Presence and abundance of Ephemeroptera and other sensitive macroinvertebrates in relation with habitat conditions in pampean streams (Buenos Aires, Argentina)*

The objective of this work was to analyse the presence and abundance of Ephemeroptera and other sensitive invertebrates in two streams in the pampean area of Argentina, which have different ecological conditions. Juan Blanco stream is a pristine system and a reference site for the area while Bunirigo stream is affected by industrial effluents coming from food industries and tanneries. Biological and physico-chemical samples were taken seasonally from each stream over two years at two sites (upstream and downstream). DO, conductivity, pH, BOD, COD, heavy metals (Pb, Cr, Cu, Cd, Zn and Hg) and nutrients were measured and biotic indices were applied to determine water quality. Among the Ephemeroptera, Caenis cf. argentina (Caenidae) and Callibaetis cf. fasciatus (Baetidae) were the dominant species with the maximun densities in Juan Blanco stream. Other macroinvertebrates found in the study area and considered sensitive were Magellomyia bruchina (Trichoptera, Limnephilidae), Campsurus major (Ephemeroptera, Polymitarcyidae), Aeshna bonariensis and Micrathyria dydima (Odonata Anisoptera) and Diplodon delodontus delodontus (Pelecypoda, Hyriidae) recorded at low number in Bunirigo downstream. The abundance of individuals for each species can be correlated with water quality variations in the study sites. Certain parameters like pH can influence the distributions patterns of C. cf. fasciatus.