analyze the building blocks of aquatic communities; and (2) those that measure "functional" integrity, meaning tools that assess processes occurring in aquatic communities (Cairns, 1977a,b; Karr, 1991, 1993). Collectively, these structural and functional measures help restore a biological focus in water quality measurements.

An unusual change in one or more structural characteristics is interpreted as evidence of ecological stress. Impairment of biological structure of aquatic communities may be indicated by the absence of pollution-sensitive taxa, dominance by any particular taxon and low overall taxa richness, or changes in community composition relative to the reference condition (Plafkin et al., 1989).

Fundamental measurements of ecosystem structure are (1) the number of species or other taxonomic units present, (2) the number (or mass) of individuals per species, and (3) the particular kinds of species present. Historically, aquatic ecologists have done a fairly good job of measuring structural aspects, although they are hampered by the inability to identify all of the species in a biotic community because of the lack of accurate, up-to-date taxonomic keys and comprehensive systematic treatises at the species level, particularly those of a regional nature. A still greater problem is the limited availability of qualified biologists capable of using even the existing taxonomic keys.

The particular structural attributes analyzed to determine biological integrity vary with the level of developmental complexity (plants, invertebrates, fish) and ecological organization (population, community, ecosystem) of the focal organism or group (e.g., Johnson et al., 1993). The most commonly used groups for determining structural responses vary with the type of aquatic habitat under investigation and the scale appropriate to the question being addressed. In lentic waters, microscopic algae and invertebrates (particularly rotifers, cladocerans, and copepods) are most commonly emphasized. These may be separated further into those suspended in the water (i.e., plankton) and those growing attached to natural or introduced substrata (i.e., aufwuchs on sticks or microscope slides, respectively). In lotic waters, the most commonly studied groups are attached algae (periphyton), macroscopic invertebrates living on the stream bottom (benthos), and fish.

The suitability of a particular group is determined by the specific environmental stressor, the generation time of the organisms, and their mobility and dispersal capabilities. For example, fish may be better indicators of physical habitat conditions in streams and benthic macroinvertebrates more indicative of water quality conditions (C. Yoder, personal communication, 1994). Algae have generation times of hours or days, macroinvertebrates