The systematic use of biological responses to evaluate changes in the environment with the intent to use this information in a water quality control program is defined as biological assessment (Matthews et al. 1982). The biological response is measured by using biological indicators, and river ecosystems were one of the first where they were used as an alternative or a complement of assessment systems based on physicochemical indicators. In the first decades of the twentieth century biological assessment of rivers mostly used simple techniques related to organic waste pollution (Hellawell 1978). This approach was used in the early 1900s by German aquatic ecologists in the development of the Saprobic indices to assess the effect of organic 448pollution on streams (Kolkwitz and Marsson 1902), and it was also applied in the United States (Forbes and Richardson 1913; Ellis 1937). In the last three decades of the twentieth century a number of approaches were developed to evaluate the ecological effects of stress on stream ecosystems using organisms (Descy 1979; Karr 1981; Armitage et al. 1983; Johnson et al. 1993).