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directly assessing the biological integrity of a site and the only method that is sensitive to toxicological influences and habitat degradation resulting from changes in land use or such instream disturbances as floods, navigation improvements, or substrate instability.

The distribution of benthic invertebrate species responds to natural and anthropogenic influences. Natural changes in physical and chemical conditions occur along the longitudinal axis of the river (Vannote and others, 1980), altering environmental variables (for example, riparian conditions, thermal regimes, discharge patterns, light penetration, channel gradients, sediment conditions, water and sediment chemistry) and causing benthic invertebrate communities to change. In addition, each location along the river continuum will contain a variety of habitats, such as riffles, pools, sloughs, bars, and backwaters, that differ in respect to substrate type and stability, current velocity, and water depth. Therefore, each location in the river has a range of natural conditions that, when coupled with environmental requirements of the invertebrate species, determine whether a given organism can live in a particular habitat at a particular point along the river continuum.

These patterns of species distribution are affected by natural and anthropogenic influences that alter the landscape (for example, wild fires, logging, earthquakes, agriculture, volcanic eruptions, and urbanization), modify hydrologic conditions (changes in evapotranspiration and runoff or construction of reservoirs and irrigation diversions), alter habitats (snagging operations, channel dredging, sedimentation, hurricanes), or add chemicals that are toxic or that elevate nutrient or organic loads. The challenge of ecological surveys in the NAWQA Program is to separate changes caused by natural and anthropogenic factors and relate them to water quality. This is accomplished by comparing distributions of organisms among sites that vary in natural and anthropogenic influences, including relatively pristine streams, and relating patterns of distribution to patterns of physical and chemical factors.

Purpose and Scope

The sampling methods and procedures presented here are intended to give guidance to study-unit biologists collecting benthic invertebrates as part of the USGS's NAWQA Program. Various sample-collection techniques, equipment, and data forms are presented for use at basic fixed sampling sites. These methods and techniques can be adapted for use in other components of the NAWQA Program, such as synoptic and case study sampling, or where needed in other programs of the USGS's Water Resources Division. Additional discussions and descriptions of benthic invertebrate sampling devices and methods are reported in Hynes (1970), Mason (1978), Rosenberg (1978), Adamus (1984), Merritt and Cummins (1984), Britton and Greeson (1988), and Klemm and others (1990).