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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-279
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-279
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Sep 2019

Submitted as: research article | 18 Sep 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Carbon dioxide and methane fluxes from different surface types in a created urban wetland

Xuefei Li1, Outi Wahlroos2, Sami Haapanala3, Jukka Pumpanen4, Harri Vasander5, Anne Ojala1,5,6, Timo Vesala1,5, and Ivan Mammarella1 Xuefei Li et al.
  • 1Institute for Atmospheric and Earth System Research (INAR)/Physics, Faculty of Science, University of Helsinki, P.O. Box 68, 00014 University of Helsinki, Finland
  • 2University of Turku, Turku, Finland
  • 3Suvilumi, Ohrahuhdantie 2 B, 00680 Helsinki, Finland
  • 4Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
  • 5Institute for Atmospheric and Earth System Research (INAR)/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, P.O. Box 27, 00014 University of Helsinki, Finland
  • 6Ecosystems and Environment Research Programme, Faculty of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65, 00014 University of Helsinki, Finland

Abstract. Many wetlands have been drained due to urbanization, agriculture, forestry or other purposes, which has resulted in losing their ecosystem services. To protect receiving waters and to achieve services such as flood control and stormwater quality mitigation, new wetlands are created in urbanized areas. However, our knowledge of greenhouse gas exchange in newly created wetlands in urban areas is currently limited. In this paper we present measurements carried out at a created urban wetland in boreal climate.

We conducted measurements of ecosystem CO2 flux (NEE) and CH4 flux (FCH4) at the constructed stormwater wetland Gateway in Nummela, Vihti, Southern Finland using eddy covariance (EC) technique. The measurements were commenced the fourth year after construction and lasted for one full year and two subsequent growing seasons. Besides ecosystem scale fluxes measured by EC tower, the diffusive CO2 and CH4 fluxes from the open-water area (Fw_CO2 and Fw_CH4, respectively) were modelled based on measurements of CO2 and CH4 concentration in the water. Fluxes from vegetated area were estimated by applying a simple mixing model using above-mentioned fluxes and footprint-weighted fractional area. The half-hourly footprint-weighted contribution of diffusive fluxes from open water ranged from 0 to 25.5 % in year 2013.

The annual NEE of the studied wetland was 8.0 g C-CO2 m−2 yr−1 with the 95 % confidence interval between −18.9 and 34.9 g C-CO2 m−2 yr−1 and FCH4 was 3.9 g C-CH4 m−2 yr−1 with the 95 % confidence interval between 3.75 and 4.07 g C-CH4 m−2 yr−1. The ecosystem sequestered CO2 during summer months (June–August), while the rest of the year it was a CO2 source. CH4 displayed strong seasonal dynamics, higher in summer and lower in winter, with a sporadic emission episode in the end of May 2013. Both CH4 and CO2 fluxes, especially those obtained from vegetated area, exhibited strong diurnal cycle during summer with synchronized peaks around noon. The annual Fw_CO2 was 297.5 g C-CO2 m−2 yr−1 and Fw_CH4 was 1.73 g C-CH4 m−2 yr−1. The peak diffusive CH4 flux was 137.6 nmol C-CH4 m−2 s−1, which was synchronized with the FCH4.

Overall, during the monitored time period, the established stormwater wetland had a climate warming effect with 0.263 kg CO2-eq m−2 yr−1 of which 89 % was contributed by CH4. The radiative forcing of the open-water exceeded the vegetation area (1.194 kg CO2-eq m−2 yr−1 and 0.111 kg CO2-eq m−2 yr−1, respectively), which implies that, when considering solely the climate impact of a created wetland over a 100-year horizon, it would be more beneficial to design and establish wetlands with large patches of emergent vegetation, and to limit the areas of open-water to the minimum necessitated by other desired ecosystem services.

Xuefei Li et al.
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Short summary
We studied the greenhouse gases fluxes in a created urban wetland in South Finland. Annual analysis showed the ecosystem has a net climate warming effect. The effect of open-water sector exceeded the vegetated sector, indicating that limiting open-water surfaces and setting a design preference for areas of emergent vegetation in the establishment of urban wetlands can be a beneficial practice when considering only the climate impact.
We studied the greenhouse gases fluxes in a created urban wetland in South Finland. Annual...
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