Australia: The Land Where Time Began

A biography of the Australian continent 

Aquatic Food Webs of the Channel Country

On floodplains like that of the Channel Country, where there are numerous small channels that are only connected in the times of flood, there are strings of very turbid waterholes. It is usually believed that aquatic food webs in this type of habitat are maintained by nutrients and energy from the mud deposited during floods and from the riparian zone, as occurs in typical rivers elsewhere. As the aquatic plants would be very limited because of the turbidity of the water reducing light penetration to allow photosynthesis.

Unexpectedly, research has shown that there is actually an abundance of life, a variety of animals that thrive in this turbid environment - snails, crustaceans and fish. The basis of this food web are algae.  Studies were carried out on permanent waterholes on the Cooper near Windorah, and on ephemeral pools in claypans on the Cooper Floodplain.

It was found that while more than 50 % of organic matter on the beds of the waterholes, the source of most of the plant material eaten by the animals originated in the algae. The organisms studied were thiarid snails (Notopala), shrimps (Macrobrachium) and crayfish (Cherax), and several native fish species. Unlike aquatic habitats in other places there were not many insects.

On there shallow margins of the waterholes there was a conspicuous band of algae. Most of this band was comprised of the filamentous cyanobacterium Schizothrix. This algal band growing on the mud surface demonstrated very high rates of primary production and respiration. The high respiration occurred at night. not from decomposition of organic matter. In the deeper water they showed much lower rates of primary production, because of the turbidity, as expected. The littoral zone was the main producer of organic carbon for consumption in the mid-channel part of the habitat. The littoral zone comprises 8 % of the total habitat, but its production level was so high that overall the waterhole was a net producer of organic carbon.

The shallow, ephemeral pools in claypans on the floodplain, filled after recent rain, had many shield shrimps (Triops) and other, smaller crustaceans and snails. The bottom ooze of these claypans proved to be of algal origin, and analysis of the aquatic animals found that the biomass carbon in all was derived from algae, not terrestrial plant matter, that is distributed over this area in flood times.

It was previously thought that as the massive floods of the Channel Country distributes large amounts of terrestrial carbon, the ecosystems of the floodplain would make use of this abundant food source, it has proved to be unimportant to the aquatic inhabitants of the floodplain. Floods are still important for the distribution of nutrients in the mud that supplies the algae with nutrients, as well as recruitment of the various organisms, and refilling the waterholes and lakes of the floodplain.

The situation regarding food webs probably applies to other arid country waterholes and lakes, such as along the Paroo River. If this is the case, taking a lot of water from these waterholes, especially if it is drawn rapidly, could damage or destroy the food webs by stranding the algal band above the water level which could totally disrupt the ecosystems in these places.

Sources & Further reading

1. Penny Van Oosterzee, The Centre - The Natural history of Australia's Desert Regions, Reed Australia, 1993
2. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
3. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000