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 (F_CH4) 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 (F_w_CO₂ and F_w_CH_4, 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 F_CH4 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 F_w_CO₂ was 297.5 g C-CO₂ m^-2 yr^-1 and F_w_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 F_CH4.

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.

The data was collected from an created urban wetland in Southern Finland (60.3272°N, 24.3369°E) during 2013 and 2014.

Greenhouse gas measurements was obtained by eddy covariance (ED) technique from a 2.9 m ED tower. The post-processing of the EC flux data has been done with EddyUH post-processing software. We used an artificial neural network (ANN) technique to gap-fill half-hourly flux data using meteorological variables.

Local weather conditions were recorded with a Vaisala WXT weather transmitter (WXT520, Vaisala Oyj, Finland) at the inlet monitoring station. Rainfall, wind speed and direction, temperature and relative humidity were recorded continuously at 10-minute interval. Photosynthetic photon flux density (PPFD) was measured with a PQS1 PAR quantum sensor (Kipp & Zonen, the Netherland). Due to instrument failure we obtained PPFD data only from 26 Jan to 7 April and from 22 July to 29 Dec 2013. The gaps were filled with PPFD data from another meteorological station nearby (60°38' N, 23°58' E) in Lettosuo, Finland.

The concentration of dissolved carbon dioxide ([CO₂]) and dissolved methane ([CH₄]) were measured with Contros HydroC™ CO₂ and HydroC™ CH₄ sensors (CONTROS Systems & Solutions GmbH, Germany). Dissolved CO₂ and CH₄ molecules diffuse from water column into the detection chamber through a thin-film composite membrane where the concentration of CO₂ and CH₄ is determined by means of IR absorption spectrometry and Tunable Diode Laser Absorption Spectroscopy, respectively.

Greenhouse gas measurements data can be found in "Nummela_QCfluxes.zip". Weather data can be found in "NummelaMeteodata_all.txt". Measurements in the water including dissolved gas concentration, water temperature, water table depth, etc. can be found in "water measurements.zip"