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 limi ting to ten the number of individuals in each taxon used in its calcul ation. This reduces detrimental effects on the index of certain fairly tolerant tars in clean streams, effects of a few tolerant tars in som ewhat polluted streams, and the effect of our inability to identify la rvae in some insect genera to species. It also greatly reduces seasona l variability, allowing use of the biotic index throughout the year wi th only a minimal decrease in the sensitivity of the index during the summer months. The EPT index was highly variable and exhibited seasona l variation in most of the streams.