Benthic macroinvertebrate communities were studied on igarapes Saraca, Carana, Agua Fria and Trombetas River on Porto Trombetas, Municipality of Oriximina, Para, Brazil (1 degree 25' and 1 deg 35'South and 56 deg 15' and 56 deg 25'West). The main objective was to investigate the taxonomic structure, composition and seasonal dynamic of benthic macroinvertebrates, with emphasis on the occurence and seasonal dynamics of benthic macroinvertebrates. Special attention was given to the occurence and seasonal distribution of Chironomidae (Diptera, Insecta) benthic larvae on rainy (May-June) and dry seasons (October-November) of 1994 and 1995, on three sample stations in each ecosystem, relating with ecological modifications of the activities of a bauxite mine. The highest taxonomic richness organisms (Chironomidae and Oligochaeta) were found on igarape Saraca, in the area affected by the dam. During the rainy season macrobenthic organisms were not found on impacted sample station of igarape Carana on both years, where just few larvae of POLYPEDILUM and CHIRONOMUS were present on dry period. On igarape Agua Fria where the negative influence of the bauxite tailling is increasing gradually, Chaoboridae (Diptera, Insecta) larvae dominated the benthic macrofauna. Moreover it was observed that the degradation of ecological conditions of this ecosystem will probably continue because the transporte of fine particules of clay by the local corrent. On the Trombetas River, discharges of mineral occur accidentally and are deposited on the sediment. The data obtained
showed that the mining activities made physical modifications (hydrodinamic changes by the dam formed by construction of a railroad, presence of bauxite tailing and/or bauxite mineral on the sediment, modifying the granulometric composition), chemical modifications (concentrations of available phosphorus, total nitrogen and organic carbon on sediment) and biological modifications (taxonomic structure and distribution of organisms). It was evidenced the possibility of using benthic macroinvertebrates as indicators of ecological conditions of these ecosystems pointing out the temporal (rainy and dry seasons of 1994 and 1995) and spatial changes (natural sample stations and impacted sample stations influenced by the activities of bauxite mining).