Benthic macroinvertebrates as indicators of ecological fragility of small Rivers ('Igarapes') in a bauxite mining region of Brazilian Amazonia

Benthic macroinvertebrate communities were studied in the igarapes Saraca, Carana and Agua Fria near Porto Trombetas, Municipality of Oriximina State of Para, Brazil (1 degrees 25' to 1 degrees 35'S and 56 degrees 15' to 56 d egrees W). The main objective was to investigate the potential use of the b enthic macroinvertebrates community as indicators of the ecological fragili ty of the igarapes located in an area of the Amazon basin influenced by bau xite mining. These aquatic ecosystems have low pH (< 4.5); low electrical c onductivity (< 90 mu S cm(-1)); low or undetectable total alkalinity; low n utrient concentrations in the sediment, which implies low primary productiv ity; low species richness; low secondary productivity; and low rates of org anic matter decomposition. The benthic macroinvertebrates utilize mainly al lochthonous organic matter. Some areas of the ecosystems investigated have been modified by bauxite tailings (especially in their sediment granulometr y), by a dam formed by railroad construction (which altered the hydrodynami cs and eliminated the terra firms vegetation), and by accidental discharges of various minerals from the railroad traffic and bauxite mining. These fa ctors have drastically modified the local benthic macroinvertebrate communi ties. The value of benthic macroinvertebrates as indicators of ecological f ragility, the health of the ecosystem investigated, and water quality is ev aluated.