During relatively stable hydrologic conditions, mature periphyton communities can develop in streams and rivers after a short period (several weeks to months) of colonization and reproduction. However, algal biomass and community composition can vary con siderably within and among sampling reaches in relation to differences of current velocity, light intensity, and water-chemistry factors. Field measurements of current velocity, temperature, and riparian canopy should be made in conjunction with all periphyton collections. Measurements of the relative availability of light to the periphyton community (for example, photosynthetically active radiation (PAR) or Secchi depth) should be measured when possible so that reach-specific factors may be distinguished from the effects of larger-scale, human and natural factors that influence the quality of water.
Many types of sampling equipment and various techniques have been developed for the collection of algal samples. The proper choice of sampling equipment and technique depends on physical conditions in the sampling reach (for example, water depth, current velocity, and habitat conditions) and whether the sample is intended to provide qualitative or quantitative data. The following sections establish guidelines for collecting qualitative and quantitative samples of periphyton from natural substrates, using artificial substrates to collect periphyton, and collecting quantitative samples of phytoplankton.
Qualitative periphyton samples are composite samples collected from all periphyton microhabitats present within the sampling reach. These samples are designed to give a thorough representation of the number of taxa (taxa richness) and identity of algal taxa present in the reach, but not their abundance. A subsample of the qualitative periphyton sample also can be used to assist with the identification or verification of diatom species and varieties in quantitative periphyton samples from the same locati on. Water quality can be assessed by interpreting autecological information, the taxon-specific physiological requirements or tolerance for defined ranges of water-quality conditions, which is known for over 3,000 algal species (Kolkwitz and Marsson, 190 8; Kolbe, 1927; Cholnoky, 1968; Palmer, 1969; Lowe, 1974; VanLandingham, 1976; Descy, 1979a and 1979b).
The objective of qualitative multihabitat (QMH) sampling is to obtain as complete a list as possible of periphyton taxa present in the sampling reach in the sampling time available, usually about 30 minutes. Qualitative algal samples should represent all possible texture and pigmentation combinations of three major periphyton growth-forms present on submerged surfaces in the sampling reach. The three major growth-forms of freshwater periphyton are operationally defined as macroalgae (morphology is visible to the eye), microalgae (morphology is microscopic, appearing as pigmented accumulations or films attached to submerged surfaces), and aquatic mosses (nonalgal periphyton, Division Bryophyta). Examples of macroalgae include filamentous growth-forms (< I> Cladophora, Spirogyra, Hydrodictyon, and Batrachospermum ), plant-like algae with leaf-like structures ( Chara and Nitella ), round or flattened colonies ( Nostoc ), gelatinous masses ( Chaetophora and T etraspora ), and short, tubular strands ( Lemanea ). It is important to collect periphyton from a variety of locations in the sampling reach, representing the range of conditions of current velocity, water depth, and riparian shading present at the time of sampling. If all periphyton growth-forms are present in the sampling reach, the QMH periphyton sample would consist of three sample containers: (A) a sample of macroalgae, (B) a composite sample of microalgae, and (C) a sample of aquatic mosses. In contrast, if neither macroalgae nor