Derivation of a biological quality index for river sites: Comparison of the observed with the expected fauna

A method for the national assessment of the biological quality of river sites is developed. Multivariate discrimination, based on site environmental characteristics, is used on a biological classification of reference sites to derive a procedure to predict the fauna to be expected in the absence of environmental stress. Various quality indices, based on a comparison of the observed with the expected fauna, are proposed. The sizes of the various sources of error and variation, and their effects on the rates of misclassification to quality bands, are examined.