An urban boreal lake basin as a source of CO2 and CH4

Up to now, carbon gas fluxes from urban lakes in the boreal zone have seldom been studied. In summer 2005 we investigated fluxes from an urban boreal lake basin in southern Finland with long history of eutrophication and anoxia. Hypolimnetic CO2 and CH4 concentrations were high compared to other boreal lakes. During the open-water period, the lake basin acted as a source of CO2 and CH4 with fluxes of 2.10 mol m-2 and 0.04 mol m-2, respectively. Despite the high oxidation rate (83%), CH4 flux was higher than in other lakes and CH4 contributed 60% to Global Warming Potential. The ratio of carbon emission to accumulation was 4, i.e. emissions were an important route for carbon departure but less so than in rural lakes. Since the lake oxygen conditions affected nutrient availability, there was a positive feedback from hypolimnion to carbon uptake, which was reflected in gas concentrations. - Highlights: → Hypolimnetic CH4 concentrations were higher in the urban lake vs rural lake. → The water basin was a steady source of CO2 and CH4. The annual GWP was due to CH4. → CH4 fluxes were high despite CH4 oxidation i.e. 83%. → Gas emission to carbon accumulation ratio was 4, thus sedimentation was important. → Limiting resources differ between urban and rural lakes during stratification. - Urbanization in the studied boreal lake was reflected in higher than normal hypolimnetic CO2 and CH4 concentrations resulting in considerable carbon gas fluxes, especially in terms of CH4.