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 provide nationally consistent ecological
information at all Basic and Intensive Fixed Sites as part of an
integrated physical, chemical, and biological assessment of water
quality.
- Intensive Ecological Assessments assess spatial and temporal variability
associated with biological communities and habitat characteristics.
- Ecological Synoptic Studies provide improved spatial resolution of
selected ecological characteristics in relation to land uses, contaminant
sources, and habitat conditions.
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.
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