Community structure of the macroinvertebrate fauna and water quality of a saline river system in south-western Australia

Many streams and rivers in the south-west of Western Australia have marginal salinities and only about half of the divertible surface water is fresh (<3 g l-1). Although salinization is recognised as the greatest threat to water resources throughout Australia, little is known of its effect on the biota. This paper gives the first quantitative description of the macroinvertebrate fauna of a salinized river system in southwestern Australia.Two sites on an intermittent stream (Thirty-four Mile Brook) and two sites on a perennial river (Hotham River), above and below the confluence with the above tributary, were sampled on three occasions for benthic macroinvertebrates. Only 68 taxa were recorded in quantitative samples from the sites; a further 11 taxa were recorded in qualitative samples from these and two additional sites on the tributary. The river system shared several faunal elements with athalassic salt lakes across southern Australia. Crustaceans, particularly the amphipod Austrochiltonia subtenuis and several species of ostracods, were numerically dominant at all sites. In contrast to the benthic fauna of most lotic systems, insects formed only a small proportion of the individuals collected from the Hotham River. Chironomids were the dominant insect group, both in terms of the number of species and individuals, particularly at one of the tributary sites where species of Tanytarsus comprised almost half of the total number of individuals. Overall, the benthic fauna was characterised by very high densities and low richness, diversity and evenness. This is likely to be a direct consequence of the poor water quality of the system where high salinities, often greater than 3 g l-1, were recorded.Classification and ordination revealed major differences in community structure of the benthic fauna between the Hotham River and its tributary. This was attributed to differences in the physical nature of the two streams, particularly substrate characteristics and stream permanence, rather than differences in water quality. Temporal differences in community structure were also apparent, but were more obvious in the Hotham River than in the tributary.