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Many small communities located in rural areas of the United States lack adequate domestic wastewater treatment plants. Constructed wetlands are being promoted as low-cost, low-maintenance systems for treatment of domestic wastewater in areas where standard septic tanks fail. In order to evaluate the efficacy of small constructed wetlands, 20 small-scale wetlands mesocosms (100 gal each) containing five plant treatments (no plants/control, Typha, Scirpus, and Juncus monocultures, and all three species combined) were planted into pea gravel at either 18 or 24 inches in depth. Each mesocosm received 5 gal/day of primary-treated domestic sewage. Mesocosms were monitored on a monthly basis over a two year period for TSS, BOD{dollar}\\sb5,{dollar} TKN, TP, and metals. Fecal coliforms, enterococci, Salmonella, Shigella, Yersinia and coliphage were also enumerated. Significant differences between influent and effluent water quality (p {dollar}<{dollar} 0.01) generally were observed in total suspended solids, 5-day biochemical oxygen demand, TKN and P. Increased dissolved oxygen and reduction in fecal coliforms and enterococci, Salmonella, Shigella, Yersinia, and coliphage populations were also observed. Effluent quality from the mesocosms planted with mixtures of the three species was frequently (p {dollar}<{dollar} 0.10) better than mesocosms containing monocultures of Scirpus or Juncus alone. Mesocosms containing Typha often equaled the mixed plantings in treatment effectiveness. Additionally, functional diversity was examined for the primary-clarified wastewater, control/unplanted and mixculture mesocosms. Patterns of potential carbon source utilization by microbial communities were analyzed as a means of differentiating among the samples. Utilization of 95 separate C sources was determined by inoculating microbial suspensions obtained from wastewater influent, and unplanted control, or planted mesocosms (containing Typha, Scirpus, and Juncus) into BIOLOG plates and quantifying color production. Distinctive patterns of C source utilization were apparent for each of the three samples, and the differences among samples were consistent of the sampling period (December 1995-December 1996). Temporal shift in C source utilization related to plant development state and environmental conditions were observed during the sampling period.