A Modification of the Biotic Index of Organic Stream Pollution to Remedy Problems and Permit Its Use Throughout the Year

The biotic index of the arthropod fauna of streams is modified by limiting to ten the number of individuals in each taxon used in its calculation. This redu~s detrimental effects on the index of certain fairly olerant taxa in clean streams, effects of a few tolerant taxa in somewhat polluted streams, and the effect of our inability to identify larvae n some insect genera o species. It also greatly reduces seasonal variability, allowing use of the biotic index throughout the year with only a minimal decrease in the sensitivity of the index during the summer months. The EPl' index was highly variable and exhibited seasonal variation in most of the streams. Because previous experience suggested that stream arthropod communi­ ties can be readily recognized in the field by their ominant genera, I initi­ ated a study in June 1972 to develop a rapid, objective method for evaluating water quality by relating it to the arthropod community structure as recog­ nized by dominant genera (Hilsenhoff 1977). Twenty-nine Wisconsin stream that were presumed to be undisturbed by human activities were sampled in late June, early September, and November, 1972, and early May, 1973. Twenty-four streams with known sources of pollution were similarly sampled from June 1973 through May 1974. These 53 diverse streams were selected to be representative of streams throughout Wisconsin. Samples were col­ lected with a D-f.rame net from riffles and with artificial substrate samplers (Hilsenhoff 1969) from runs or deep rimes.