Long-term changes in the aquatic macroinvertebrates of the upper South Esk River, Tasmania, with an emphasis on the mayflies (Ephemeroptera)

Context There are few long-term studies of the effects of land-use changes and climate change on Australia’s aquatic biota. In this study, the recent macroinvertebrate assemblages at three sites (one historically polluted by mine effluent and two ‘controls’ upstream of it) on the upper South Esk River (Tasmania) are compared with detailed surveys of the same sites conducted 50 years earlier. Aims To assess possble long-term changes in a south-east Australian river’s aquatic biota in the face of changing climate and land use patterns. Methods ‘Kick sampling’ was used to sample benthic aquatic animals over a 3-year period, which were sorted to the family level for most taxa and the genus level for mayflies (Ephemeroptera) and the results then compared to a similar data set collected for the same sites in the mid-1970s. Key results At the two control sites, macroinvertebrate densities, as well as a biotic index of water quality, have approximately halved relative to the 1970s, although with increased abundances of dipterans and oligochaetes and fewer gastropods, trichopterans and acarids. The densities of the leptophlebiid mayfly Austrophlebioides, previously the most abundant mayfly genus present and an apparent indicator of clean water, have collapsed and have largely been replaced by the environmentally more tolerant Atalophlebia. Macroinvertebrate densities at the previously polluted site remain low, suggesting continuing long-term effects of metal pollution. Conclusions The analyses overall suggest a shift from a biotically diverse and ‘clean, cold’ river in the 1970s to one that is increasingly disturbed. Implications The source of the system stresses is unclear, given mixed land use in the catchment, but observations suggest that erosional siltation could be a contributing factor.