National Water-Quality Assessment (NAWQA) Program

Design of the National Water-Quality
Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz

Ecological Studies

Ecological Studies are an integral part of the approach used by NAWQA to assess water quality. Information on biological communities and habitat characteristics contributes to the conceptual model of factors that affect water quality and to improved understanding of the relations among physical, chemical, and biological characteristics of streams (Gurtz, 1994). The following primary strategies are used for Ecological Studies: Fixed-Site Reach Assessments and Intensive Ecological Assessments are done at a few sites in each Study Unit to provide an initial evaluation of the linkages among physical, chemical, and biological conditions across a wide range of Environmental Settings within and among Study Units. Ecological Synoptic Studies provide a much more complete geographic representation within a Study Unit but for a more limited set of physical, chemical, and biological characteristics.

Fixed-Site Reach Assessment

Descriptions of biological communities and habitat conditions are essential for an overall assessment of the status of water resources. At all Basic and Intensive Fixed Sites, biological communities (fish, invertebrates, algae), and habitat conditions are described and recorded. These data are used to improve understanding of relations among aquatic biological communities and the physical, chemical, and hydrologic conditions associated with selected Environmental Settings. Basic and Intensive Fixed Sites provide the best opportunity to satisfy this objective because of the nationally consistent strategies for collecting data on water chemistry and hydrologic conditions at these sites.

Site Selection

Fixed-Site Reach Assessments are done at all Basic and Intensive Fixed Sites except those in which ecological conditions are not representative because of local geomorphology or other natural or human influences. Indicator Sites are selected with the goal of keeping stream size, gradient, and geomorphic characteristics in a relatively narrow range while meeting the other objectives of the Basic and Intensive Fixed Sites. Consistency in these attributes among sites is important for making comparisons of ecological characteristics and facilitates interpretation of other water-quality measurements, such as sediment concentration and transport of constituents associated with sediment.

Sampling Strategy

The unit of sampling for Ecological Studies is the sampling reach, which is a part of the stream where stream, bank, and flood-plain habitat features are representative of the local area, and near the fixed-site location where chemical data are collected. Collections of biological communities and characterizations of riparian and instream habitat conditions are made for at least one sampling reach at each Basic and Intensive Fixed Site. Reach length is defined at each site by a combination of factors, which include stream geomorphology and meander wavelength (Meador, Hupp, and others, 1993). For wadeable streams, the acceptable range for sampling reach length usually ranges from 150 to 500 m, but longer reaches may be necessary in nonwadeable streams. Criteria for minimum and maximum reach lengths are used to provide a sampling-reach length sufficient to ensure the collection of representative samples from, for example, the fish community.

Three taxonomic groups--fish, invertebrates, and algae--are sampled because they respond differently to various environmental stresses. Fish are valuable biological indicators of long-term water-resource conditions because they are long lived (years to decades) and have considerable economic value and public interest. Benthic invertebrates (aquatic insects, mollusks, crustaceans, worms) have life cycles (from months to a few years) that are intermediate between fish and algae, have close association with streambed sediments, and can be used for characterizing changes in water quality over small spatial areas. Algae respond quickly (within days to weeks) to changes in their environment and serve as valuable biological indicators of rapid changes in water-resource conditions.

Representative samples of the fish community are collected from the stream reach by using a combination of sampling methods to determine species presence and abundance (Meador, Cuffney, and Gurtz, 1993); the two primary sampling methods used are electrofishing and seining. Fish are identified as to species, length and weight are recorded, and the presence of external anomalies, which include skeletal deformities, eroded fins, lesions, tumors, diseases, and parasites, is noted.

Three types of benthic invertebrate samples are collected in each sampling reach (Cuffney and others, 1993b). Semiquantitative samples provide information on the abundance of taxa present in two targeted habitats--one that is expected to support the highest number of taxa within the reach (for example, riffles or woody snags) and a depositional habitat (for example, pool). In addition, a qualitative sample from all instream habitat types in the reach provides a more complete list of taxonomic groups present at the time of collection. All samples are composites collected throughout the entire reach.

The algal community is sampled from each of the habitats targeted for benthic invertebrates and a multihabitat composite sample is also prepared (Porter and others, 1993). All algal samples are collected in a semiquantitative manner.

