This thesis examines the inter-annual variability in growing season net ecosystem CO2 exchange (NEE) at a subarctic sedge fen located near Churchill, Manitoba, Canada. Landscape-scale NEE and the energy and water balance of the fen were studied during five growing seasons between the years 1994 and 1999. Inter-annual variability in NEE was large and ranged from a net source to the atmosphere and a net sink to the ecosystem. Estimates of landscape-scale gross ecosystem production (GEP) and ecosystem respiration (ER) indicate that GEP is largely responsible for the inter-annual variability in NEE. Annual estimates of NEE indicate that, during the present period, the fen is losing carbon nearly three times faster than its long-term historical accumulation rate of approximately -11 g CO2 m-2 y-1 . The position of the polar jet stream and synoptic scale weather patterns has an important control on the timing of snowmelt, leaf emergence, and development of the vegetation during early spring. The 1997-1998 El Niño phenomenon caused a warm and wet spring that lengthened the growing season and increased carbon acquisition at the fen. Spring season hydroclimatological conditions play a key role in determining the source and sink strength of the wetland. Community-scale NEE measurements revealed that sedge-moss communities account for the majority of the CO2 exchange at the fen. Community relationships of GEP and ER indicate that changes in community spatial distribution within the fen could nearly triple the net carbon acquisition of this wetland. Model simulations show that the sensitivity of NEE to changing environmental conditions varies inter-annually depending on the initial conditions of the wetland. Simulations using climatic change scenarios indicate that warmer air temperatures will increase carbon acquisition during wet years but may act to reduce wetland carbon storage in years that experience a large water deficit.