Environmental measurements are required to determine the quality of ambient water, the character of effluents, and the effects of pollutants on aquatic life. The EMSL-CI conducts an Agency-wide quality assurance program to assure standardization and quality control of systems for monitoring water and wastewater. The effectiveness of measures taken to maintain and restore the biological integrity of the Nation's surface waters is dependent upon our knowledge of the changes in the taxonomic composition of aquatic life caused by discharges of toxic substanes and other pollutants, and upon the level of our understanding of the complex relationships that prevail in aquatic ecosystems. The periphyton is the community of organisms growing on natural or artificial substrates in water. This community is of particular value in accessing changes in water quality in streams since it is stationary and thus is continuously exposed to the water flow.