Benthic macro-invertebrates as indicators of ecological fragility of small rivers ('igarapés') in a bauxite mining region of Brazilian Amazonia

Benthic macroinvertebrate communities were studied in the igarapés Saracá, Caranã and Água Fria near Porto Trombetas, Municipality of Oriximiná, State of Pará, Brazil (1°25' to 1°35'S and 56°15' to 56°W). The main objective was to investigate the potential use of the benthic macroinvertebrates community as indicators of the ecological fragility of the igarapés located in an area of the Amazon basin influenced by bauxite mining. These aquatic ecosystems have low pH (< 4.5); low electrical conductivity (< 90 µS cm-1); low or undetectable total alkalinity; low nutrient concentrations in the sediment, which implies low primary productivity; low species richness; low secondary productivity; and low rates of organic matter decomposition. The benthic macroinvertebrates utilize mainly allochthonous organic matter. Some areas of the ecosystems investigated have been modified by bauxite tailings (especially in their sediment granulometry), by a dam formed by railroad construction (which altered the hydrodynamics and eliminated the terra firma vegetation), and by accidental discharges of various minerals from the railroad traffic and bauxite mining. These factors have drastically modified the local benthic macroinvertebrate communities. The value of benthic macroinvertebrates as indicators of ecological fragility, the health of the ecosystem investigated, and water quality is evaluated