Macro-invertebrates are animals without backbones that are big enough to see with the naked eye. Because they are widespread and come in a lot of different shapes and sizes, macro-invertebrates are often used as indicators of the health of streams and other water bodies. Known to their friends as ‘water bugs’, aquatic macro-invertebrates include insects such as water beetles, backswimmers, water boatmen, mayflies, dragonflies, caddisflies and midges; crustaceans such as shrimp, yabbies and water fleas; shell fish such as freshwater mussels; and various worms.
We hope that the number and diversity of macro-invertebrates in the Majura dams will increase as the habitat becomes more varied. The new plants, rocks and branches should provide more places to hide from the hungry mosquito fish that are currently present in large numbers. We also hope that the water will become clearer with less soil being washed into the dams during rain. This will make it easier for the macro-invertebrates that need to filter their food from the water.
Macro-invertebrates were sampled just before the first plantings in October 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 sampling sheets for more detail. We used a simple scoring system known as SIGNAL2. Bigger scores indicate a more diverse and healthier water body. The Upper Dam had a score of 4.0 and the Lower Dam a score of 6.1. These scores are pretty good for dams but we hope that we will see an increase over time.
Gunther Theischinger, a dragonfly expert with the NSW Department of Environment and Conservation, identified our first dragonfly nymph as Hemicordulia tau a common vagrant of SE Australia. He has kindly offered to identify future specimens and suggests that we look out for another species of Hemicordulia, a Diplacodes species, Orthetrum calendonicum, Hemianax papuensis and perhaps the damselflies Ischunura aurora, Ischunura heterosticta and Xanthagrion erythroneurum.
How will new macro-invertebrates get to our dams? Some may already be there in small numbers. As they become more abundant we are more likely to find them. The adults of many aquatic insects can fly. We hope that they will notice how attractive our dams are becoming and stop to lay their eggs. Other macro-invertebrates use other forms of travel such as the feet of water birds. We will just have to wait and see.
The plan is to have two main macro-invertebrate samples per year – one in spring and one in autumn. Observations will also be made by Blue Gum School and others. The results will be posted on the website. For more information on aquatic macro-invertebrates consult The Waterbug Book by John Gooderham and Edward Tsyrlin (CSIRO Publishing).Examples of water bugs can be viewed at: http://www.waterwatch.org.au/library/module-3/introduction.html#fig