Interpreting Stream Physical Characteristics in Index of Biotic Integrity Classifications: General Similarities and Specific Differences

Abstract

AbstractFish assemblage‐based indices of biotic integrity (IBIs) are commonly used by many states. These IBIs contain a core of common metrics and a few metrics that are specific to the local environmental setting and fish assemblage. This implies a consistency of responses as degradation increases, but the responses are modified by the local environmental setting. These implications lead to two hypotheses: (1) stream physical characteristics, such as substrate composition, geomorphologic composition, and hydraulic characteristics, will differ among integrity classes regardless of geographic location or environmental setting and (2) the differences in stream physical characteristics among integrity classes change due to the environmental setting. To test these hypotheses, fish assemblages and stream physical characteristics were examined across a priori defined degradation gradients in four diverse environmental settings by comparing species traits (preferences for substrate, preferences for geomorphic channel units, and locomotion morphology) and stream physical characteristics (substrate composition, geomorphic channel unit abundance, and stream hydraulic characteristics) among three IBI classes (high, moderate, and low). Across all environmental settings combined, Euclidean distances of species traits normalized to local reference conditions indicated differences in substrate preferences among each of the three integrity classes: substrate particle size preferences differed among all three integrity classes, geomorphic channel unit preference differed between the high‐integrity sites and the other sites, and locomotion morphology differed between low‐integrity sites and the other sites. These results indicate that degradation occurs in each of the three physical stream characteristics regardless of environmental setting. Within the individual environmental settings, there were some specific differences in the changes in substrate preferences, geomorphic unit preference, and locomotion morphology among integrity classes. Within individual environmental settings, differences in stream physical characteristics among integrity classes generally reflected the same differences as indicated by differences in fish assemblage composition.