AbstractAdaptation by natural microbial communities may be operationally defined as an increase in the biodegradation rate of a chemical as a result of exposure to the material. The comparative adaptation of sediment‐associated and water microbial communities to the sorptive cationic surfactant dodecyltrimethylammonium chloride (C12‐TMAC) was studied in continuous‐flow, settled‐sediment microcosms. In microcosms initially exposed to an input concentration of 0.1 mg/L C12‐TMAC, biodegradation activity in the water column increased 20‐fold after approximately 10 to 15 d. As input concentrations of C12‐TMAC entering the microcosm were increased, water column biodegradation activity also increased, even though measured water column concentrations remained relatively constant, due to biodegradation and/or adsorption. When C12‐TMAC input was reduced to zero, microbial activity returned to control levels. Sediment biodegradative activity was unaffected by input concentrations of C12‐TMAC below 0.5 mg/L. When the input concentration was increased to 10 mg/L, biodegradative activity increased by at least 10‐fold. Once adaptation had occurred in the sediments, the process was relatively unaffected by reductions in the concentration of C12‐TMAC entering the microcosms. Thus, relative to processes in the water column, the presence of sediments can influence both the onset of adaptatio
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