Intensive Fixed Sites are the part of the fixed-site design where all water-column sampling strategies are employed, as well as all levels of bed-sediment and tissue (see Bed-Sediment and Tissue Study), and aquatic ecological assessment sampling. They are the same as the Basic Fixed Sites except for the addition of enhanced sampling frequency and the addition of dissolved-pesticide analyses for 1 year. The Intensive Indicator Fixed Sites within each study unit are selected to represent land-use settings that are most critical for addressing pesticide contamination issues.
The sampling strategy at each Intensive Fixed Site is the Basic Fixed-Site strategy with the addition of analyses of dissolved pesticides for all samples and a higher frequency of interval and extreme-flow sampling during selected seasonal periods. Intensive sampling and analyses usually are done for a 1-year period, after which sampling and analyses return to the same strategy as that of Basic Fixed Sites. Seasonal periods selected for high-frequency sampling are the most critical periods for concentrations and transport of dissolved pesticides and nutrients. The periods are identified from information on chemical-use patterns, seasonal climatic conditions, irrigation practices, and existing water-quality data. During high-frequency sampling, fixed-interval sampling typically is weekly, or, if the basin is large, biweekly. The fixed-interval sampling is supplemented with extreme-flow samples of equal or greater number than those for standard operation of Basic Fixed Sites (Gilliom and others, 1995, p. 17).
There are four major components of the Fixed-Site Reach Assessment conducted by NAWQA at all of the Intensive (and Basic) Fixed Sites. These components include; algal, benthic invertebrates, fish-community assessments and aquatic-habitat evaluations. 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 (Gilliom and others, 1995, p. 24). In addition, each aquatic community provides an indication of what has been occurring in the stream over a different time scale. Algal communities tend to respond to changes in water quality relatively quickly (hours to days), aquatic insects somewhat slower (days to weeks), and fish communities, which are responding to changes in algal and invertebrate communities as well as directly to water-quality changes, over a longer period (weeks to months). An evaluation of aquatic habitat is necessary to determine if community responses are due to chemical changes in the water or physical changes in flow, channel morphology, or other habitat changes.
Algae and aquatic insects are sampled concurrently in fast flowing water in a known sample area. Additionally, a qualitative sample of both communities is collected throughout the stream reach to include a broader range of habitats and taxa. Fish were sampled with electrofishing gear appropriate for the stream reach being sampled. The entire stream reach was fished except in the largest rivers where near shore habitats were sampled.
Habitat measures were collected along transects. Both instream and riparian measures are made and included instantaneous velocity and depth, substrate analysis, canopy cover, erosion patterns, flood plain and bank composition and structure, and instream fish-habitat components. Microhabitat measures were also taken at each algae and aquatic benthic invertebrate sampling point and included instantaneous velocity, embeddedness, and substrate particle sizes at each algae and aquatic benthic invertebrate sampling point.
Additional information regarding sampling methodology is available in NAWQA Method and Guideline Protocol publications. A map of the drainage basins and site locations of the Intensive Fixed Sites is available for viewing: