Years are not brothers: Two-year comparison of greenhouse gas fluxes in large shallow Lake Võrtsjärv, Estonia

- Two-year comparison of annual CH4 and CO2 flux patterns revealed strong variability.
- Highest emission peaks occurred in early spring and late autumn.
- In 2010 the net GHG efflux was more than twenty times higher caused by pelagic CO2.
- Main causes were different ice and snow conditions and phytoplankton growth patterns.
- Helophyte GHG fluxes are important for a lake GHG budget despite of small zone area.

SummaryTwo-year comparison of methane (CH4) and carbon dioxide (CO2) fluxes from different habitat zones in Lake Võrtsjärv revealed strong zonal, seasonal and annual variabilities. On average, largest emissions of both gases per unit area occurred in the helophyte zone. Fluxes of CO2 exceeded those of CH4 in both pelagic and littoral zones. In both years, emission peaks occurred in early spring and late autumn, but were considerably higher in 2010, which caused a more than twenty times higher net efflux. Annual differences in carbon emission were mostly accounted for by variable pelagic CO2 fluxes caused by different ice and snow conditions in spring and phytoplankton growth patterns in summer and autumn. On an annual basis, carbon uptake predominated over release in the pelagic zone in 2009, whereas an opposite balance occurred in 2010. An almost continuous emission from the helophyte zone outbalanced the pelagic uptake in 2009, resulting in a positive net efflux from the lake as a whole. In 2010, when the pelagic area was also mostly emitting carbon, the efflux from the helophyte zone constituted 36% of the total. Multiple linear regression analysis showed that the pelagic flux of CO2 could be best described by opposite changes in phytoplankton biomass and pH (R2 = 0.68), whereas the best descriptors for the whole lake methane emission (R2 = 0.52) were sediment temperature (positive), saturation level of dissolved oxygen and depth (both negative). Strong annual differences in net C emissions of Võrtsjärv confirm the necessity of inclusion longer observation period, compared to widely used one year/ice-free period, in cases of lakes in climate zones with large seasonal variabilities. The results also confirm the need for including GHG emissions from helophytes (e.g. Phragmites australis) to the lake's net GHG budget, even if they cover a relatively small area.

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