An Improved Biotic Index of Organic Stream Pollution

Major improvements were made in using a biotic index of the arthropod fauna to evaluate organic stream pollution. All tolerance values were reevaluated, many were changed, and the scale for tolerance values was expanded to 0-10 to provide greatcr precision. Keys to larvae of Ceratopsyche have been developed and tolerance values for species in this important genus are provided. Sorting of samples in the laboratory instead of in the field is recommended, and directions for processing and evaluating samples are included. A "saprobic index" (Pantel and Buck 1955) and a "biotic indcx" (Chutter 1972) werc proposed for evaluating the water quality of streams through a study of their fauna. I introduced a similar biotic index (Hilsenhoff 1977) that used only arthropods for evaluation, thus simplifying collecting, sorting, and identification. It was based on a sample of 100 or more arthropods collected from a riffle area. This index is a measure of organic and nutrient pollution. which causes lower dissolved oxygen levels, cspecially at night during thc summer and after hcavy rain. Lowered levels of dissolved oxygen in turn affect the ability of each specics of arthropod to survive in a particular stream. For the purpose of calculating the biotic index, every species or genus was assigned a tolerance value of 0-5, with 0 assigned to species most intolerant of organic pollution and 5 assigned to the most tolerant species. Intermediate values were assigned to species intermediate in their tolerance of organic pollution. The biotic index is an average of tolerance values for all individuals collected from a site. Initially the index was used mostly in Wisconsin. In 1979 and 1980 the Wisconsin Department of Natural Resources (DNR), in cooperation with the University of Wiscon­ sin, used the index to evaluate more than 1000 stream sites in spring and autumn. Personnel in my laboratory identified all the arthropods and calculated biotic index values for all sites. Experience from this cooperative study and several other studies enabled me to publish new recommendations for using the biotic index, revised tolerance values, and regional keys to species in important genera (Hilsenhoff 1982). Recently, additional improvements have been made in the biotic index. Most important are an expansion of the scale of tolerance values to 0-10 to provide greater precision, a reevaluation of all tolerance values, and inclusion of tolerance values for many additional species. Procedures for sampling and sorting were updated, and a discovery that Simulium vittatum is really two genetically distinct species (Rothfels and Featherston 1981) with differing ecological requirements (Adler and Kim 1984) altered my recommendation for dealing with these sibling species. Adequate correction factors for current, temperature, IRe search supported by the College of Agricultural and Life Sciences, University of Wiscon­ sin-Madison, and by Hatch Research Project 2785. 2Department of Entomology, University of Wisconsin, Madison, WI 53706. 1 Hilsenhoff: An Improved Biotic Index of Organic Stream Pollution Published by ValpoScholar, 1987 32 THE GREAT LAKES ENTOMOLOGIST Vol. 20, No.1 and seasonal differences are needed, and studies to correct these deficiencies will be completed soon. REASSIGNMENT OF TOLERANCE VALUES Initial tolerance values for each species were based mostly on a study of 53 Wisconsin streams in which physical and chemical parameters were evaluated to determine the degree of organic and nutrient pollution in each stream (Hilsenhoff 1977). Tolerance values for species not found or occurring rarely in these streams were based on their occurrence in other streams and their association with species to which tolerance values had been asigned. Data from more than 2000 samples collected in the 1979-80 cooperative study with the DNR were used to reevaluate tolerance values by comparing the tolerance value of each species with the average biotic index value of streams in which that species most commonly occurred. A description of the procedure that was used ean be obtained from the author. It became apparent that intermediate values would increase precision, so the 0-5 scale of tolerance values was expanded to 0-10 to accommodate intermediate values while retaining whole numbers for ease in making calculations. New tolerance values assigned to 359 species or genera found in the DNR samples are listed in Appendix I. Forty-nine additional species that were not collected from the 1979-80 study streams, mostly because their life cycles precluded their being present in spring or fall samples, were assigned tolerance values based on our knowledge of streams in which they occurred. Previous experience was also used to adjust tolerance values of nine species that were found in less than five samples from the study streams, and subsequent experience resulted in adjusting values of Asellus, Crangonyx and Hyallela.