
- nutrient enrichment and eutrophication
- high water turbidity and siltation
- pesticide contamination of water and sediments
- localised heavy metal pollution
- increasing acidity of inland waters
- cold-water pollution.
The effects on aquatic ecosystems of three of the major water quality issues are described in greater detail below.
Pesticides have been regularly detected in drainage water from irrigated land and in some rivers and streams draining irrigated land (see Water quality and sources of pollution).
Organochloride insecticides (e.g. DDT, Dieldrin, Lindane) were the most commonly used pesticides until recently. However, they are being replaced by organophosphate insecticides and herbicides that are less toxic to native species. Some pesticides (and other organic and heavy contaminants) can accumulate in the body tissues of aquatic biota and may have chronic effects such as reduced growth, impaired reproduction and changed behaviour. A reduction in fish reproduction can be as effective in reducing populations as acute pollution events resulting in mass fish kills (Wager & Jackson 1993).
Apart from fish kills, there is very little information on the effect of pesticides on aquatic ecosystems. Since 1996, more information on the effects of pesticides on individual Australian species has been derived from laboratory studies, but there is no information on impacts on whole aquatic ecosystems and food webs.
As discussed in Fish kills earlier in this report, 20 fish kills have been attributed to pesticides in the inland waters of New South Wales since 1990. A recent study of Sydney's inland sewage treatment plants found that the concentrations of some pesticides in their effluent were high enough to have an ecological impact (SKM 2000e).
Bowmer et al. (1998) have defined areas where additional research and monitoring is required to determine the full impact of pesticides on aquatic ecosystems in the irrigation areas of south west New South Wales. These include:
- the impact of pesticides on the major rivers
- pesticides entering waterways during storms
- pesticides in run-off from dryland farms
- crop specific data on pesticide use
- geographically more widespread monitoring
- calculation of pesticide loads
- monitoring of pesticides in sediments
- impact of pesticides on aquatic food webs
- impacts of pesticides additives.
Thermal pollution can be either an increase or a decrease in temperature. Increases in temperature are usually associated with the use of water for cooling an industrial process and the subsequent discharge of the water. Coal-fired electricity generation plants are common sources of 'warm' thermal pollution, though most now have their own cooling water ponds which negate the need to discharge heated water to inland rivers and lakes. Some industrial processes also use water for cooling.
Thermal pollution from cold water is increasingly being recognised as having a significant ecological impact. Cold-water pollution occurs downstream of dams and water storages that stratify, resulting in 'warm' water at the surface and 'cold' water at the bottom. Many dams and storages have single off-takes at the base of the dam wall so it is often 'cold' bottom water that is discharged. River systems that are regulated by multiple dams, such as the Murray- Darling Basin, are especially affected by cold-water pollution.
Cold-water pollution has been proposed as one of the most important causes of the reduction in the range and abundance of native freshwater fish species in New South Wales river systems (Lugg, unpub.). Cold-water pollution affects native fish by depressing water temperature by at least 10C for more than 300km downstream of the dam. Colder water reduces food availability, reduces growth and survival of fish, stops native fish spawning, and allows better adapted cold-water non-native species (trout) to have a competitive advantage.
Three thousand kilometres of rivers in New South Wales are thought to be affected, including the Murray, Tumut, Murrumbidgee, Lachlan, Belubula, Macquarie, Namoi, Peel, Gwydir, Dumaresq, Hunter, Patterson, Hawkesbury, Shoalhaven and Brogo rivers (NSW EPA 2001).
There is no information on the impact of cold-water pollution in other areas of Australia; however, it is likely that similar impacts on fish (and other components of freshwater ecosystems) may be occurring downstream of dams in many regulated rivers.
While some inland waters are naturally brackish or saline, dryland salinity and/or irrigation induced salinity has increased the instream salinity of many river systems. High salinities can have toxic effects on aquatic plants and animals (mainly because of interference with osmoregulation) and indirect impacts such as contributing to the loss of habitat and food species.
