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Quantitative periphyton samples of macroalgae (for example, filamentous assemblages of Cladophora ) require sampling from relatively larger areas than suggested for microalgae in order to provide a characterization of conditions in the sampling reach. Estimates of macroalgal biomass can be valuable for water-quality modeling and eutrophication-process studies, such as the effect of benthic macroalgae on diel cycles of dissolved-oxygen concentrations, pH, and alkalinity in the water of nutrient-enriched streams and rivers. A limitation of quantitative collection methods for macroalgae is that the microalgal community component can be severely under-represented or absent. Therefore, a quantitative microalgal sample should be collected in conjunction with the macroalgal sample to assess the autecological character of the periphyton community.

Quantitative samples are collected to determine the biomass of macroalgae that is attached to a defined area of the streambed. Sample-collection methods described for macroalgae are also applied to quantitative sampling of aquatic mosses. A qualitative sample of the macroalgal or aquatic moss assemblage should also be collected for species identification if the taxon is not recognizable in the field. Periphyton biomass can be measured as dry mass (DM), ash-free dry mass (AFDM), or photosynthetic-pigment content (for example, chlorophyll a and b (CHL) concentrations).

Quantitative samples of macroalgae can be collected with benthic invertebrate sampling gear, such as a Surber sampler, Hess sampler, or box sampler (Cuffney and others, 1993), or with other devices, such as a cylindrical coring device or template, that defines a measured area of stream bottom. The sampling device is placed over a representative macroalgal assemblage, and algae within the template of the sampling device are removed by hand or with the use of a brush or scraper. Quantitative macroalgal samples also can be collected by scraping or brushing algae from the surface of representative rocks and estimating surface area by the foil-template method.

Sample processing methods for macroalgae differ with respect to the nature of the biomass measurement. If macroalgal samples are to be analyzed for AFDM, pour off any residual stream water from the sample, place the sample in a plastic bag with a sample label, chill the sample, and transport the sample to the laboratory. Record the area of the macroalgal sample on the field data sheet (fig. 5) and on the sample label. If weather conditions permit, the macroalgal sample can be air dried during the site visit; the dried sample is placed in a plastic bag or other container with a sample label. Air-dried samples of macroalgae do not require chilling. Determinations of AFDM can provide an inexpensive estimate of algal biomass in a stream reach, indicating relative differences in loads of nutrients and other water-chemistry constituents among streams in a basin. If project personnel have access to an analytical balance, drying oven, and muffle furnace, AFDM can be determined by laboratory methods describe d in Britton and Greeson (1988) or Clesceri and others (1989).

The biomass of macroalgae also can be estimated by determining the CHL content of the periphyton assemblage. This determination is particularly appropriate for studies designed to address the effects of benthic algal processes on water quality, such as relations with instream concentrations of dissolved oxygen, alkalinity, and pH. Processing of a macroalgal sample for CHL analysis is accomplished by (1) obtaining a representative subsample volume from the total volume of the macroalgal sample, (2) collecting the sub- sample volume on a glass-fiber filter (Whatman GF/F or equivalent) using a filtration