There is a natural variation in the ability of plant and animal species and ecosystems to tolerate salinity. The ecosystems most likely to be affected by salinity are rivers and riverbanks, wetlands, and vegetation in lower parts of the landscape.

Native vegetation

Some plants, such as mangroves, have adapted to saline conditions. These plants, called halophytes, grow successfully in saline environments because they have specially adapted mechanisms that allow the plant to deal with the presence of excess salt.

However, for most plants, rising groundwater and increasing salinity have a negative impact on plant growth and seed germination. As soil salinity increases, plants find it increasingly difficult to extract water from the soil. High levels of salt can create an imbalance of plant nutrients in the soil and some salts are toxic to certain plants. Waterlogging causes poor soil aeration, starving plants of oxygen.

The immediate effects of salinity on plants that are not adapted to salt include leaf drop, leaf burn, stunted growth, poor seed germination and tree death. But, salinity also affects the health of vegetation communities, as fewer young plants survive to adulthood to replace the previous generation. The composition of vegetation communities may also change as salt tolerant plants become dominant in salt affected areas.

The National Land and Water Resources Audit, Australian Dryland Salinity Assessment 2000, lists the assets at risk of from dryland salinity in NSW. They include large areas of forests, managed protection areas, nature conservation areas, pasture land, remnant vegetation and wetlands.

The loss of vegetation can compound salinity problems by increasing groundwater recharge and salt mobilisation, which in turn, results in greater losses of vegetation.

The Discussion paper: Offsets, salinity and native vegetation investigates how the negative impacts of clearing native vegetation might be offset by separate actions with positive impacts. Offset actions could include improving the management of existing native vegetation, restoring or regenerating an area of degraded vegetation, or revegetating a previously cleared area.

Aquatic ecosystems

The impact of salt on natural systems is generally poorly understood, but studies suggest that adverse impacts are likely to occur in rivers and associated ecosystems when salt concentrations reach 1500 EC units over periods of time. The Murray-Darling Basin Salinity Audit reports that the threshold of 1500 EC units is currently exceeded in the Macquarie Marshes four per cent of the time. But this is predicted to rise to 23 per cent of the time by 2100, unless changes are made to the way natural resources are managed.

A joint project between DIPNR and the Murray Darling Freshwater Research Centre is investigating the relationship between increasing salinity and the structure of biological communities that develop from aquatic plant seeds and zooplankton eggs stored in the sediments of wetlands and rivers. The preliminary results indicate that salinity above 1500 EC units reduces the diversity and abundance of aquatic plant and invertebrate communities. However, plant and animal communities from different wetlands responded to different levels of salinity in different ways.

Scientists studying the Murray-Darling Basin (Metzeling et al 1995) found that many adult (fresh water) fish could tolerate up to 10g/L of salt, but it was not known if they would be able to produce healthy offspring, or if their young would survive.

Increasing levels of river salinity can also affect vegetation growing along riverbanks. Riparian vegetation plays an important role in maintaining water quality by reducing surface run-off and stream bank erosion. Erosion can contribute sediment to rivers and streams, causing further decline in water quality.

Caption: The death of salt sensitive plants and animals can disrupt the entire food chain within an aquatic ecosystem. Mammals, like the Australian platypus, might be able to tolerate higher levels of salinity, but their food supply, which includes freshwater invertebrates such as shrimp, worms, yabbies molluscs and insects, may be severely affected by increasing levels of salinity.