Spatial heterogeneity of water surface carbon dioxide and methane fluxes in a subarctic catchment

Water surface fluxes of carbon dioxide (CO₂) and methane (CH₄) show significant variation in space and time. Spatial variability in flux rates can, for example, be introduced by gradients in bathymetry, coverage by vegetation with different community structure, or lateral influx from connected groundwater bodies. The resulting variability in carbon cycle processes makes it difficult to estimate net lake flux budgets based on a low number of small scale sampling points, while integrated signals from coarser resolution observations are difficult to interpret as they combine multiple source and sink types. Due to a lack of observational data, especially at high spatial resolution, uncertainties of water-air fluxes of CO₂ and CH₄ in freshwater ecosystems are therefore large. This is problematic, especially in regions of high northern latitudes, where the density of inland waterbodies is very high.

To better capture spatially heterogeneous flux patterns, we measured the surface carbon fluxes accompanied by meteorological, hydrochemical and bathymetric measurements with the BlueMinerva, an autonomous floating platform,across a network of lakes during the StordalenX25 campaign in northern Sweden.

We obtained more than 1,000 spatially distributed flux estimates over a measurement period of two weeks. In comparison to terrestrial fluxes in the mire, CO₂ and CH₄ fluxes from freshwater were low. For CO₂ fluxes, fluorescent dissolved organic matter and pH were the strongest drivers overall, while the specific conductivity at the water surface explained most of the CH₄ flux variability across the network according to a random forest model. Furthermore, flux patterns may also be influenced by the fraction of each lake’s area covered by macrophytes, as derived from satellite imagery. For both gases, differences of flux estimates between the studied lakes were significant, which is particularly interesting for four of the lakes which are interconnected by small channels. 

Overall, this study demonstrates the importance of resolving heterogeneous carbon fluxes at small spatial scales for accurately estimating associated carbon budgets.