Distribution of benthic intvertebrates along a disturbed section of the La Trobe River, Victoria: an analysis based on numerical classification

Abstract

The benthic invertebrates of the banks and main channel at 10 sites along 100 km of the lower reach of the La Trobe River, which flows entirely through agricultural and industrial areas, were quantitatively sampled every two months between May 1979 and March 1981; 23 chemical variables were measured concurrently. In all, 337 taxa were collected. Normal and inverse classifications of the faunal data with two similarity indices (Czekanowski, Canberra Metric) indicated that the uppermost two sites (upstream of the industrial areas) with a rich fauna were clearly distinct from the more depauperate downstream sites; these latter sites could be divided into two groups (main channel samples) or four groups (bank samples). In both habitats, eight groups of common (>0.5% of total numbers) taxa were evident: in each habitat, there were one or two groups of resistant taxa abundant at all sites (mostly Chironomidae), one or two groups of opportunistic taxa common at disturbed sites only (Caenidae, Corbiculidae, Ecnomidae, Chironomidae), and two groups of sensitive taxa that were most abundant at the two uppermost sites (Leptophlebiidae, Baetidae, Ecnomidae, Elmidae, Helodidae, Chironomidae, Ceratopogonidae, Hydracanna). Oligochaeta were abundant at all sites and as a group were considered resistant. Multiple discriminant analysis of the previously established site groups with 17 of the chemical variables indicated that high values for conductivity distinguished the most downstream site groups, which had the poorest fauna; the effect of this factor on the fauna was apparently indirect. The analysis also indicated that a decrease in suspended solids at site groups on an impounded section and an increase in nutrients at a site immediately downstream of the input of treated sewage were associated with changes in the fauna. The release of heated water (<25ºC) from a power station at one of the sites on the impounded section had little effect on the fauna.