Macro-invertebrate sampling: Spring 2006

This is the third time that macro-invertebrates have been sampled in the Majura Dams as part of Project Dragonfly. They were first sampled in spring 2005 to provide a benchmark against which future samples could be compared. As well as our first dragonfly nymph, we found quite a variety of other macro-invertebrates including water boatmen (true bugs), mayfly nymphs, caddis flies, midge larvae, yabbies and freshwater shrimp. A second sample was taken in autumn 2006. See earlier reports for more detail. This third sample, in spring 2006, is twelve months after the initial sample and twelve months after restoration work commenced.

We are using a simple scoring system known as SIGNAL2. Bigger scores indicate a more diverse and healthier water body. The scores for each dam are shown in Figure 1. It is premature to draw any conclusions about trends over time as the scores are sensitive to sampling conditions and seasonal fluctuations. The total number of macro-invertebrates caught has tended to increase (Figure 2) and the range of species has remained more or less the same.

Six dragonfly larvae were found in the upper dam this time and one in the lower dam. All appear to be the common species Hemicordulia tau with Gunther Theischinger kindly confirming the identification. Gunther has recently published the definitive book on Australian dragonflies. According to Gunther, many species of dragonfly are unable to emerge as adults without a suitable vertical surface on which to climb so the new plants that are surviving despite the drought will be important for our dragonflies’ future.

The mosquito fish (Gambusia) that were extremely abundant in previous samples were not seen at all in the lower dam at this sampling time. They were present in the upper dam. The lower abundance of fish may be one of the reasons for the larger number of macro-invertebrate specimens caught in the lower dam.

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Figure 1. Signal2 scores for each of the three sampling periods so far. A higher score indicates as more diverse and healthier water body. The changes over time are probably due to seasonal and sampling variability rather than any real trend.

Figure 2. The number of specimens caught in each of the three sampling periods so far.

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