Importance of Flood Flows to the Productivity of Dryland Rivers and Their Floodplains

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Final Report
Prof P.M. Davies, Prof S. E. Bunn and Ms F. Balcombe
Environment Australia, 2003

Workshop for Agency Partners

A workshop was facilitated via a special sessions forum on drought conducted in Albury during 2001. This had a wide attendance and many agency partners were present. The abstract of the paper presented follows:

Carbon metabolism of a dryland river: wet and dry comparisons.

Peter M. Davies1 and Stuart E. Bunn2
CRC for Freshwater Ecology, 1Department of Zoology, The University of Western Australia and 2Centre for Catchment and In-Stream Research, Griffith University.

Cooper Creek in the Channel Country of western Queensland, like many Australian inland floodplain rivers, is characterised by anastomosing channels that interconnect only during highly episodic flooding. Outside of flooding, the river is restricted to highly- turbid permanent waterholes. Despite the turbidity, a highly productive band of algae and diatoms (predominantly the epipelic blue-green algae Schizothrix arenaria), was restricted to shallow littoral zones. Measurements of carbon metabolism using perspex chambers in-situ showed rates in this littoral zone were extremely high at some sites(~3500mgCm-2day-1); over two-orders of magnitude greater than the mid-channel benthos. After local rainfall, many isolated claypans on the floodplain quickly turned green; indicating significant algal growth. Subsequent experiments were conducted to simulate the effects of flooding and measure the subsequent productivity of floodplain soils. Dry soils were collected from the floodplain and wetted within swimming pools placed on the edge of Cooper Creek. These soils had rates of gross primary production (GPP), after only five days, of about 800 mgCm-2day-1. Another experiment where dry floodplain soils were collected and returned to the laboratory, showed that only hours after wetting, algal material was evident. In situ measurements of chlorophyll fluorescence on these soils indicated significant algal growth on many of these samples after three days. A large flood in March 2000 inundated much of the Cooper Creek floodplain (about 16% of the catchment). During flooding, both the newly-inundated benthos and water column had elevated rates of carbon respiration (R24) with GPP increasing with the period of inundation. When floodwaters started to recede, rates of GPP were over 1300 mgCm-2day-1 and the system had switched to be a net producer of carbon (GPP/R24>1). Scaling up the results for area showed that the amount of aquatic carbon produced by a day of floodplain inundation was equivalent to 82 years of aquatic production during the dry.