A NEW METHOD TO ESTABLISH SCORING CRITERIA OF THE INDEX OF BIOTIC INTEGRITY

Karr's Index of Biotic Integrity (IBI) was applied in north central Io wa streams to examine the technique's utility for measuring environmen tal quality in agriculturally-influenced streams and to develop method ological improvement. During 1988 and 1989, fish were collected in the Boone River and Lizard Creek systems, and physical and chemical measu rements of stream environmental conditions were taken. A modified two- step procedure was proposed for establishing the scoring standard of t he 12 IBI metrics. First, a simple linear regression relationship of t he metric variable on log drainage area was calculated (p = 0.1). If a significant correlation was found, a Maximum-Species-Richness Line (M SRL) was determined by calculating the 95% Y-intercept as its intercep t, and then plotted by the original regression slope. If a nonsignific ant relationship was obtained, the upper and lower 5% data points were deleted to define the upper and lower boundaries of the remaining poi nts with horizontal lines. Finally, the area below the calculated 95% MSRL and the area between the two horizontal lines were equally trisec ted into above average (5 point), average (3 point), and below average (1 point) categories. After the scoring criteria were established, th e IBI value of each sampling site in Boone River and Lizard Creek syst ems was calculated. The new scoring procedure resolved the problem of the commonly adopted ''eye-fit'' method through generating only a sing le MSRL for a given data set. In Lizard Creek, IBI results were genera lly consistent with evaluations of physical and chemical environmental quality. During the drought period, variations of IBI scores reflecte d the dynamics of environmental conditions in the sampling sites. Addi tionally, the IBI showed greater temporal consistency than another com monly used analytical tool, the Shannon-Weiner Diversity Index. Thus, the new scoring procedure seems to advance the effectiveness of IBI in assessing the environment quality of lotic waters. Numerical species metrics associated with species diversity were more important than pro portional ones in contributing to the IBI. Drought conditions througho ut the study period influenced some of the data and analytical results .