Stream Health Index for the Puget Sound Lowland

The NRCSE was established in 1996 at the University of Washington. Center activities include workshops and other research activities in areas of statistical methodology related to scientific problems in the environmental and ecological sciences. SUMMARY Stream health is often measured by the multimetric benthic index of biotic integrity (B-IBI). For Puget Sound Lowland (PSL) streams, the B-IBI comprises ten metrics which quantify the well-being of benthic inhabitants of the stream. Each metric is converted to a score of 1, 3, or 5, where a higher value indicates a healthier stream with respect to the metric. Summing the metric scores yields the B-IBI. Stream health is then rated as very poor, poor, fair, good, or excellent according to the index value. Different metrics may be measured on different scales. A scoring scheme is therefore required to standardize values across metrics. Conventional scoring for the B-IBI metrics requires subjective and space/time-dependent input on the cutoff points for the discretization of each metric. In contrast, simple statistical standardization (centering and division by the standard deviation) appears to be more natural, is non-study-specific, and maps the metric space onto a continuous scale centered at 0. Comprising the same ten metrics of the PSL B-IBI, our stream health index for the Puget Sound Lowland (SHIPSL) is the sum of all statistically standardized scores. We use benthic taxonomy data on field samples from 1997 to develop a SHIPSL-based rating scheme of grades A to F for stream health. Bootstrap simulations are used to compare the performance of SHIPSL to B-IBI in reflecting PSL stream health. Results show that SHIPSL is a more efficient measure of stream health. Without sacrificing information on biotic integrity, SHIPSL reduces bias and variability of the health index, and eliminates sensitivity of the rating to slight changes in the metric scoring scheme.