Macro-invertebrate sampling: Autumn 2006

Macro-invertebrates were 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, caddisflies, midge larvae, yabbies and freshwater shrimp. See the earlier report for more detail.

We sampled again in autumn 2006. The abundance of macro-invertebrates can vary a lot according to the season – hence our plan is to have two samplings per year – one in spring and one in autumn.

We are using a simple scoring system known as SIGNAL2. Bigger scores indicate a more diverse and healthier water body. In autumn 2006, the Upper Dam had a score of 6.0 and the Lower Dam a score of 5.5. In the previous spring (2005), the scores were 4.0 and 6.1 respectively. The differences between spring and autumn are most likely to be due to season rather than any real change in the dams. The lower dam, in particular, had many fewer macro-invertebrates in autumn compared with the spring sampling.

Three dragonfly larvae were found in the upper dam this time, so now we have evidence of dragonflies in both dams. We also found several small yabbies and, surprisingly, three small mayfly larvae, which have a long winter ahead of them. There was no sign of the true bugs (the backswimmers and water boatman) that were abundant in the spring. They have probably flown off before it got too cold, possibly leaving eggs ready to hatch in spring.

The mosquito fish (Gambusia) are so abundant that it is surprising that any macro-invertebrates survive at all. As the new plants grow and provide more places for the macro-invertebrates to hide, their life should become a little easier and we might start to see a few new faces.

Jean Chesson
Friends of Mt Majura

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Upper Dam

Lower Dam