Importance of Flood Flows to the Productivity of Dryland Rivers and Their Floodplains
Prof P.M. Davies, Prof S. E. Bunn and Ms F. Balcombe
Environment Australia, 2003
The results of this study show the importance of floodplain inundation to important ecological processes in arid zone rivers. The basic ecological economy of these systems is 'boom' and 'bust', corresponding to extended dry periods and floodplain inundation during major flood events.
Despite the high natural turbidity, permanent river waterholes in this study had a highly productive band of filamentous algae restricted to the shallow littoral margins. Rates of primary production in this zone are among the highest recorded for streams and rivers in Australia and remain high, even during the winter months. Stable isotope analysis has revealed that this bath-tub ring of algae is the major source of energy driving the food web in the permanent waterholes, and supports large populations of snails, crustaceans and fish.
Factors that influence the distribution and productivity of the bath-tub ring of algae are likely to have a pronounced effect on ecosystem function. For example:
- Rapid drawdown of water in river waterholes (e.g. pumping for irrigation) may expose the shallow band of algae. Although algae appear to be tolerant of desiccation, repeated exposure is likely to limit primary production and also reduce availability of this food resource to aquatic grazers.
- Uncontrolled access of stock and feral animals to the margins of river waterholes can physically disturb the algal zone. We have found that even a moderate level of disturbance significantly lowers algal production and recovery to pre-disturbance levels takes many days. Extensive and repeated disturbance of the margins of waterholes by stock and feral animals could limit algal production and threaten the very food base of snails, crustaceans and fish.
- The toxic effects of agricultural herbicides on aquatic algal are poorly understood, even though several chemicals (e.g. atrazine) are routinely found in inland rivers. Sub-lethal effects (e.g. reduced primary production) may also be important.
Large floods transform a dryland river into a vast, slow-moving wetland, triggering benthic (and pelagic) algal production across the inundated floodplain. At the height of a recent flood on the Cooper, a model estimated that the amount of algal carbon produced on the floodplain during a single day of inundation was equivalent to 82 years of aquatic production in the permanent waterholes during the dry. This boom of production on the floodplain was accompanied by a proliferation of aquatic invertebrates, especially small crustaceans. We recorded ten species of fish on the floodplain of the Cooper reaching an average biomass of over 1 tonne km-2. The diets of all fish species were dominated by aquatic sources and with a greater range of dietary items than that recorded in the dry. Some of this floodplain production undoubtedly returns to river waterholes as fish biomass once floodwaters recede. However, given the small area of permanent waterholes (3.2 km2) in this region compared with inundated floodplain (several thousand km2), much of the aquatic production must either be exported downstream or retained on the floodplain.
Small floods and in-channel flows periodically connect populations of aquatic organisms and provide an opportunity for dispersal. Altered flow regimes and changed land management affect patterns of connectivity of waterholes (refugia), and are likely to lead to population declines and loss of biodiversity.