Quantification of Carbon Emissions from Lakes and Streams in a Subarctic Catchment

Northern inland waters are generally supersaturated in CO2 and CH4 and are therefore sources of atmospheric carbon. The importance of aquatic carbon emissions in northern ecosystems is however poorly understood. We measured the concentration of CO2 and CH4 monthly in 27 lakes (14.5-0.1 ha) during the ice-free season (May-Oct) and under ice just before ice break-up. In streams (width: 1-9 m) CO2 was measured biweekly and CH4 monthly at 23 locations. Diffusive emissions of carbon to the atmosphere were estimated from partial pressures and gas exchange coefficients for respective system. The carbon emission from lakes during ice break-up in spring was estimated from accumulated gases under ice during winter. Our results show that all aquatic systems were net sources of carbon to the atmosphere on an annual basis. There was, however, also considerable spatial and temporal variability. The carbon emission during ice break-up was the most important efflux event from lakes accounting for about one quarter of the annual lake CO2 emission. During the ice-free season lakes ranged from being net sources to being net sinks of CO2, resulting in low average summer CO2 emission from lakes compared to the total annual aquatic emission from the catchment. Instead, streams, even though they only account for approximately 4% of the total aquatic area, dominated the annual aquatic CO2 emission from the catchment. The diffusive flux of CH4 were significant from both streams and lakes but were in total of less importance compared to the diffusive CO2 flux.