Threats to wetlands
Dead river red gum woodland with chenopod understorey, northern Macquarie Marshes. Photo: Bill Johnson, OEH
Wetlands are among the most threatened ecosystems in Australia and the world. Many wetlands in NSW have been significantly altered or destroyed by diversion or drainage of their water, and major disturbance to their catchments. In inland NSW, for example, there has been a 50% decline in the area of the Macquarie Marshes
and a 75% decline in the area of the Gwydir wetlands
. In coastal NSW, 60% of wetlands have been lost or degraded over the past 200 years.
Wetlands also have to cope with competition and impacts from introduced plants and animals, and increasingly from the impacts of climate change such as sea level rise and altered rainfall patterns.
The major threats to wetlands are:
- river regulation and water diversion
- development and catchment disturbance
- introduction of weeds and pest animals
River regulation and water diversion
River regulation and water diversion is the principal threat to wetlands in NSW. Dams, weirs and other diversion structures have been constructed on rivers to mitigate floods and make water available for urban, industrial and agricultural uses, and the timing and volume of river flows have been altered to service those uses.
Many NSW wetlands are receiving decreased flows of water from rivers and overland runoff, less often and at unnatural times of the year. Some wetlands have become completely isolated from the river systems that once nourished them, sometimes causing those areas to become permanently dry and disappear. Other wetlands are used to store water for town supply or irrigation. As a result, the wetlands no longer dry out as they would naturally during drought. This may lead to the formation of acid sulfate soils.
River regulation and water diversion affect many aspects of wetlands, including their plants and animals, hydrology, water quality and geomorphology (their physical shape in the landscape). As a result, 'alteration to the natural flow regimes of rivers and streams and their floodplains and wetlands' has been identified as a key threatening process in NSW. Insufficient flows to inland wetlands can result in the loss of aquatic and semi-aquatic plant communities and the death of animals that rely on natural water flows, and also reduce opportunities for waterbirds to breed.
The installation and operation of in-stream structures that alter natural flow regimes of rivers and streams has also been listed as a key threatening process under the Fisheries Management Act 1994. Weirs and other structures create barriers for fish attempting to migrate upstream or downstream, and can make them easy prey for birds when they gather downstream of barriers.
Development and catchment disturbance
Direct impacts on wetlands from development include clearing of wetlands for urbanisation and other coastal development; clearing of wetlands for agriculture, for example, cropping and grazing; and changes in the hydrology (e.g. runoff) and nutrient levels of wetlands from adjacent developments such as housing, roads and other infrastructure.
Changes in a catchment's land use due to urbanisation and agriculture and other types of development can have profound effects on the functioning of wetlands. Principal impacts include greater fluctuations in river flows, increased sedimentation, increased nutrients, more rapid changes in water salinity, increases in pests and weeds, and outbreaks of harmful organisms such as blue-green algae.
Downstream of development there can be increases in the loads of nutrients (such as nitrogen and phosphorus) and suspended solids entering waterways in runoff after rain. For example, in coastal wetlands, the extra nutrients are rapidly taken up by some types of aquatic plants, particularly macroalgae and phytoplankton. The numbers of these plants increase, and they become dominant over plants living on the sediment, such as seagrasses and benthic microalgae. These changes in plant dominance have profound effects on the food chain.
Blue-green algal blooms, which can affect both inland and coastal rivers and wetlands, make water unsuitable for urban and agricultural uses and recreational activities, and are toxic to some aquatic plants and animals. On inland rivers such as the Darling blue-green algal outbreaks can occur after long periods of low rainfall and minimal river flows. In coastal wetlands such as Myall Lakes, blue-green algal blooms can occur after nutrients have been washed from the catchment in high rainfall, and the salinity of the lakes is sufficiently reduced.
Introduction of weeds and pest animals
Weeds and pest animals compete with native wetland species and habitats, and may replace them altogether. Common weeds and pests in coastal wetlands include lantana, salvinia, caulerpa and pigs, which can affect water quality and destroy habitats through digging and wallowing. Weeds and pests in inland wetlands include the introduced plant lippia, pigs and European carp, which can displace native fish in rivers and wetlands.
Fire in reed beds, northern Macquarie Marshes. Photo: Bill Johnson, OEH
Climate change will affect wetlands and the rivers that supply water to them through changes to rainfall and increased temperature and evaporation. This will reduce surface and groundwater supply and put increased pressure on the plants and animals that rely on these sources of water.
Extraction of water and modification to flow regimes has already had a considerable impact on biodiversity in many rivers and wetlands; the combination of climate change impacts and human impacts could be much greater than the sum of those pressures.
Human-caused climate change has been listed as a key threatening process under the Threatened Species Conservation Act 1995. In response to that listing, OEH has prepared Priorities for Biodiversity Adaptation to Climate Change, which identifies the challenges and actions for addressing the impacts of climate change on biodiversity. The ecosystems expected to face the most severe impacts from climate change are:
- those along the coast and in alpine areas
- the inland wetlands and woodlands of south-western NSW
- marine systems
- some fire-sensitive ecosystems.
Changing water regimes and wetland habitat on the Lower Murrumbidgee floodplain
Download this report, which traces 140 years of disruption to the Lower Murrumbidgee's native environments as a result of dams, levee banks and other irrigation devices.
Guidelines for managing cropping on lakes in the Murray-Darling Basin
Briggs, S. and Jenkins, K. (1997)
Page last updated: 13 March 2013