For the purpose of NAWQA ecological surveys, a composite RTH periphyton sample is prepared by combining periphyton collections from five locations within the sampling reach into a single sample. At each location, periphyton is brushed and withdrawn from a minimum of one SA from each of five representative (relative to size and periphyton abundance, texture, and color) rocks. Periphyton collections from a total of five SA's, representing a total sampling area of approximately 15 cm 2 are made at each location. Periphyton collections from the five locations are composited to form the RTH periphyton sample, representing a total of 25 SA's and a total area sampled of approximately 75 cm 2. However, if periphyton abundance is very sparse, algal biomass from additional rocks or SA's must be collected in order to provide sufficient material for laboratory analysis. The diameter of the SA (typically 1.8 to 2.0 cm) is measured from rocks that have been sampled at the beginning and at the end of sampling activities at a sampling reach to ensure there has been no change during the sampling period. The area sampled (typical ly 2.5 to 3.1 cm 2 ) is calculated and recorded on the field data sheet (fig. 5). The area sampled and the total number of SA's collected for the quantitative periphyton sample are recorded on a sample label (fig. 3) and on the associated field data sheet.
When periphyton cannot be sampled with the SG-92 sampler (for example, because the rock surface is irregular and the sampler cannot be sealed to the rock without leakage), quantitative periphyton samples can be collected from the sampling reach by scraping or brushing the entire surface of representative rocks and determining the surface area by the foil template method. Individual rocks are placed into a small plastic tub with a small amount of native stream water; periphyton are scraped or brushed from the rock surface, and the algae-water suspension is poured into a sample container. The rock surface is covered with a sheet of aluminum foil, and the foil is trimmed to match the area where algae were attached. The trimmed foil template is placed into a labeled collection envelope and submitted with the quantitative periphyton sample. The surface area of the foil template can be estimated with a planimeter or by mass. Alternatively, graph paper can be used to estimate surface area colonized by perip hyton. The surface area is determined by summing the number of graph squares contained in the replicate and multiplying by the area of one graph square. Additional methods for estimating the surface area of rocks are discussed by Graham and others (1988 ).
Collecting representative periphyton samples from gravel-sized epilithic microhabitat can be difficult because the rocks are smaller than the diameter of the SG-92 periphyton sampler, and the foil-replicate method would likely introduce substantial error into estimates of the surface area colonized by periphyton. The suggested quantitative method for sampling periphyton attached to gravel substrates involves the use of a 15.24-cm (6-in.) diameter PVC cylinder, or a similar device that serves as a templat e to outline a measured area of streambed (for example, a Surber sampler). The sampling device is placed in a representative stream location, and individual gravel-sized rocks are retrieved from inside the sampling device and placed in a small plastic container. A small volume of native stream water is added to the container, and periphyton are removed from all rock surfaces with a stiff-bristled toothbrush. The algae-water mixture is poured into a sample container, and the area enclosed by the sampling device (approximately 182 cm 2 for the PVC cylinder) is recorded on the sample label and on the field data sheet.
Quantitative periphyton samples may be collected from bedrock microhabitats with a PVC pipe sampler functionally similar to the SG-92 periphyton sampler. The pipe sampler is constructed from a 61-cm (2-ft length of thick-wall PVC pipe, with an inside dia meter of