[1] Most estimates of diffusive flux (F) of methane (CH 4 ) and carbon dioxide (CO 2 ) from lakes are based on single-point flux chamber measurements or on piston velocity (k) modeled from wind speed and single-point measurements of surface water gas concentrations (C aq ). We analyzed spatial variability of F of CH 4 and CO 2 , as well as C aq and k in 22 European lakes during late summer. F and k were higher in the lake centers, leading to considerable bias when extrapolating single-point chamber measurements to whole-lake estimates. The ratio of our empirical k estimates to wind speed-modeled k was related to lake size and shape, suggesting a lake morphology effect on the relationship between wind speed and k. This indicates that the error inherent to established wind speed models can be reduced by determining k and C aq at multiple sites on lakes to calibrate wind speed-modeled k to the local system. Citation: Schilder, J., D. Bastviken, M. van Hardenbroek, P. Kankaala, P. Rinta, T. Stötter, and O. Heiri (2013), Spatial heterogeneity and lake morphology affect diffusive greenhouse gas emission estimates of lakes, Geophys. Res. Lett., 40,[5752][5753][5754][5755][5756]

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Methane (CH 4 ) and carbon dioxide emissions from lakes are relevant for assessing the greenhouse gas output of wetlands. However, only few standardized datasets describe concentrations of these gases in lakes across different geographical regions. We studied concentrations and stable carbon isotopic composition (d 13 C) of CH 4 and dissolved inorganic carbon (DIC) in 32 small lakes from Finland, Sweden, Germany, the Netherlands, and Switzerland in late summer. Higher concentrations and d 13 C values of DIC were observed in calcareous lakes than in lakes on non-calcareous areas. In stratified lakes, d 13 C values of DIC were generally lower in the hypolimnion due to the degradation of organic matter (OM). Unexpectedly, increased d 13 C values of DIC were registered above the sediment in several lakes. This may reflect carbonate dissolution in calcareous lakes or methanogenesis in deepwater layers or in the sediments. Surface water CH 4 concentrations were generally higher in western and central European lakes than in Fennoscandian lakes, possibly due to higher CH 4 production in the littoral sediments and lateral transport, whereas CH 4 concentrations in the hypolimnion did not differ significantly between the regions. The d 13 C values of CH 4 in the sediment suggest that d 13 C values of biogenic CH 4 are not necessarily linked to d 13 C values of sedimentary OM but may be strongly influenced by OM quality and methanogenic pathway. Our study suggests that CH 4 and DIC cycling in small lakes differ between geographical regions and that this should be taken into account when regional studies on greenhouse gas emissions are upscaled to inter-regional scales.