Groundwater Dependent Ecosystems
Groundwater dependent ecosystems (GDE) are a diverse and important component of biological diversity. The term GDE takes into account ecosystems that use groundwater as part of survival, and can potentially include wetlands, vegetation, mound springs, river base flows, cave ecosystems, playa lakes and saline discharges, springs, mangroves, river pools, billabongs and hanging swamps. The groundwater dependence of ecosystems will range from complete reliance to those that partially rely on groundwater, such as during droughts. The degree and nature of dependency will influence the extent to which ecosystems are affected by changes to the groundwater system, both in quality and quantity.
The dependency of ecosystems on groundwater is based on one or more of four basic groundwater attributes (SKM, 2001):
- flow or flux - the rate and volume of supply of groundwater;
- level - for unconfined aquifers, the depth below surface of the water table;
- pressure - for confined aquifers, the potentiometric head of the aquifer and its expression in groundwater discharge areas; and
- quality - the chemical quality of groundwater expressed in terms of pH, salinity and/or other potential constituents, including nutrients and contaminants.
The response of ecosystems to change in these attributes is variable. There may be a threshold response in some cases, whereby an ecosystem collapses completely if conditions change to a critical level. An example is mound spring communities supported by groundwaters of the Great Artesian Basin (GAB). These would cease to exist if pressures in the GAB fell to the point where there was no further surface discharge at the spring. In other cases a more gradual change in the health, composition and/or ecological function of communities is expected as, for example, may occur with increasing groundwater salinity or contaminant concentration.
Types of groundwater dependent ecosystems
Terrestrial vegetation:
Terrestrial vegetation may depend to varying degrees on the diffuse discharge of shallow groundwater, either to sustain transpiration and growth through a dry season or for the maintenance of perennially lush ecosystems in otherwise arid environments.
Wetlands
Groundwater dependent wetland ecosystems are those that are at least seasonally waterlogged or flooded. Hatton and Evans (1998) considered that wetlands provide the most extensive and diverse set of potentially dependent ecosystems in Australia. Examples of groundwater dependant wetland ecosystems include mesophyll palm vine forests, paperbark swamp forests and woodlands, swamp sclerophyll forests and woodlands, swamp scrubs and heaths, swamp shrublands, sedgelands and mound springs ecosystems. The diversity of groundwater dependent wetland ecosystems means that each of the four key groundwater attributes can play some role in their dependency.
River base flow systems
Stream flows may have a groundwater discharge as a baseflow component in many river reaches in Australia. This groundwater baseflow component may be vital to the character and composition of in-stream and near-stream ecosystems.
Aquifer and cave ecosystems
Hypogean life exists in a continuum through different types of karstic, cave, porous and fissured aquifers.
Terrestrial fauna
Groundwater dependent fauna have a reliance on groundwater that is not based on the provision of habitat, but as a source of drinking water. Groundwater, as river baseflow or discharge into a spring or pool, is an important source of water across much of the country, particularly in northern and inland Australia and other areas with semi-arid climate. Its significance is greater for larger mammals and birds, as many smaller animals can obtain most of their water requirements from respiration. Pastoralists in inland Australia have made extensive use of groundwater to supply drinking water to grazing stock. In addition to watering stock, groundwater is also used by native fauna (eg kangaroos) and pest and feral animals. Provision of water has allowed larger populations of both wildlife and pest animals to be sustained than would otherwise be the case.
Estuarine and near shore marine systems
These types of ecosystems are the marine counterparts of the terrestrial ecosystems and can include coastal mangroves and salt marshes, coastal lakes, sea grass beds and marine animals. Some marine and estuarine animals depend on groundwater discharge to provide a suitable habitat or an appropriate environment in which species of plants and/or animals they eat will prosper. Groundwater discharge may be in the form of direct off-shore discharge or baseflow into streams that discharge to the ocean. Examples of groundwater dependent fauna include crocodiles, turtles, fish and macro-invertebrates (Hatton and Evans 1998).
Relevant Links
Department of the Environment and Heritage Environmental Water Requirements for Groundwater Dependent Ecosystems
National Rivers Consortium Framework for assessment of environmental water requirements for GDEs
Australian Journal of Botany special issue 54(2) on Groundwater Dependant Ecosystems (GDEs)
Tas DPIW Groundwater Dependent Ecosystems
International Association of Hydrogeologists Commission on Groundwater Dependent Ecosystems
References
Hatton T, Evans R, 1998. Dependence of ecosystems on groundwater and its significance to Australia. LWRRDC Occasional Paper No 12/98.
SKM, 2001. Environmental Water Requirements of Groundwater Dependent Ecosystems. Technical Report Number 2, Sinclair Knight Mertz for Environment Australia.