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habitats for semi-quantitative sampling should be investigated prior to committing to the use of artificial substrates for semi-quantitative sampling of the richest-targeted habitat.

All nets and screens used in the collection of semi-quantitative samples must have a mesh size of 425 Ám (USA Standard Testing Sieve, ASTM number 40 or Tyler equivalent 35 mesh). Samples are elutriated, sieved, and split in the field to reduce the bulk o f the composite sample to less than 0.75 L. Samples collected and processed in this manner are placed in appropriate sample containers, preserved in 10-percent formalin, and properly labeled.

Recommendations for Sampling Benthic Invertebrates

Specific, nationally consistent recommendations on which habitat type to select for semi-quantitative sampling in each study unit are difficult to devise, particularly for selection of the RTH. The instream habitat types presented in figure 2 represent a generalized classification scheme, broadly based on habitat features of relevance to benthic invertebrates, that can be used for local, regional, and national aggregation of data. However, this classification scheme cannot account for local factors, such as substrate condition, depth, current velocity, hydrologic management, and accessibility, that may make one habitat type more suitable under one set of site conditions than another. For example, in large rivers with unstable erosional zones that lack snags or macrophyte beds, pool habitats may represent the faunistically richest habitat as well as the habitat where exposure to particle-borne contaminants is greatest. Under these circumstances, pool habitats are sampled as the RTH and a contrasting ha bitat type is chosen as the DTH. The selection of the appropriate instream habitat type for RTH and DTH sampling is based on national guidance supplemented with information derived during the retrospective data analysis, input from the study-unit liaison committee, consultation with the regional biologists and North Carolina Ecology Group, and reconnaissance sampling.

National guidance on choosing appropriate instream habitat types (fig. 4) is provided by prioritizing the elements within each level of the hierarchy used to define the matrix of instream habitat types. Characteristics of the hierarchy are ranked from hi ghest priority (1) to lowest priority (6). Based on these rankings, the highest priority for RTH sampling is a riffle, main-channel, natural-bed instream habitat type, whereas the lowest priority is given to a pool, island-margin, slough instream habitat type. Similarly, the highest priority for DTH sampling is a pool, main-channel, natural-bed instream habitat type, and the lowest priority is given to a riffle, island-margin, manufactured-bed instream habitat type. These priorities apply only within a level of the hierarchy, such as major channel feature, and not across levels, such as comparing priority levels for channel margins with woody snags. The study-unit biologist determines which of the instream habitat types present in the sampling reach b est meets the objectives of semi-quantitative sampling with regard to local conditions and available resources. The guidance provided in figure 4 represents a starting point for determining the appropriate habitat type to sample within a study unit.