A macroinvertebrate multimetric index to evaluate the biological condition of streams in the Central Amazon region of Brazil

Managers, researchers and technicians involved in the conservation and management of water resources in the Central Amazonia need a robust tool to assess biological quality in aquatic ecosystems. To provide such a tool, we developed a multimetric index based on stream macroinvertebrate data. We collected samples from eight reference (undisturbed) streams and 12 streams altered by deforestation and domestic sewage during two distinct seasons (dry and rainy) in the municipality of Manaus, Brazil. Metric candidates to compose the index were tested for: Range, temporal variability (stability), sensitivity in separating disturbed from reference streams, correlation with the anthropogenic disturbance gradient and natural stream variability and redundancy between metrics. Our final index included seven metrics: family, Trichoptera and Ephemeroptera–Plecoptera–Trichoptera (EPT) richness as richness measures, EPT percent abundance as a measure of composition, EPT/Chironomidae ratio and sensitive-taxa richness as tolerance measures and percent abundances of gathering-collectors and shredders as trophic measures. All metrics were scored relative to their range quartiles. The final index, derived from the sum of all metric scores (0–70), was divided into five sub-ranges to represent distinct levels of biological quality in streams (bad, poor, regular, good and excellent).