IN-STREAM HABITAT UNIT CLASSIFICATION - INADEQUACIES FOR MONITORING AND SOME CONSEQUENCES FOR MANAGEMENT

Habitat unit classification can be a useful descriptive tool in hierar chical stream classification. However, a critical evaluation reveals t hat it is applied inappropriately when used to quantify aquatic habita t or channel morphology in an attempt to monitor the response of indiv idual streams to human activities. First, due to the subjectivity of t he measure, observer bias seriously compromises repeatability, precisi on, and transferability of the method. Second, important geomorphic an d ecological changes in stream habitats are not always manifested as c hanges in habitat-unit frequency or characteristics. Third, classifica tion data are nominal, which can intrinsically limit their amenability to statistical analysis. Finally, using the frequency of specific hab itat unit types (e.g., pool/riffle ratio or percent pool) as a respons e variable for stream monitoring commonly leads to the establishment o f management thresholds or targets for habitat-unit types. This, in tu rn, encourages managers to focus on direct manipulation or replacement of habitat structures while neglecting long-term maintenance or re-es tablishment of habitat-forming biophysical processes. Stream habitat m anagers and scientists should only use habitat unit classification to descriptively stratify in-stream conditions. They should not use habit at unit classification as a means of quantifying and monitoring aquati c habitat and channel morphology. Monitoring must instead focus on dir ect, repeatable, cost-efficient, and quantitative measures of selected physical, chemical, and biological components and processes spanning several scales of resolution.