The surface of bogs is commonly patterned and composed of different vegetation communities, defined by water level. Carbon dioxide (CO2) dynamics vary spatially between the vegetation communities. An understanding of the controls on the spatial variation of CO2 dynamics is required to assess the role of bogs in the global carbon cycle. The water level gradient in a blanket bog was described and the CO2 exchange along the gradient investigated using chamber based measurements in combination with regression modelling. The aim was to investigate the controls on gross photosynthesis (PG), ecosystem respiration (RE) and net ecosystem CO2 exchange (NEE) as well as the spatial and temporal variation in these fluxes. Vegetation structure was strongly controlled by water level. The species with distinctive water level optima were separated into the opposite ends of the gradient in canonical correspondence analysis. The number of species and leaf area were highest in the intermediate water level range and these communities had the highest PG. Photosynthesis was highest when the water level was 11 cm below the surface. Ecosystem respiration, which includes decomposition, was less dependent on vegetation structure and followed the water level gradient more directly. The annual NEE varied from -115 to 768 g CO2 m(-2), being lowest in wet and highest in dry vegetation communities. The temporal variation was most pronounced in PG, which decreased substantially during winter, when photosynthetic photon flux density and leaf area were lowest. Ecosystem respiration, which is dependent on temperature, was less variable and wintertime RE fluxes constituted approximately 24% of the annual flux..