Australia: The Land Where Time Began

A biography of the Australian continent 

Floodplain Inundation

An immediate surge of nutrients is released when the floodplain soils are wet, usually by an ephemeral flood. This nutrient release has 3 main components.

• partly from nutrients deposited in the previous flood
• partly from biological and physical effects of wetting dry soil
• partly from the breakdown of organic matter accumulated since the previous flood

In the soil are many small freshwater species that have been dormant since the drying after the previous flood, when the flood arrives the dormancy of these creatures is broken. The soil bacteria have been respiring aerobically in the dry soil, water cuts off the oxygen supply, so they switch to anaerobic respiration, then release nutrients into the water in forms usable by other organisms. In the floodwaters the biological productivity peaks quickly but falls off slowly until it reaches a new plateau. Standing cannot support the same rapid biological growth as fresh floodwater. The species found in the floodplains are adapted to the sudden nutrient surges at flood time.

The tiny aquatic species that are the basis of food webs on the floodplains proliferate rapidly within hours of the soil being wet by the flood. At this time many opportunistic species arrive, often  from great distances, to take advantage of the ephemeral bounty. When the water overflows the main channels and spreads out over the floodplain al the isolated waterholes and channels are connected, allowing the spread of aquatic species throughout the system. The plants and animals begin their breeding cycles, needing to complete them before the water is gone. The fact that floodplains act as sources or sinks for different materials have important effects on the riverine ecosystems. The health on the aquatic organisms such as fish, and whether or not the blooms of cyanobacteria produce toxic chemicals, depends on how the materials are distributed.

When the rivers are not artificially controlled they contribute water, nutrients and sediment to floodplains. The floodplains supply carbon, living organisms and water treatment of the floodwater, and breeding grounds for plants and animals. Rivers that are separated from their flood plains are impoverished and degradation because the floodplain supply food and life to the rivers and acts like a filter for the river, removing pollutants and excess nutrients that can lead to toxic algal blooms.

Floodplains are biologically rich, having 100-1000 times more species than the rivers that supply the water to them. Habitat diversity is supported by billabongs, intermittent lakes, backwaters, anabranches, swamps and wetlands. Billabongs that are spaced out across a floodplain contain a high number of endemic species, in spite of being connected in flood times. Some have been likened to islands where speciation occurs. The fact that endemic species persist means that removing a billabong would lead to extinctions.

Sources & Further reading

1. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000