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Algal Data from Selected Sites in the Upper Colorado River Basin, Colorado, Water Years 1996-97 By Scott V. Mize and Jeffrey R. Deacon Abstract Algal community samples were collected at 15 sites in the Upper Colorado River Basin in Colorado as part of the National Water-Quality Assessment Program during water years 1996-97. Sites sampled were located in two physiographic provinces, the Southern Rocky Mountains and the Colorado Plateaus, and represented agricultural, mining, urban, and mixed land uses and back- ground conditions. Algal samples were collected once per year during low-flow conditions. Quantitative algal samples were collected within two targeted instream habitat types including a taxonomically richest-targeted habitat and a depositional-targeted habitat. This report presents the algal community data collected at the fixed sites in the Upper Colorado River Basin study unit. Algal data include densities (abun- dance of cells per square centimeter of substrate) and biovolumes (cubic micrometers of cells per square centimeter of substrate) for the two habitat types. Quality-assurance and quality-control results for algal samples indicate that the largest sampling variability tends to occur in samples from small streams. INTRODUCTION The National Water-Quality Assessment (NAWQA) Program is a long-term program of the U.S. Geological Survey (USGS) that is designed to describe the status and trends in the quality of the Nation's surface- and ground-water resources and to provide an understanding of the natural and human factors that can affect the quality of these resources (Leahy and others, 1990). The program is interdisci- plinary and integrates biological, chemical, and phys- ical data to assess the Nation's water quality at local, regional, and national levels. Biological community surveys (algae, macroinvertebrates, and fish communi- ties) are one of the few means of directly assessing the biological integrity of a site and provide one approach that is sensitive to changes in water chemistry and physical habitat (Meador and Gurtz, 1994). The Upper Colorado River Basin (UCOL) was one of the study units selected for the National Water- Quality Assessment. Fourteen surface-water fixed sites (Spahr and others, 1996) and one additional biological sampling site were selected for the collec- tion of algal community data. The surface-water fixed- site network was designed to integrate the water chem- istry and biological information at the site. Algal samples were collected once per year during low-flow conditions. Water quality can be characterized by evaluating the results of qualitative and quantitative measure- ments of the algal community. The algal component of NA WQA biological community surveys is designed primarily to characterize the species distribution and community structure of benthic algae (periphyton) and their relation to water quality. The species composition and community structure of algae provide evidence of physical and chemical conditions present in a stream over time ranging from weeks to months. Quantitative periphyton samples were collected from two instream habitat types at each site and included a taxonomically richest-targeted habitat (RTH) and a depositional- targeted habitat (DTH). Specific locations of the RTH and DTH habitats within a stream reach are used to characterize the community within the sampling reach and are consistent with those selected for other NAWQA biological community surveys (Porter and others, 1993). Abstract