Browsing by Author "Erkkilä, Kukka-Maaria"

Freshwaters are a source of carbon to the atmosphere in the form of methane (CH4) and carbon dioxide (CO2). Global estimates of the freshwater contribution to the carbon budget are often based on a water boundary layer model (BLM) with gas transfer coefficient k calculated depending solely on wind speed. According to comparison studies, this model gives underestimated emissions and should not be used for more reliable results. A widely used flux measurement method over lakes is the floating chamber (FC) method. FC measures surface flux from a very small area of the lake, so it may not be representative of the whole ecosystem. Measurements are relatively cheap and easy, but also laborious and sporadic. Instead of measuring just a specific point on the lake, eddy covariance (EC) technique provides continuous flux measurements over a much larger source area (footprint). EC systems have been widely used over land areas, but are now growing their popularity in the lake community as well. The aim of this study was to compare EC, FC and BLM methods for CO2 and CH4 fluxes over a boreal lake. The measurements were made at a small dimictic Lake Kuivajärvi in Hyytiälä (Juupajoki, Southern Finland) during an intensive field campaign in September 2014. Manual FC measurements were done at four measurement spots in the EC footprint area 2-3 times a day for catching spatial and temporal variability. Gas transfer velocity for BLM was calculated according to three different parametrizations. Results indicate that BLM fluxes calculated based on water convection and wind driven turbulent gas exchange compare quite well with EC measurements while the model based solely on wind speed is a clear underestimate. FC measurements show about 1.7 times larger flux values than EC. The comparison is more clear for CH4 than CO2 fluxes. The greatest values of CH4 fluxes were measured near the shore, while CO2 flux did not show any spatial variability. After the lake started its autumn mixing, CH4 flux showed a diurnal variation with highest values measured during daytime. There was no diurnal variation before mixing. CO2 flux on the other hand showed diurnal variation only when calculated according to the BLM method.