There is a general lack of information on the tolerance of freshwater biota to increasing and more variable salinity (ANZECC/ARMCANZ 2000b). There are, however, some general comments that can be made. Although there are salt-tolerant freshwater biota and some species can cope with short-term increases in salinity, most freshwater biota would be adversely affected by long-term increases in salinity. There will also be salinity impacts on the biological and physical components of river habitats (e.g. loss of riparian vegetation and increased soil erosion) that may affect individual species. For example in Western Australia more than 80% (by length) of stream fringing zones are seriously degraded by salinity in the cleared agricultural areas of the state and many wetlands are threatened or already in decline (DEP 1998).
Adverse biological impacts on Australian freshwater ecosystems have been measured when the instream salinity exceeds 1500 EC units (ANZECC 1992). The new Australian and New Zealand Guidelines for Fresh and Marine Water Quality suggest that significant impacts on aquatic ecosystems may result from an increase in salinity of 500 EC units (ANZECC/ARMCANZ 2000a). Based on these two guidelines, freshwater ecosystems in the Mallee and western regions of Victoria and most of south-west Western Australia are already affected. Freshwater ecosystems of the Warrego, Condamine-Balonne, Border, Lachlan, Bogan, Macquarie, Castlereagh and Namoi rivers could be significantly affected by 2050 based on predictions of increasing salinity (MDBMC 1999).
Wetlands are particularly susceptible to the affects of salinity. Lowland or floodplain wetlands are affected by salinisation through river regulation, irrigation-induced groundwater rises, increased extractions from the river, floodplain disposal of saline drainage water and groundwater rises due to land clearing (and associated dryland salinity). These threats differ significantly from those in upstream wetlands, where the major cause of salinisation is the evaporation of saline inflow water.
Of the 100 000 ha of the Murray River floodplain, 26 000 hectares are currently affected by salt, with between 30 000 and 50 000 ha predicted to be further affected by 2050 (MDBMC 1999). Major floodplain wetlands experiencing some effects from increasing salinity are the Chowilla Wetlands in South Australia and the Avoca Marshes in Victoria. Other major wetlands in the Murray-Darling Basin that are at risk from increasing salinity are Great Cumbung Swamp, Macquarie Marshes and the Gwydir Wetlands (MDBMC 1999). Nationwide it is estimated that a total of 80 important wetlands are already affected by salinity and this will rise to 130 by the year 2050 (NLWRA 2001b). Many riparian habitats (especially wetlands) can contain endemic species and communities that are at risk from salinisation (e.g. south-west Western Australia). Loss of these communities would cause a reduction in biodiversity in these areas.
The annual cost of salinity to the environment is estimated to be $40 million a year (Hayes 1997).
Case Study 8: Status of a Ramsar site - Chowilla floodplain
The effects of changed flows in the Murray-Darling Basin on the Chowilla floodplain have been discussed previously. The combination of river regulation and poor land management are the key threatening processes to the health of the Chowilla floodplain.
The Chowilla Anabranch is a tributary system of the Murray River. It is located around 20 km from Renmark (South Australia) and straddles the border between South Australia and New South Wales. It comprises a network of creeks, channels, lakes and swamps, all of which have high conservation values, and the area is listed under the Ramsar Convention. The importance of the Chowilla floodplain to Australia was highlighted by its selection by the Ramsar Convention Bureau as the Australian case study of the Wise Use of Wetlands, and by its inclusion in the UNESCO Man and the Biosphere program in 1992 (Sharley & Huggan 1995).
Values of the area include remnant stands of river red gums, extensive areas of black box woodland, habitat for breeding aquatic species, large waterbird colonies, good fish habitat and habitat for rare or endangered species. Key species include 8 threatened plant species, 9 threatened bird species, 20 species of breeding waterbirds and 17 species of fish (Sharley & Huggan 1995).
The floodplain is a naturally occurring salt discharge site; however, river regulation and European land management practices have raised the groundwater level under the floodplain by up to three metres. Chowilla Creek is the largest single contributor of salt to the River Murray and after flood events peak inflows of salt from Chowilla into the river can exceed 1000 tonnes per day (Sharley & Huggan 1995).
Consequences of this highly saline environment include severe dieback of vegetation, deterioration of the soil structure and decreased water quality downstream. These, in turn, have impacts on the floodplain biota.
The floodplain is gaining some respite through a project that is lowering the saline water table by pumping and disposal of saline groundwater off site. This approach will halt the ecological degradation of the floodplain and allow the re-establishment of healthy tree communities. In turn, this will provide positive outcomes for the aquatic biota of the wetland.