This is the fourth year of sampling macro-invertebrates 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. Since then samples have been taken each autumn and spring. This report covers autumn and spring 2008.
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. The scores have fluctuated over time with no obvious trend. Sometimes the upper dam scores higher and sometimes the lower dam scores higher. The range of species has remained much the same. Both dams continue to have a relatively healthy and diverse macro-invertebrate fauna.
The total number of macro-invertebrates caught is sensitive to sampling conditions and should be interpreted cautiously (Figure 2). The lower dam shows a marked fluctuation with season with fewer macro-invertebrates in autumn and more in spring. The upper dam does not show this seasonal effect. The most noticeable difference between seasons is the appearance in spring of corixids (water boatmen) and notonectids (backswimmers). At the end of summer they may fly away or die off possibly leaving eggs to overwinter.
No dragonfly larvae were caught this year. Next year we might do some more intensive sampling to see where they are hiding. The caddis flies are proving to be versatile animals. They live in a case that they construct from sticks or various bits of plant material. They appear in our dams in a variety of guises ranging from slender green stems, smallish twigs, flattish leaf litter to rather bulky collections of sticks. We do not know if they are all the same species with eclectic fashion tastes or different species each with their own style.
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