multihabitat samples, together with semi-quantitative samples, are used to develop a detailed list of the taxa present within the sampling reach at the time of collection. Semi-quantitative sampling is used to measure community structure, expressed as relative abundance of each taxon, within standardized instream response habitat types.
Two types of standardized response habitats are sampled semi-quantitatively, a faunistically "richest" (richest-targeted) and a contrasting "depositional" (depositional-targeted) habitat. The "richest" habitat is standardized, within certain limits, on the basis of the anticipated ability of the habitat to support the taxonomically richest benthic invertebrate community within the sampling reach. The "depositional" habitat is standardized as an instream habitat usually characterized by low current velo city and fine-sediment deposition. Typically, habitats selected for richest-targeted habitat sampling are those characterized by coarse substrates and high current velocities. However, in some circumstances, such as in some large rivers, fine-grained, slow-flowing "depositional" habitats may represent the most stable and faunistically "richest" habitats within the sampling reach and are sampled as the richest-targeted habitat. Relevant site information, sampling information, and microhabitat characteristics are recorded on separate field data sheets.
Typically, a single sampling reach at each basic fixed site is sampled once during a NAWQA Program cycle. However, at a subset of basic fixed sites, more intensive sampling is done to assess spatial variability among sampling reaches at a site and short-term temporal variability. Spatial variability is estimated by establishing three sampling reaches at a site and sampling all of them once during 1 year. The spatial variability assessment is usually timed so that the amount of supporting physical and chemical data is maximized. Short-term temporal variability is assessed by choosing one of the three sampling reaches and sampling it once a year for 3 consecutive years. Sites are chosen for the intensive multiple-reach and multiple-year sampling with the intent of encompassing the important sources of variability within the study unit. Such sites include reference conditions, heavily impacted sites, major land uses, and major physiographic areas of the study unit.
Each sampling reach is characterized by collecting and compositing multiple samples for qualitative and semi-quantitative sampling. Qualitative samples are composited with the objective of representing all instream habitat types accessible within the sam pling reach. Semi-quantitative targeted habitats are characterized by collecting and compositing a minimum of five samples from each of the appropriate habitat types. Benthic invertebrates are collected using the sampling method and equipment that are most appropriate for the specific instream habitat type being sampled. Dip nets, kick nets, grabs, seines, and hand-picking of substrates are recommended for obtaining qualitative samples. Slack samplers, Ponar grabs, Surber samplers, large coring devices, and artificial substrate samplers are suggested for obtaining semi-quantitative samples.
Sample processing in the field focuses on reducing the volume of composited samples to a manageable level while maximizing the number of large and rare taxa collected. Reductions in sample volume are accomplished by removing coarse organic and inorganic debris, removing large and rare taxa to a separate sample container, and elutriating, sieving,