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A sample volume of 1 L is sufficient for samples collected from productive, nutrient-enriched rivers as indicated by water color. In contrast, a larger sample volume is required for phytoplankton samples collected from unproductive, low-nutrient rivers a s indicated by water transparency. Phytoplankton samples collected in conjunction with water-chemistry sampling at basic fixed sites are taken with a depth-integrating sampler, such as a D-77; the phytoplankton sample is taken directly from the churn spl itter (Ward and Harr, 1990). Alternatively, quantitative phytoplankton samples can be collected with a water-sampling bottle (Kemmerer, Van Dorn, or another type) or with a pump (Sournia, 1978; Britton and Greeson, 1988; Clesceri and others, 1989).

Phytoplankton samples can be collected at basic fixed sites over time, similar to the approach for collecting water samples for chemical and physical determinations. Seasonal changes in the abundance and composition of phytoplankton occur on temporal scales measured in weeks or months (Reynolds, 1984). Although samples collected at 4- to 6-week intervals will generally represent seasonal patterns of algal species succession, it is desirable to sample more frequently during critical water-quality periods, such as during substantial algal blooms, seasonal taste and odor episodes, and periods when diel changes of dissolved-oxygen content may adversely affect aquatic life and other water-quality issues.

The determination of chlorophyll a and b concentrations in phytoplankton communities is a study-unit option; however, such analyses are warranted for the purposes of (1) understanding the relation between algal biomass and short-term changes in dissolved-oxygen content, pH, and alkalinity of the water, (2) use in water-quality modeling efforts, and (3) improving an understanding of the sources and loads of carbon in river systems.

If chlorophyll is not to be determined, the entire sample is preserved with buffered formalin for identification and enumeration. Information concerning the site and sampling procedures is entered onto a field data sheet (fig. 6) analogous to that used for periphyton samples. Information recorded on the sample label (fig. 3) includes total sample volume.

For chlorophyll determinations, an unpreserved subsample is withdrawn from the phytoplankton sample, and the aliquot is filtered onto a glass-fiber filter, similar to the procedure described for periphyton samples. A subsample volume of 50 mL is adequate for most phytoplankton samples; however, a larger subsample may be required for samples with exceptional water clarity. Sufficient subsample volume should be filtered to ensure that adequate algal biomass is retained on the filter. Filters are wrapped in aluminum foil, placed into a sample bottle or container, and immediately placed on dry ice. The subsample volume is recorded on the sample label (fig. 3) and on the field data sheet (fig. 6), and the filters are sent to the analytical laboratory for a nalysis.


Processing methods for quantitative algal samples vary with the type of sample and the number of algal measurements required for the ecological study. Processing includes preservation and affixing an appropriate sample label. Some samples require decant ing to remove excess water. Optional samples for determinations of chlorophyll and ash-free dry mass require filtration.