Benthic algae (periphyton) and phytoplankton communities are characterized in the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program as part of an integrated physical, chemical, and biological assessment of the Nation's water quality. This multidisciplinary approach provides multiple lines of evidence for evaluating water-quality status and trends, and for refining an understanding of the factors that control water quality. This is accomplished by integrated sampling at locations chosen to represent combinations of natural and human factors that are important in influencing the water quality at local, regional, and national scales.
The sampling unit for algal community characterization is the sampling reach, a length of stream that contains multiple examples of the predominant geomorphic features (for example, two riffle-pool sequences) that characterize a stream or river segment. Each sampling reach is characterized using a combination of qualitative and quantitative algal samples. For the purpose of NAWQA ecological surveys, algal samples typically are collected in conjunction with benthic invertebrate samples. Periphyton sampl es are collected from the surfaces of natural substrates (periphyton microhabitats) in relation to the presence of microhabitats in the sampling reach and the selection of habitats for benthic invertebrate sampling. To address specific study-unit issues, phytoplankton samples can be collected at large river locations during periods of stable hydrologic conditions. The choice of sampling locations and the occurrence of different instream habitat types in the sampling reach determine the types of qualitat ive and quantitative algal samples that are collected. The character of periphyton microhabitats in the sampling reach determines the types of sampling devices and methods used for collecting representative algal samples.
Water quality can be characterized by evaluating the results of qualitative and quantitative measurements of the algal community. The species composition and community structure of algae provide evidence of physical and chemical conditions present in as tream, over time scales ranging from weeks to months. Qualitative periphyton samples are collected to document the occurrence of algal taxa in all available periphyton microhabitats present in the sampling reach. The purpose of qualitative sampling is to develop a detailed list of the taxa present in the reach at the time of collection. The qualitative multihabitat periphyton sample is prepared by compositing collections of periphyton from instream habitat types present in the sampling reach. Quantita tive periphyton samples are collected to measure algal community structure within targeted instream habitat types--a taxonomically "richest" habitat and a depositional-targeted habitat. The specific locations of the richest-targeted and deposit ional-targeted habitats within a stream reach are consistent with those selected for invertebrate sampling. Relevant site information, sampling information, and microhabitat characteristics are recorded on algae field data sheets. Estimates of algal bio mass (chlorophyll content and ash-free dry mass) are optional measures that are useful for interpreting water-quality conditions at some study units.
Benthic algal communities are collected from natural substrates, using the sampling method that is most appropriate for the predominant periphyton microhabitat in the designated habitat. Qualitative samples are collected from all instream habitat types in the sampling reach. Quantitative targeted habitats (richest and depositional) are sampled by collecting and compositing a minimum of five periphyton samples from each designated instream habitat type. The use of artificial substrates can be considered for nonwadeable stream locations, or when uniformity of substrate is an important consideration for detecting