Wetland cycles

From Tropical Topics newsletter, No. 68 June, July 2001, produced by Stella Martin from Queensland's Environmental Protection Agency. Download the PDF to read the whole issue.

The importance of floods

magela creek

Floodplains on the lower Magela Creek in Kakadu National Park, NT.

Recently The Australian Geographic magazine published a letter from a reader who suggested damming the large rivers of northern Queensland and diverting the water into Murray-Darling system. This, he argued, would serve to increase the flow of the Murray and also help control flooding in the northern States.

Television news coverage of flooding emphasises only the ill-effects on humans. It overlooks the fact that the natural systems of the Australian tropical savannas have evolved with weather extremes which constitute the climatic norm. Plants and animals are adapted to cope with, and even flourish, with annual inundations and droughts.

Each year, at the end of the dry season, floods bring life to the savanna lands. Rivers break their banks, linking up to create great sheets of water which spread out across the flood plains. Water pours into drying swamps and waterholes, reconnecting billabongs to their parent rivers, filling lagoons and travelling on to provide water to areas which have had no direct rainfall. Water tables are replenished, providing a reservoir of underground water.

The first floods serve to cleanse the waterways. They scour sediments out of river channels, maintaining the depth of waterholes and keeping rivers navigable for boats. The rushing water also flushes organic matter out of billabongs and lagoons where a build up of nutrients might otherwise provide ideal conditions for potentially toxic algal blooms. Sediments and organic matter are carried downstream and deposited over the flood plain, replenishing nutrient levels and revitalising these areas for another growing season.

The arrival of water triggers breeding among many animals. Crustaceans, frogs and insects breed quickly, in turn attracting predators and fuelling a surge of growth right up the food chain. Floods not only stimulate spawning in fish of the savanna waterways, but also provide them with the means to migrate as they link up the rivers and lagoons. The bigger the flood the further they go, resulting in better genetic mixing and colonisation of new areas.

Although sometimes destructive to human endeavours, floods bring immense benefits. Floodplains, replenished by nutrients and water, provide feed for cattle, while temporary and permanent floodplain water allows the pastoral industry to make full use of grazing lands. Underground water is drawn on by farmers and urban dwellers throughout the Dry. Good floods, with large numbers of fish spawning, mean good catches in subsequent years.

Even the marine environment benefits. Estuaries are kept healthy, nutrients introduced by the floods providing food for many young marine fish and prawns; breeding for many species is timed accordingly. There is a direct correlation between big floods and large catches of banana prawns. Sediments washed to the coast also go on to nourish our beaches.

We need our floodwaters in northern tropical Australia. They can cause major disruption at times, but they are part of the natural system which, without them, would be in trouble.

Drying out

How does the drying out of a wetland lead to an increase in ducks? It doesn’t seem very logical, but a chain of related events creates a connection. 

As the area dries, it is colonised by grasses and other terrestrial plants. However, when water returns, these plants drown. The dead grasses, along with the remains of aquatic plants, fish and invertebrates which perished during the dry period, form a rich organic layer. This, in turn, leads to a population explosion of invertebrates which feed on this nutritious detritus. Notable among these are rice midge larvae, which have been recorded in densities as high as 15,000 per square metre. This ready food source encourages the adult ducks to breed. Then, by the time the ducklings hatch, the adult midges have emerged to form dancing clouds on the water surface, just at duckling bill height — an easy catch for these hungry learners.

The extreme variations in rainfall patterns in Australia mean that many wetlands dry out on a regular basis. This benefits not only ducks. Studies have shown that these ephemeral wetlands are, biologically, among the most productive and most diverse.

A variety of water levels suits a variety of plants. As one type of vegetation is succeeded by another it leaves behind tubers and seeds. Many of these can withstand long dry periods, ready to sprout as water levels rise. On the other hand, others may respond to increased light, available when water levels drop.

High water levels, which never drop, are detrimental to many plants. River red gums, which grow close to water and can withstand flooding, will nonetheless die if high water levels not fall. Seedlings of paperbarks and other waterside trees need a period of low water levels to become established while the seeds of cumbungi (Typha spp), produced in thousands from the bull-rush-like flower heads, can only germinate in moist mud which has become exposed. Similarly, the female flowers of ribbonweed (Vallisneria gigantea) must be able to protrude above the water surface, when levels are low, for pollination to take place.

A number of wetland animals also require dry periods as part of their lifecycle (see Surviving the Dry). Various small freshwater crustaceans lay eggs which are not only resistant to drought but, in some cases, actually require a period of desiccation before the eggs will develop. Some of these eggs are so small they are dispersed by wind during dry periods. Some long-necked turtles lay eggs below the water, development only beginning when falling water levels expose the eggs (see Waterholes).

It is a mistake to think that a wetland must always be wet. Studies have shown that the cycle of flooding (see Threats to Wetlands) and drying is probably the single most important factor in maintaining wetland health. If water levels are held constant (or if a wetland is drained) the natural diversity and productivity suffer.