Page 09 16. Some invertebrate species can be used as indicator species to assess the level of pollution by organic waste in freshwater systems. Species within the taxa Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) are particularly sensitive to pollution. These taxa are referred to collectively as EPT taxa. The presence of these species can be used to assess water pollution levels by calculating an EPT index using the formula shown. The value obtained allows the water quality to be assessed using Table 1. EPT index = total number of EPT taxa present × 100 total number of taxa present Table 1 EPT index Water quality >50% excellent 35–49% good 25–34% moderate <25% poor A study sampled the invertebrate species in a stretch of river and the results are shown in Table 2. Table 2 Taxa Number of individuals Striped stonefly 6 Dragonfly 5 Simulid blackfly 9 Net-spinning caddisfly 6 Bloodworm 8 Flathead mayfly 3 Damselfly 7 The data suggests the water quality of this river is A excellent B good C moderate D poor. [Turn over