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Abstract. Pristine boreal mires are known as substantial sinks of carbon dioxide (CO2) and net emitters of methane (CH4). Bogs constitutea major fraction of pristine boreal mires. However, the bog CO2 and CH4 balances are poorly known, having been largely estimatedbased on discrete and short-term measurements by manual chambers and seldom using the eddy-covariance (EC) technique. Eddy-covariance (EC) measurements of CO2 and CH4 exchange were conducted in the Siikaneva mire complex in southern Finland in2011–2016. The site is a patterned bog having a moss–sedge–shrub vegetation typical of southern Eurasian taiga, with several ponds near the ECtower. The study presents a complete series of CO2 and CH4 EC flux (FCH4) measurements and identifies theenvironmental factors controlling the ecosystem–atmosphere CO2 and CH4 exchange. A 6-year average growing season(May–September) cumulative CO2 exchange of −61 ± 24 g C m−2 was observed, which partitions into mean totalrespiration (Re) of 167 ± 33 (interannual range 146–197) g C m−2 and mean gross primary production (GPP) of 228 ± 46(interannual range 193–257) g C m−2, while the corresponding FCH4 amounts to 7.1 ± 0.7 (interannual range6.4–8.4) g C m−2. The contribution of October–December CO2 and CH4 fluxes to the cumulative sums was notnegligible based on the measurements during one winter. GPP, Re and FCH4 increased with temperature. GPP and FCH4 did not show any significant decline even after asubstantial water table drawdown in 2011. Instead, GPP, Re and FCH4 were limited in the cool, cloudy and wet growing season of2012. May–September cumulative net ecosystem exchange (NEE) of 2013–2016 averaged at about −73 g C m−2, in contrast with the hot anddry year 2011 and the wet and cool year 2012. Suboptimal weather likely reduced the net sink by about 25 g C m−2 in 2011 due toelevated Re, and by about 40 g C m−2 in 2012 due to limited GPP. The cumulative growing season sums of GPP and CH4 emissionshowed a strong positive relationship. The EC source area was found to be comprised of eight distinct surface types. However, footprint analyses revealed that contributions of differentsurface types varied only within 10 %–20 % with respect to wind direction and stability conditions. Consequently, no clear link betweenCO2 and CH4 fluxes and the EC footprint composition was found despite the apparent variation in fluxes with wind direction.

Carbon balance of a Finnish bog: temporal variability and limiting factors based on 6 years of eddy-covariance data