Modeling natural environmental gradients improves the accuracy and precision of diatom-based indicators

Abstract

AbstractDiatom-based indicators can contribute significantly to comprehensive assessments of stream biological conditions. We used modeling to develop, evaluate, and compare 2 types of diatom-based indicators for Idaho streams: an observed/expected (O/E) ratio of taxon loss derived from a model similar to the River InVertebrate Prediction And Classification System (RIVPACS) and a multimetric index (MMI). Modeling the effects of natural environmental gradients on assemblage composition is a key component of RIVPACS, but modeling has seldom been used for MMI development. Diatom assemblage structure varied substantially among reference-site samples, but neither ecoregion nor bioregion accounted for a significant portion of that variation. Therefore, we used Classification and Regression Trees (CART) to model the variation of individual metrics with natural gradients. For both CART and RIVPACS modeling, we restricted predictors to natural variables unaffected by or resistant to human disturbances. On average,...