Water

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

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

Algal Growth on Floodplain Soils

A trial laboratory experiment on algae in dry soils was designed to determine:

  1. if viable algal material is present in dried-out western Queensland floodplain soils, and
  2. how fast algal turfs develop on these substrates.
Algae growing on flood plain soils on the Warrego River, near Cunnamulla.

Algae growing on flood plain soils on the Warrego River, near Cunnamulla.

In November 1999, dried-out soil samples were collected from ten sites located on western Queensland floodplains using a metal soil core sampler (Table 1). Five replicate samples were taken at each site. Samples were transported and stored in dark plastic containers. The top 0.5 - 1.0 cm layer, forming the distinctive surface 'crust', was separated from the dry floodplain soil cores. The 'crusts' were then placed onto the filter paper in the petri dishes. Where no distinct 'crust' was present, loose pieces of dry soil were placed onto the filter paper. Filtered Cooper Creek water (vacuum filtration through 0.7 mm glass-fibre prefilters) was added to each petri dish to a level just below the rim, and lids were replaced. Samples were exposed to full insolation during the length of the experiment. The experiment was run from February 8 until February 14, 2000.

Visual inspection after 2.5 hrs from the start of the experiment revealed that the intact surface crusts of the W1, P1 and D2 samples were already turning a bright green colour. On February 11, 3 days after the start of the experiment, algae were forming a distinct green layer on the surfaces of 12 samples (Table 2 and Figure 13). Fluorescence measurements for these samples are summarised in Table 2. At the end of the experiment (February 14), algae were present in a total of 24 samples (Table 3; no fluorescence measurements were taken on this date since the fluorometer was unavailable). Microscopic examination of the epipelic algal material revealed that it was composed almost exclusively of the filamentous blue-green alga Schizothrix arenaria (Division Cyanobacteria, Family Oscillatoriaceae). As is the case with other members of the Oscillatoriaceae, heterocysts (= site of N fixation) and akinetes (resting spores) are absent in Schizothrix.

Table 1. Soil cores collected from western Queensland, November 1999.
Site Code (each 5 reps A - E) Region Coordinates
Cooper Creek site 1
(midchannel)		 CC1 Windorah S 25°23.695
E 142°46.420
Cooper Creek site 2
(midchannel floodrunner)		 CC2 Windorah S25°23.761
E 142°46.463
Cooper Creek site 3
(floodplain)		 CC3 Windorah S 25°26.271
E 142°48.002
Cooper Creek site 4
(channel)		 CC4 Windorah S 25°25.546
E 142°43.920
Cooper Creek site 5
(floodplain Mayfield)		 CC5 Windorah S 25°25.542
E 142°43.975
Cooper Creek site 6 CC6 Tanbar S 25°52.101
E 141°53.183**
Diamantina site 1b D1   S 25°42.423
E 139°57.155
Diamantina site 2 D2 Rocky crossing pool S 25°41.516
E 139°50.356
Warrego R W1 Tinnenburra S n/a
E n/a
Yowah Crk (Paroo R tributary) P1 Near Eulo S 28°18.925
E 144°55.048
** NB: coordinates for campsite next to Tanbar pool
Figure 13: Algal assemblage on soil after two days

Figure 13: Algal assemblage on soil after two days

Table 2. Results of fluorescence measurements on February 11. Epipelic algae were present in all samples listed.
Site Replicate sample (A – E) Fluorescence yield (F) Maximum yield(Fmax) Photosynthetic yield (y)
CC1 B 50 102 0.51
CC2 A 133 232 0.426
CC2 D 133 206 0.354
CC6 D 4 8 0.5
D2 A 134 226 0.407
D2 E 78 163 0.355
W1 A 179 287 0.376
W1 B 107 151 0.291
W1 C 50 90 0.444
W1 D 134 206 0.349
P1 D 88 145 0.393
P1 E 75 106 0.292
Table 3. Samples with epipelic algae at the end of the experiment (February 14).
Site Replicate sample (A - E)
CC1 A, B, C, D, E
CC2 A, B, C, D, E
CC3 A, B, C, D, E
CC6 D
D2 A, E
W1 A, B, C, D
P1 D, E
The vast floodplain of Cooper Creek (near WIndorah).

The vast floodplain of Cooper Creek (near WIndorah).

Isotope data and carbon to nitrogen ratios are consistent with the view that the source of floodplain soil carbon is of algal origin. The importance of algal carbon supporting food webs contrasts early views of rivers as detritus based ecosystems. Figure 14 shows a dry season waterhole food web constructed using stable isotope signatures.

Figure 14: Dry season waterhole food web.

Figure 14: Dry season waterhole food web.

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