Abstract. Many wetlands have been drained due to urbanization, agriculture,
forestry or other purposes, which has resulted in a loss of their ecosystem
services. To protect receiving waters and to achieve services such as flood
control and storm water 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 Southern
Finland in the boreal climate. We conducted measurements of ecosystem CO2 flux and CH4
flux (FCH4) at the created storm water wetland Gateway in Nummela,
Vihti, Southern Finland, using the eddy covariance (EC) technique. The
measurements were commenced the fourth year after construction and lasted
for 1 full year and two subsequent growing seasons. Besides ecosystem-scale fluxes measured by the EC tower, the diffusive CO2 and CH4 fluxes from the open-water areas (FwCO2
and FwCH4, respectively) were modelled based on
measurements of CO2 and CH4 concentration in the water.
Fluxes from the vegetated areas were estimated by applying a simple mixing
model using the above-mentioned fluxes and the footprint-weighted fractional
area. The half-hourly footprint-weighted contribution of diffusive fluxes
from open water ranged from 0 % to 25.5 % in 2013. The annual net ecosystem exchange (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 areas, exhibited strong diurnal cycles during
summer with synchronized peaks around noon. The annual
FwCO2 was 297.5 g C-CO2 m−2 yr−1 and FwCH4 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 storm water
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 areas exceeded that of the vegetation
areas (1.194 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.
Carbon dioxide and methane fluxes from different surface types in a created urban wetland