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(Lang and Austin, 1984), styrofoam floats (Flint and others, 1977), and polyurethane foam substrates (Cairns and others, 1983).

The choice of an appropriate exposure period for artificial substrates is dependent on water temperature, intensity and duration of light, nutrient status of the stream, current velocity, and the objectives of the study. If artificial substrates are used as a substitute for sampling of natural substrates, the exposure period must be long enough for the algal community to develop fully, generally 1 to 2 months. Therefore, it is necessary to place artificial substrates in identified sampling reaches well before the anticipated date of the ecological survey. The length of the exposure period, hydrologic conditions during the exposure period, and sampling season can affect the extent and taxonomic composition of periphyton growth on artificial substrates ( Biggs, 1988). However, if sampling is conducted in nutrient-rich streams during the warm weather season, and if the study objective is solely to compare water-quality responses of the periphyton community among reaches, an exposure period from 3 to 4 weeks is generally sufficient. Regardless of the length of time selected as the exposure period, artificial substrates should be exposed for the same time period in all sampling reaches of an ecological study. The date of substrate placement and retrieval, as well as the total time of exposure, is indicated on the field data sheet (fig. 5) and on the sample label (fig. 3).

Quantitative Phytoplankton Samples

Phytoplankton are algae that are buoyantly suspended in the water column of streams, rivers, reservoirs, and lakes. Phytoplankton are passively transported by currents and turbulent mixing, and they respond to physical and chemical conditions present at the time of sample collection; that is, they reflect water-quality conditions of the water mass in which they occur (Clesceri and others, 1989). Although certain phytoplankton taxa are useful for assessing taste and odor problems in domestic water supplies or for determining the origin or recent history of a water mass, phytoplankton are somewhat less useful than periphyton for indicating and integrating water-quality changes relative to a fixed sampling location, particularly in wadeable streams and rivers. The plankton of many streams consist of benthic algal species that have been dislodged from periphyton microhabitats as a result of biological processes (McCormick and Stevenson, 1989; Barnese and Lowe, 1992) or physical disturbance, such as scouring. Benthic diatoms also may occur in the plankton of streams as a result of taxon-specific immigration, reproduction, and emigration processes (Korte and Blinn, 1983; Stevenson, 1983). However, potamoplankton communities (those that resemble phytoplankt on communities of lakes) may develop in larger streams and rivers during periods of relatively stable hydrologic conditions, particularly in large, impounded rivers (Lowe, 1980; Round, 1981).

Phytoplankton species frequently have been used as indicators of eutrophication and other water-quality conditions in streams and lakes (Williams, 1964; Palmer, 1969; Reynolds, 1984; Charles, 1985). Physiological processes of planktonic algae also affect the quality of water, particularly with respect to water transparency (or color), alkalinity, pH, dissolved nutrients and organic carbon, and dissolved concentrations of oxygen and carbon dioxide. Certain algal taxa are toxic when ingested by fish or ot her animals (Palmer, 1977), and microbial decomposition of massive phytoplankton blooms often can result in the depletion of dissolved oxygen, leading to fish kills from asphyxiation.

Quantitative phytoplankton samples are obtained by collecting a representative whole-water sample of sufficient volume to ensure adequate phytoplankton biomass for analysis.