TOWARDS DIAGNOSTIC BIOTIC INDEXES FOR RIVER MACROINVERTEBRATES

The construction of biotic indices that use macroinvertebrates to asse ss pollution and other anthropogenic disturbances of rivers and stream s often requires that each taxon be assigned a number indicating its l evel of sensitivity. A problem in constructing such indices is that in dividual taxa may vary quite widely in sensivity, depending on the nat ure of the particular disturbance. One possible means of overcoming th is problem is to construct a suite of indices, each assembled using se nsitivity numbers targeted to a particular impact. In order to test th is idea, we sampled macroinvertebrates from rivers in south-eastern Au stralia subjected to three different types of anthropogenic disturbanc e: operation of large darns, discharge of effluent from municipal sewa ge treatment plants, and contamination by metals originating from hist orical mining. Using macroinvertebrate data from sampling sites with v arying levels of exposure to disturbance, we developed sensitivity num bers for macroinvertebrate families for individual rivers and combinat ions of rivers with the same disturbance type. Sensitivity numbers cal culated for individual families differed significantly according to di sturbance type in several cases. Gastropod molluscs (family Thiaridae) were tolerant of dam effects but sensitive to sewage and metals, wher eas coenagrionid damselfly nymphs, elmid beetles and ostracods were mo st tolerant of sewage. Corydalid alderfly larvae, leptophlebiid mayfly nymphs, lestid damselfly nymphs, libellulid dragonfly nymphs and scir tid beetle larvae were most tolerant of metals. Indices constructed us ing sensitivity numbers for a particular disturbance were generally mo st responsive to that disturbance, but there was considerable generali ty in responses as well as substantial variability between different r ivers with the same disturbance type. In particular, macroinvertebrate communities at sites downstream of dams responded quite variably, pro bably because of substantial differences in release regimes. We conclu de that the approach has merit but requires considerable further devel opment and testing, as well as consideration of the levels of specific ity and diagnostic strength that are appropriate or achievable.