mostly descriptive and autecological through the first half of the twentieth century, focusing on the distribution, abundance, and taxonomy of stream organisms (Cummins et al., 1995). The development of stream ecology as an independent discipline is fairly recent (occurring mostly over the past 25 years) and is an outgrowth of various initiatives (described below) that began in the 1950s and 1960s. Hynes' (1970) classic book The Ecology of Running Waters may be regarded as the first book on stream ecology. Even today, many stream ecologists identify more with the discipline of ecology than with limnology.

Paradigms of Subdisciplines Within Stream Limnology

Organizing paradigms have been part of all the scientific disciplines that collectively constitute the field of stream limnology for many decades, as illustrated by the following examples.

The concept of the watershed as the basic unit in hydrology dates back at least to the 1920s (Horton, 1931; Platt, 1993), and the watershed perspective has been used both in organizing hydrologic concepts and in data collection. Classification of flowing waters according to stream order, based on their relative position in the typically dendritic (treelike) or hierarchial network in which streams are connected in a large river basin, is a long-standing organizing principle in both hydrology and geomorphology. Chaos theory and the concept of fractal dimensions have stimulated much recent research in stream geomorphology.

The modeling of streams as plug-flow reactors has been an organizing paradigm of sanitary and environmental engineering since the development of the first water quality model—Streeter and Phelps' 1925 model for dissolved oxygen concentrations in streams receiving point-source discharges of sewage.

The use of benthic invertebrates as indicator organisms for organic pollution and the division of streams into zones of pollution and recovery based on the presence of indicator species or groups of organisms have been major driving forces in stream biology almost since the inception of the field. Numerous classification schemes were developed under this organizing principle, starting with the European "Saprobien" system near the beginning of the twentieth century (Kolkwitz and Marsson, 1908, 1909). This paradigm stimulated much of the biological research on the structure of stream communities through the middle of this century. It also can be considered a precursor to broader indicator or classification systems such as Karr's IBI (index of biological integrity) (Karr et al., 1986, 1987) and other indices of biodiversity and biological integrity (Karr, 1991), which are currently popular topics in stream ecology.

Ecological energetics also has been an organizing concept for research in stream ecosystems for almost 40 years—since H. T. Odum's classic



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