Mound Springs in Arid Australia

Points covered:

• Where are mound springs found?
• What are the unique flora and fauna of mound springs?
• Why are mound spring inhabitants endangered?

Words to know:

• ARID - climate with less than 250 millimetres annual rainfall, high evaporation and sparse vegetation.
• TERRESTRIAL - pertaining to or living on land.
• AQUIFER - water-holding sandstone.
• ENDEMIC - native to one area or continent only.
• SEDGES - rush or grass-like plants from the family Cyperaceae.
• CYANOBACTERIA - blue-green algae.

Arid Australia is probably the last place that you would expect to find many kinds of unique freshwater animals. However, natural springs called mound springs, that are fed by the Great Artesian Basin, are home to a diverse array of unique and unusual aquatic invertebrates and fishes. Furthermore, they are also important habitats for plants, birds and other terrestrial animals in an otherwise barren landscape.

Artesian water, stored in deep aquifers, reaches the surface through fault lines in the overlying rock. Where it breaks through the ground, in Queensland and South Australia, it forms mounds from the sediments and salts that are deposited by the spring water as it evaporates. Mound springs are often small hills, and in South Australia, they form an arc of about 400 kilometres along the south-western edge of the Great Artesian Basin.

The combination of plants and animals around mound springs are found nowhere else in the world. Most springs have a luxuriant growth of sedges that help stabilise the sediment and provide shelter for the animals. Reeds and grasses, green algae, Cyanobacteria, salt-tolerant species such as the succulent samphire and a rare, endemic species of button grass grow in or around the springs. Some invertebrates found in the springs include small crustaceans such as phreatocid isopods, amphipods, prawns and ostracods, which are sometimes endemic to one group of springs. Fishes, such as desert gobies, are also common in many mound springs.

Mound springs are home to over 40 species of small freshwater snails that live nowhere else, with many species found only in a single spring. A significant number of these snail species are now considered to be endangered because many of the springs are located on pastoral land, where they are threatened by trampling stock, modification of the spring flow or unsustainable use of artesian water. Many springs have disappeared in the last 100 years as a result of water extraction from the Great Artesian Basin, probably resulting in the extinction of unique species before they were even discovered.

Winston Ponder
Malacology
Australian Museum