Qualitative and semi-quantitative sampling is conducted within each sampling reach associated with a basic fixed site. Qualitative sampling, which involves collecting invertebrates from as many different instream habitats as possible, is intended to provide a list of taxa present in the sampling reach. Semi-quantitative sampling is intended to provide a measure of the relative abundance of each taxon present in two contrasting habitat types within the sampling reach. These samples, along with the corresponding chemical and physical data, are used to (1) characterize the community within the sampling reach, (2) compare reaches among environmental settings, (3) compare changes in communities over time, and (4) couple physical and chemical water-quality characteristics with biological characteristics.
More intensive sampling is conducted at a subset of four to six sites to assess spatial variability among reaches and short-term temporal variability at a site. At these IEA sites, three sampling reaches are established to represent environmental conditions associated with the basic fixed site. One sampling reach is sampled in each of 3 successive years to estimate short-term temporal variability. Two additional sampling reaches are sampled in 1 year to assess the magnitude of reach-to-reach variability. Sampling at IEA sites supplements ecological surveys by providing a means to assess and compare variability locally, regionally, and nationally. These sites are chosen, to the extent possible, with the intent of characterizing variability across the range of conditions exemplified by the study unit.
In the example presented in figure 1, the sampling reach containing the basic fixed site (sampling reach B) is sampled in all 3 years to assess short-term temporal variability. The other two reaches (A and C) are sampled during only 1 year, typically the first or second year of sampling, to assess reach-to-reach variability. The study-unit biologist selects which one of the three sampling reaches to use for multiple-year sampling based on such criteria as ease of access and representativeness of the sam pling reaches.
The national scope of the NAWQA Program ensures that characterizations of benthic invertebrate communities are done in streams and rivers where physical and chemical characteristics vary widely in response to local and regional differences in environmental settings and human influences. Consequently, no single sampling technique or device is appropriate for all sites and instream habitats across the Nation. Therefore, a variety of techniques and sampling gear is recommended, based on the type of sample being collected (qualitative or semi-quantitative) and the physical conditions at the collecting site, such as water depth, current velocity, and bed materials. Likewise, no "standard," nationally consistent instream habitat, such as a riffle, exists. Consequently, sampling focuses on a qualitative characterization of the sampling reach supplemented with semi-quantitative characterization of standard "response" habitats. Standard response habitats include the taxonomically richest instream habitat with in the sampling reach and a contrasting faunistically impoverished habitat, which is typically, but not always, a slow-flowing, depositional habitat.