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The collection of representative, quantitative periphyton samples from natural substrates is preferred but presents special sampling challenges that directly affect the accuracy and precision of various structural estimates of algal standing crop. Because algal colonization (immigration and reproduction) is affected by numerous factors, such as light intensity, depth, velocity, and substrate texture, the distribution of periphyton in flowing streams is typically heterogeneous, or patchy. Although the use of artificial substrates (glass slides, clay tiles, or other introduced materials) may reduce the variability associated with natural substrates, careful sampling of natural substrates is likely to yield more complete information regarding the structure of the periphyton community and relations with benthic herbivores (invertebrates and fish).

Artificial substrates can be considered for stream reaches where natural substrates cannot be sampled because of safety issues or habitat inaccessibility, or when uniformity of substrate surfaces is an important consideration for interpreting water quality. However, quantitative algal data from artificial substrates are not directly comparable to data from natural substrates. Methods for using artificial substrates are discussed later in this document.

Epilithic periphyton microhabitats

Quantitative periphyton samples of microalgae are collected from rocks or other solid, flat surfaces with the NAWQA periphyton sampling device (SG-92) (fig. 4). The SG-92 is a modified syringe sampling device similar to those described by Cushing and oth ers (1983), Britton and Greeson (1988), and Aloi (1990). The SG-92 is constructed by cementing (cyanoacrylate adhesive) an O-ring (inside diameter, 2.06 cm [13/16 in.]; outside diameter,
2.70 cm [1 1/16 in.]) to the flanged end of the barrel of a 30-mL syringe. Periphyton brushes
(fig. 4) are constructed by affixing (epoxy adhesive) small, circular sections of bristles from a stiff-bristled toothbrush to the ends of 0.64-cm [1/4-in.] diameter plastic rods. The length of the bristles is trimmed to approximately 4 mm, and the perip hyton brushes are discarded when the bristle length decreases to 1 mm.

When quantitative periphyton samples are collected in conjunction with benthic invertebrate sampling, rocks with algal cover representative of the invertebrate sampling locations are chosen without intentional bias; extreme conditions, such as extremely d ense or sparse algal cover, are avoided. If benthic invertebrate samples are not collected, rocks are collected from five locations representative of the epilithic microhabitat distributed throughout the sampling reach, again avoiding extremes of algal cover and physical site conditions. Stream depth and velocity are measured at the location where quantitative periphyton samples are collected, and the measurements are recorded on the field data sheet (fig. 5). At each sampling location, a minimum of five rocks are placed in a plastic tub below the water surface to reduce loss of periphyton. The rocks are transported in the tub to a convenient sample-processing area.

Quantitative RTH periphyton samples are collected by removing attached algae contained within the circular sampling area (SA) of the SG-92 sampler with a periphyton brush. The SG-92 is firmly sealed to the substrate by pressing the O-ring of the SG-92 as sembly against the rock and rotating the assembly approximately one quarter turn before dislodging periphyton with the brush. The algae-water mixture is withdrawn from the
SG-92 with a 5-mL hand pipettor and placed into a sample container. Filtered stream water is then added to the SG-92, and the procedure is repeated until all algal biomass has been removed from the SA.