Years are not brothers: two-year comparison of greenhouse gas fluxes in large shallow Lake Võrtsjärv, Estonia
Abstract:
Two-year comparison of methane (CH4) and carbon dioxide (CO2) fluxes from different habitat zones inLake Võrtsjärv revealed strong zonal, seasonal and annual variabilities. On average, largest emissions ofboth gases per unit area occurred in the helophyte zone. Fluxes of CO2exceeded those of CH4in bothpelagic and littoral zones. In both years, emission peaks occurred in early spring and late autumn, butwere considerably higher in 2010, which caused a more than twenty times higher net efflux. Annual dif-ferences in carbon emission were mostly accounted for by variable pelagic CO2fluxes caused by differentice and snow conditions in spring and phytoplankton growth patterns in summer and autumn. On anannual basis, carbon uptake predominated over release in the pelagic zone in 2009, whereas an oppositebalance occurred in 2010. An almost continuous emission from the helophyte zone outbalanced the pela-gic uptake in 2009, resulting in a positive net efflux from the lake as a whole. In 2010, when the pelagicarea 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 CO2could be best described by oppo-site changes in phytoplankton biomass and pH (R2= 0.68), whereas the best descriptors for the wholelake methane emission (R2= 0.52) were sediment temperature (positive), saturation level of dissolvedoxygen and depth (both negative). Strong annual differences in net C emissions of Võrtsjärv confirmthe 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 needfor 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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