Small open top chambers (0.8 m .times. 1.0 m) were developed to maintain elevated CO2 concentrations in three plant communities in a brackish marsh ecosystem. Mean annual CO2 concentrations were 350 .+-. 22 .mu.l l-1 in chambers which received no added CO2 and 686 .+-. 30 .mu.l l-1 in chambers with elevated CO2 concentrations. Light quality was not affected in the photosynthetically active wavelengths but the chamber reduced light quantity by 10%. Night-time air temperatures inside the chamber (Ti) averaged 2.degree. C above air temperature outside the chamber (To) due to heating from the air blowers. Air temperature profiles through the plant canopy and boundary layer showed that daytime temperature differences (Ti - To) were greater than night-time differences and this day/night difference also depended on the plant community. Effects of the chamber on the micro-environment of the plant communities resulted in a significant growth enhancement in the plant community dominated by the C3 sedge Scirpus olneyi Grey but not in the other two communities.