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quantitatively in shallow, wadeable streams, collecting benthic algal samples from these microhabitats can be difficult in nonwadeable streams and rivers, generally requiring the use of a dredge or divers. If the DTH selected for NAWQA benthic invertebra te sampling is dominated by rocks or aquatic macrophytes, then epilithic or epiphytic periphyton microhabitats should be sampled rather than epipelic or epipsammic microhabitats.

Epipsammic or epipelic periphyton microhabitats can be prevalent throughout entire sampling reaches of streams in certain regions of the United States (notably Coastal Plain streams), as well as in streams that receive drainage from unstable soils or land -disturbance activities. Most benthic primary production occurs near the margins of these streams, where there is sufficient light penetration to support the growth of algae. Periphyton are visible as thin, pigmented (green, gold, or brown) organic accumulations on the surface of coarse or fine sediments. These periphyton communities are inherently unstable; the algal community structure reflects water-quality conditions over a period of days to weeks, depending on antecedent hydrologic conditions. The periphyton community structure also is influenced by streambed-sediment chemistry conditions at the time the sample is collected. Quantitative periphyton samples should represent conditions in the stream margins where algae are actively growing, rather than conditions in the stream thalweg where streambed scouring and lack of light penetration limit algal growth to a greater extent than do water-chemistry conditions.

Quantitative periphyton samples are collected from the upper 5- to 7-mm layer of coarse (epipsammic microhabitats) or fine (epipelic microhabitats) streambed sediments in depositional areas of a sampling reach. The top half of a disposable 47-mm plastic petri dish is gently pushed into the streambed sediment, and a small Plexiglas sheet or stainless-steel spatula (fig. 4) is slipped under the petri-dish half, sealing the sample inside the petri dish. The sealed petri-dish half is gently lifted from the stream bottom, briefly agitated (below the water surface) to remove material trapped around the edge of the petri dish, and the sample is rinsed into a sample container. The petri-dish half encloses a sampling area of approximately 17 cm 2. The DTH periphyton sample is prepared by compositing five replicate petri-dish samples collected in the sampling reach, representing a total sampling area of approximately 85 cm 2. The total sampling area is included on the sample label and o n the field data sheet (fig. 5).

Using Artificial Substrates to Collect Periphyton

Artificial substrates can be considered for sampling reaches when natural substrates cannot be sampled because of inaccessibility of the microhabitat, cost of sample collection, or safety issues associated with the collection of representative samples (for example, the use of divers in large, fast-flowing rivers). Artificial substrates can also reduce the heterogeneity (or patchiness) of algae occurring on natural substrates and can be used to compare water quality among streams with disparate periphyton microhabitats. However, data from artificial substrates cannot be compared with data from natural substrates because of differences in factors that contribute to algal processes and differences in the age of the periphyton community. If artificial subs trates are considered for one or more stream reaches in a basin, it is recommended that they be used at all sites so that meaningful water-quality interpretations can be made.

Artificial substrates for periphyton include benthic substrates (for example, rocks, bricks, clay tiles, glass or plastic rods, and wood dowels) and suspended substrates (for