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differences or changes in water quality. Phytoplankton samples may be collected in large nonwadeable streams and rivers to meet specific study-unit objectives.

Algal samples are labeled with a unique 16-character code that identifies the study unit (stream location), sampling date, type of sample, subtypes, sample number, and sample element code. Optional algal samples for the determination of chlorophyll conce ntrations or ash-free dry mass are processed in the field, placed on dry ice, and submitted to the National Water-Quality Laboratory in Arvada, Colorado. Samples for the identification and enumeration of algal taxa are preserved with buffered formalin an d shipped to a contract laboratory for analysis. The Biological Quality-Assurance Unit has responsibility for laboratory quality assurance and quality control, entry of contractor data into the national data base, and storage and maintenance of taxonomic collections. The Biological Quality-Assurance Unit also monitors the accuracy of taxonomic identifications and precision of enumeration by analyzing split samples from 10 percent of the study-unit sampling reaches.