Following guidelines for establishing the size and location of the sampling reach (Meador, Hupp, and others, 1993) should lead to the inclusion of the majority of instream habitat types typical of the stream at a given location. Rarely will all 51 instre am habitat types be present. Consequently, it is a good practice to use the instream habitat type matrix during site reconnaissance to determine which habitats are present and what type of equipment is needed to sample each one. Because qualitative sampling involves all instream habitat types that are present and accessible within the sampling reach, no guidance is required for selecting one habitat type over another, as contrasted with semi-quantitative sampling. However, the types of habitats present and their abundance within the sampling reach are important in determining the proper approach to qualitative multihabitat (QMH) sampling.
Because of the complexity and uncertainties associated with reach-based proportional qualitative sampling, the NAWQA Program limits the objective of qualitative sampling to the development of a taxonomic list of organisms present within the sampling reach . Consequently, roughly equivalent effort (for example, time spent sampling, number of samples collected, or area sampled) is put into sampling each habitat type. This approach maximizes the representation of small habitats and prevents the over-represe ntation of areally extensive habitat types. The instream habitat types sampled and the sampling gear are recorded on the qualitative-sampling field data sheet.
The selection of appropriate instream habitat types for semi-quantitative sampling requires a considerable amount of guidance to determine which of the many habitat types present within a sampling reach are sampled. Semi-quantitative samples supplement q ualitative samples by providing data on the presence and relative abundance of invertebrates in two contrasting instream response habitats: (1) a habitat that supports, in the absence of human influences, the richest assemblage of invertebrates within the sampling reach (for example, a riffle in shallow, coarse-grained, high-gradient streams; or snags in sandy-bottomed, Coastal Plain streams); and (2) a habitat where organisms are most likely to be exposed to sediment-borne contaminants for extended periods of time, typically a depositional area such as a pool where particulate-borne contaminants tend to accumulate.
Characterizing communities in these two contrasting instream habitat types is important because these habitat types differ widely in the duration and pathways of contaminant exposure and potential for species loss. The species-rich habitat is expected to be highly sensitive to water-quality changes because it can support a diverse assemblage of species that display a wide range of sensitivities to water-quality changes. Therefore, the community in this habitat has the potential for greater change than less species-rich communities. However, species-rich habitats typically occur in erosional areas and may not be sites where exposure to sediment-borne contaminants, in either concentration or duration, is greatest.
The fauna of depositional areas are of interest because, even though they are generally less rich taxonomically, they are probably exposed to greater concentrations of sediment-borne contaminants for longer periods of time than the fauna of erosional areas. Consequently, communities in depositional areas may respond to contaminants before the