Catastrophic disturbances to stream ecosystems: volcanism and clear-cut logging

Clear-cutting and fire are major forms of dis-turbance presently affecting streams in forestedlands. These events have recurrence intervalson the order of decades to a few centuries.Repeated volcanic eruptions also occur in tec-tonically active zones such as the Pacific North-west on time scales of centuries to millenia.Catastrophic disturbances, such as volcanism orclear-cut logging, can affect stream ecosystemsin a variety of ways. Historically, research onthe impacts of disturbances has been conductedon the basis of their on-site short-term impactrather than within the context of integrated,basin-level analyses. Even local impacts havenot been adequately assessed from a streamecosystem perspective. In most cases, physicaland chemical attributes of water and some bioticparameters are narrowly emphasized, while oth-er important determinants of the stream ecosys-tem are neglected.
Two of the most neglected components instream ecosystem research are physical habitatand microbial processes within the sediment. Inthis paper, we outline the importance of physicalhabitat and how it is altered by catastrophicdisturbance. The interaction of organic materialswith physical habitat determines the kinds andrates of microbial processes in streams. We willconfine our examples to the Pacific Northwestof the United States. However, these principlesapply generally to forested streams everywhere.Our goal is to present a context for sampling andinterpreting microbial activity in old-growth, re-cently clear-cut, and volcanically impacted wa-tersheds.