Benthic macroinvertebrates as indicators of water quality of southern Chile rivers

The Damas River Hydrographic Basin (40degrees39'S, 72degrees23' W) presents an intense agricultural and cattle ranching activities in 78.2 % of its surface, which has resulted in important inputs of nutrients into the aquatic ecosystem. The objectives of this study were: (a) to analyze the spatial distribution of the benthic macrofauna in the Damas basin, and (b) to determine the feasibility of using the Families Biotic Index (FBI) for the evaluation of water quality. The samplings were conducted in January 1998 using a Surber net (0.09 m(2)) at 15 sites, distributed along the main course of the river and its tributaries. A total of 77 taxa was recorded, being the most diverse Plecoptera (16 %), Trichoptera (16 %), Diptera (14 %) and Ephemeroptera (12 %). A marked tendency to a decrease of specific richness was observed from the head of the basin towards downstream. The abundance, biomass and FBI presented an inverse trend to the species richness. The FBI was significantly and positively correlated with total phosphorous (r(2) = 0.71), temperature (r(2) = 0.66), nitrite (r(2) = 0.56), electrical conductivity (r(2) = 0.50), biological oxygen demand (r(2) = 0.46) and total nitrogen (r(2) = 0.46) and negatively correlated with dissolved oxygen (r(2) = 0.53). All of these variables are closely associated with the intense use of fertilizers in the basin. The results obtained suggest that the FBI is an appropriate indicator for determining the water quality of rivers of agricultural and cattle ranching basins in southern Chile.