Three nontaxonomic indices; ATP/Chlorophyll <u>a</u>(ATP/Chla), ATP/ADP, and Chlorophyll <u>a</u>/Pheopigment (Chla/Pheo) were compared to the taxonomic measures of species diversity (d) and species richness as indicators of stress in aquatic environments. Field and laboratory microcosm responses of indigenous microbial communities exposed to municipal sewage treatment plant (STP) effluent were monitored. The STP effluent produced increased adenylate concentrations, ATP/ADP and ATP/Chla ratios, and decreased Chla, Chla/Pheo, d, and species richness relative to upstream reference communities. Nontaxonomic responses were consistent in four separate field tests. Significant differences in responses were discernible in 3 d when communities were transferred from reference to polluted sites. Chla/Pheo decreased more rapidly than other measurements. The predictive capability of laboratory flow through microcosm tests was examined by simultaneously transferring communities from upstream reference sites to downstream field sites and to various dilutions of field effluent in the laboratory. / Master of Science