Sediment respiration and lake trophic state are important predictors of large CO 2 evasion from small boreal lakes

We show that sediment respiration is one of the key factors contributing to the high CO 2 supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (<100 km 2 ) and 32 lakes with the highest total phosphorus (P tot ) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO 2 concentrations plotted against depth and time demonstrate strong CO 2 accumulation in hypolimnetic waters during the stratification periods. The relationship between O 2 departure from the saturation and CO 2 departure from the saturation was strong in the entire data set ( r 2 =0.79, n =2 740, P <0.0001). CO 2 concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO 2 concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake‐area‐weighted annual CO 2 evasion based on our 177 randomly selected lakes and all Finnish lakes >100 km 2 (Rantakari & Kortelainen, 2005) was 42 g C m −2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO 2 evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.