ABSTRACT

Multi-metric indices (MMIs) are a measure of a combination of characteristics of biological communities and are used as indicators of water quality and ecological health. Although MMIs for algal communities have been developed for specific regions of the United States, none of the indices have national applicability. The MMIs described in this report were developed by the National Water-Quality Assessment Program of the U.S. Geological Survey to assess the overall health of benthic algal communities in U.S. streams and rivers within five geographic regions that encompass the conterminous United States.

The traditional procedure for developing MMIs (also referred to as indices of biological integrity) is to select individual metrics that, separately, can distinguish between undisturbed sites (selected for this study as reference sites) and predetermined disturbed sites. The metrics are then combined into a single index. In addition to traditional approaches for selecting individual metrics, the current study used stepwise logistic regressions to select sets of metrics that best predicted whether sites were in an undisturbed or a disturbed condition. Multi-metric indices and logistic regression models were developed for five regions of the United States using calibration datasets and were evaluated using independent validation datasets. Applying the regional MMIs to validation sites, the percentage of sites correctly classified as “reference” or “disturbed” ranged from 66 to 92 percent. Most often, only two or three metrics, typically percentages of individual organisms belonging to diatom taxa indicative of reference or disturbed calibration sites, were needed to distinguish between reference and disturbed validation sites. The classification accuracy of these indices did not improve when non-diatom metrics based on the autecological characteristics of algal taxa were included. The autecological metrics that were used to differentiate reference and disturbed sites were based on the assignment of the same diatom taxa to various ecological categories and therefore were redundant. Individual autecological metrics that measure the effect of specific stressors are needed, however, to identify potential causes of impairment. The regional MMIs developed in this study were applied at National Water-Quality Assessment Program sampling sites to assess the overall biological health of algal communities. The assessment of algal communities in urban and agricultural land-use settings indicated increased stress to ecological health when compared to communities in other land-use settings.

First posted June 27, 2011