Habitat characterizations of channel, bank, and flood-plain features follow a spatial hierarchy that incorporates basin, stream segment, stream reach, and sample descriptors (Meador, Hupp, and others, 1993). Basin descriptors are recorded as part of the Environmental Framework of each site and include such variables as ecoregion, physiographic province, geology, soils, climate, and land use. Stream-segment data are obtained from geographic information system databases and topographic maps and include information on stream meandering, gradient, elevation, and water-management features. Habitat characterizations at the reach scale include geomorphic channel units, such as riffle, run, and pool, as well as physical features of the channel, bank, and flood plain and observations of dominant species of woody vegetation and macrophytes. Habitat characteristics that are associated with individual biological samples include substrate particle size, water depth, and velocity.

Fixed-Site Reach Assessments are done at least once at each Basic and Intensive Fixed Site during the 3 years of the intensive data-collection phase. Sampling of biological communities is conducted during similar hydrologic and seasonal conditions for all fixed sites in the Study Unit. Scheduling of sample collection takes into consideration several factors, which include hydrology, life histories of aquatic species, accessibility of sites, and timing of major human activities.

Sample Analyses

Samples of fish, invertebrate, and algal communities are identified to the lowest practical taxon, preferably species, to obtain information on taxonomic composition and abundance. Procedures have been developed for processing and quality assurance of samples (Cuffney and others, 1993a). The Biological Quality-Assurance Unit at the U.S. Geological Survey National Water-Quality Laboratory monitors taxonomic data, which include coordinating verification of identifications, establishing taxonomic voucher collections, developing and maintaining computer databases, and collaborating with other agencies in sharing taxonomic data and coordinating databases.

Intensive Ecological Assessment

Intensive Ecological Assessments are done at a subset of Basic and Intensive Fixed Sites in each Study Unit to provide information on spatial and temporal variability of biological communities and habitat characteristics. An understanding of background variability is critical to interpreting the natural and human factors that influence ecological conditions.

Sites for Intensive Ecological Assessments (typically three to four sites) are chosen to represent a range of water-quality conditions, stream sizes, and habitat conditions within each Study Unit. Reach-to-reach variability is estimated at these sites by sampling multiple (minimum of three) reaches that are located so that each represents similar water-quality conditions to the fixed site. Year-to-year variability is described by sampling one of the three reaches during each year of the 3-year intensive data-collection phase. Sampling and sample-processing strategies for each reach are identical to those for the Fixed-Site Reach Assessments.

Ecological Synoptic Studies

Ecological Synoptic Studies are short-term investigations of specific ecological characteristics within all or part of a Study Unit. Their roles in the NAWQA study design are similar to those for Water-Column Synoptic Studies--to provide improved spatial resolution compared with fixed-site sampling and to evaluate the spatial distribution of selected ecological characteristics in relation to causative factors, such as land uses, contaminant sources, or instream habitat conditions. Ecological Synoptic Studies supplement information from the more comprehensive data collected at Basic and Intensive Fixed Sites by targeting specific and more narrowly defined conditions for ecological characterization at more locations.

Ecological Synoptic Studies usually focus on the relation of selected Environmental Setting characteristics to selected biological community characteristics. For example, the distribution of fish species may be assessed for small and moderate-sized streams within urban, agricultural, and undeveloped settings. The design is coordinated wherever possible with that of Water-Column Synoptic Studies in the same Study Unit and among Study Units to address questions of regional or national interest. Priorities for Ecological Synoptic Studies are evaluated based on the potential to address relations among physical, chemical, and biological characteristics; the areal extent of a water-quality problem or biological issue; and the degree to which the design contributes to regional understanding and current National Synthesis topics. Results of the Retrospective Analysis, information on land-use patterns and local priority issues, and ecological information obtained from field reconnaissance of sites are used to determine the design.

Most Study Units complete one to two Ecological Synoptic Studies during the second and third years of the 3-year intensive data-collection phase. Although, the strategy for site selection, sampling, and analysis is issue specific and usually include a subset of the biological components sampled at the fixed sites, nationally consistent methods are used so results can be compared among Study Units.

Continue to Ground-Water Study Design, Glossary, or return to Contents

Maintainer: h2o Webserver Team
U.S. Department of the Interior, U.S. Geological Survey
Persistent URL:
Page Contact Information: Contact USGS
Last modified: Friday, January 11 2013, 12:03:54 PM
FirstGov button  Take Pride in America button