Summer greenhouse gas fluxes in different types of hemiboreal lakes.

Lakes are considered important regulators of atmospheric greenhouse gases (GHG). We estimated late summer open water GHG fluxes in nine hemiboreal lakes in Estonia classified under different lake types according to the European Water Framework Directive (WFD). We also used the WFD typology to provide an improved estimate of the total GHG emission from all Estonian lakes with a gross surface area of 2204 km representing 45,227 km of hemiboreal landscapes (the territory of Estonia). The results demonstrate largely variable CO fluxes among the lake types with most active emissions from Alkalitrophic (Alk), Stratified Alkalitrophic (StratAlk), Dark Soft and with predominant binding in Coastal, Very Large, and Light Soft lakes. The CO fluxes correlated strongly with dissolved CO saturation (DCO) values at the surface. Highest CH emissions were measured from the Coastal lake type, followed by Light Soft, StratAlk, and Alk types; Coastal, Light Soft, and StratAlk were emitting CH partly as bubbles. The only emitter of NO was the Alk type. We measured weak binding of NO in Dark Soft and Coastal lakes, while in all other studied lake types, the NO fluxes were too small to be quantified. Diversely from the common viewpoint of lakes as net sources of both CO and CH, it turns out from our results that at least in late summer, Estonian lakes are net sinks of both CO alone and the sum of CO and CH. This is mainly caused by the predominant CO sink function of Lake Peipsi forming ¾ of the total lake area and showing negative net emissions even after considering the Global Warming Potential (GWP) of other GHGs. Still, by converting CH data into CO equivalents, the combined emission of all Estonian lakes (8 T C day) is turned strongly positive: 2720 T CO equivalents per day.