Greenhouse gas fluxes from the eutrophic Temmesjoki River and its Estuary in the Liminganlahti Bay (the Baltic Sea)

Abstract

We studied concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the eutrophic Temmesjoki River and Estuary in the Liminganlahti Bay in 2003–2004 and evaluated the atmospheric fluxes of the gases based on measured concentrations, wind speeds and water current velocities. The Temmesjoki River was a source of CO2, CH4 and N2O to the atmosphere, whereas the Liminganlahti Bay was a minor source of CH4 and a minor source or a sink of CO2 and N2O. The results show that the fluxes of greenhouse gases in river ecosystems are highly related to the land use in its catchment areas. The most upstream river site, surrounded by forests and drained peatlands, released significant amounts of CO2 and CH4, with average fluxes of 5,400 mg CO2–C m−2 d−1 and 66 mg CH4–C m−2 d−1, and concentrations of 210 μM and 345 nM, respectively, but N2O concentrations, at an average of 17 nM, were close to the atmospheric equilibrium concentration. The downstream river sites surrounded by agricultural soils released significant amounts of N2O (with an average emission of 650 μg N2O–N m−2 d−1 and concentration of 22 nM), whereas the CO2 and CH4 concentrations were low compared to the upstream site (55 μM and 350 nM). In boreal regions, rivers are partly ice-covered in wintertime (approximately 5 months). A large part of the gases, i.e. 58% of CO2, 55% of CH4 and 36% of N2O emissions, were found to be released during wintertime from unfrozen parts of the river.