Landscape-scale net ecosystem CO2 exchange (NEE) and the energy balance of a subartic fen were studied during five growing seasons near Churchill, Manitoba. Interannual variability in NEE was large and ranged from a net sink of -235 g CO2 m-2 in 1996 to a net source of +76 g CO2 m-2 in 1994. Annual estimates of CO2 exchange indicate that during the present period the fen is losing carbon nearly 3 times faster than its long-term historical gain of about -11 g CO2 m-2 yr-1. Our estimates suggest that gross ecosystem photosynthesis may be more variable than ecosystem respiration on diurnal, seasonal, and interannual timescales. Our data strongly indicate that an early snowmelt combined with wet and warm conditions during the spring period lead to large carbon acquisition even when drier conditions were experienced over the majority of the growing season. The phenological stage of the vegetation relative to the climatic conditions experienced is an important cause of the interannual variability in NEE. An accurate representation of phenology in climate models is, therefore, critical to the success of forecasting the carbon budgets of northern wetlands.