Estimating CO2 evasion from streams connected to a boreal lake : comparing three different methods

Sammanfattning: There are different reservoirs of Carbon (C) on Earth and these reservoirs are closely linked to each other by the transport of C across different interfaces. The freshwater environment is one of the interfaces and an active component of the global C cycle. A considerable amount of C is lost through degassing (evasion) of carbon dioxide (CO2) and methane (CH4) from surface waters globally. Evasion is a vertical flux of gasses between the water surface and the atmosphere. Much of the efforts of determining the evasion component have been on lakes and large rivers while less focus has been spent on headwater streams. Small streams represent a large part of the interface between the soil and the aquatic environment. Peat and boreal forest soils are often rich in C, so the streams, which drain this landscape, are transporting and cycling a large amount of C in different forms. The aim of this study was to estimate the CO2 evasion in streams connected to a boreal lake, and to compare three different methods for measuring evasion from those streams. The CO2 evasion was calculated by a mass balance method, a gas transfer velocity method and a gas transfer coefficient method. The evasion by the gas transfer velocity and the gas transfer coefficient methods were determined using a volatile gas tracer. The CO2 evasion by the mass balance method was calculated from the differences in CO2 concentrations along a stream reach. The study was conducted in the catchment of Lake Gäddtjärn, which is situated in the boreal landscape of central Sweden. Four stream sites were selected and the measurements were done twice. The results of this study showed that the evasion of CO2 from those boreal streams ranged from 2.6 to 12.9 kg C m-2 yr-1 based on stream surface area. The evasion rates were largely controlled by variability in discharge and stream morphology. The method comparison showed that the gas transfer velocity and gas transfer coefficient methods showed similar results, on average 15% difference in CO2 evasion rate. It also showed that the mass balance method is not a good option for those kinds of stream systems. The study conclude that evasion of CO2 from boreal streams is a significant flux and hence an important term in the landscape C cycle.