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The influence of spatial and temporal environmental variations on rates of organic pollutant biodegradation were assessed by using heterotrophic uptake kinetics. These studies were conducted at three sites, representing the gradient from freshwater to estuarine to marine systems. Of the compounds tested, total uptake Vmax rates decreased in the order of nitrilotriacetic acid, m-cresol, chlorobenzene, and 1,2,4-trichlorobenzene. In general, the freshwater site exhibited the highest uptake rates, with somewhat lower rates at the estuarine site. Rates at the marine site were much lower than at the other sites, except during the winter. Metabolic rates at both the freshwater and estuarine areas were significantly decreased during periods of low water temperature. Rates at the marine site were relatively uniform throughout the year. Linear regression analysis was used to compare m-cresol biodegradation rates to characteristics of the microbial community, which included direct microscopic counts, CFU counts, and cellular incorporation of amino acids. The observed rates did not consistently correlate well with any of the measured characteristics of the microbial community.