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
Primary Production on the Floodplain
Floodplain inundation is a highly episodic, unpredictable feature of the hydrology of many arid zone rivers. During floods, water inundates vast areas of the landscape and consequently has the potential to 'switch on' substantial algal growth (Figure 2). To quantify this algal growth primary production on the floodplain was quantified in the open water and on the inundated soils. This was done using perspex metabolism chambers to measure in-situ rates of respiration (carbon consumption) and of primary production (growth of algal carbon). Initially high rates of benthic respiration (R24;24 hour rates of ecosystem-level carbon consumption) were recorded (mean=284 mg.C.m-2day-1) presumably a response to elevated bacterial activity on the newly-inundated, nutrient-enriched soils. During this period, rates of benthic gross primary production (GPP; gross primary production = growth of carbon over 24 hours) were very low (mean=16 mg.C.m-2day-1).
With increasing duration of the flood, benthic primary productivity became more important. After two weeks inundation rates of mean GPP increased to 1366 (mg.C.m-2day-1). Similarly rates of benthic respiration increased to 696 mg.C.m-2day-1. Initially, there was little relationship between GPP and R24 (r=-0.1) and mid-flood (r=0.28). However there was a significant coupling between GPP and R24 at the later stages of the flood (r=0.94, p
The rates of open water GPP have been expressed as per m2. We believe almost all the phytoplankton productivity takes place near the surface of the water. Therefore we have expressed open water production per area rather than per volume. Again, we consider the measured rates to underestimate total rates as it was difficult to measure GPP in the surface water without including water from further down in the water column.
Figure 2: Newly inundated flood in Copper Creek near Currareva. The photo shows area of high ground (often ancient dunes) which are a refuge to terrestrial fauna during floods.
The areal extent of waterholes during the dry and landscape inundated during the 2000 flood, in the upper and mid-regions of Cooper Creek, were mapped using satellite imagery and GIS. This mapping enabled an accurate and detailed assessment to be made of the areal extent of 'wetted area' (Figures 3 and 4).
The waters on the inundated floodplain were enriched with nutrients. The high concentrations were associated with suspended particulates in the water column. These nutrients contributed to stimulating the enormous plankton growth observed on the floodplain during the initial phase of the flood. Flood waters were the most likely means for deposition and exchange of nutrients to newly inundated soils. Floating mats of cyanobacteria were observed at several sites. Cyanobacteria are generally able to fix atmospheric nitrogen.
Cooper Creek floodwaters 2000
The nitrogen (N2) fixation rates of micro-organisms in sediment were measured at some inundated floodplain sites and surface water samples (Figure 5). The acetylene reduction method was employed with the reduction of acetylene to ethylene used to measure nitrogenase activity. The rate of acetylene reduction was then used to estimate natural rates of N2 fixation. There was no measurable N2 fixation at any site. However, floating mats of cyanobacteria were nitrogen fixers.
Measuring nitrogen fixation in sediments.
Rates of primary production following floodplain inundation are shown for the benthos (Figure 6a) and water column (Figure 6b). The estimates are most likely conservative and will be refined by further breakdown of the inundated area into specific productivity zones. Some areas were 'hot spots' of production e.g. Jundah, while in other areas waters were too deep to allow substantial benthic productivity e.g. waterholes.