Quantification of methane fluxes from water bodies on the floodplain forest ecosystem level

Freshwaters (rivers, streams, ponds, reservoirs) are a well-recognized source of methane (CH₄) characterised by large spatiotemporal variability. However, the determination of the role of water bodies for net CH₄ exchange on forest ecosystem scale is very scarce. Our study aimed to determine the importance of individual emission pathways for total CH₄ fluxes from streams and to verify the possible identification and quantification of CH₄ fluxes from water bodies on the floodplain forest ecosystem level. In 2020, we measured CH₄ fluxes by diffusion and ebullition from a lowland stream flowing through the lowland broadleaf mixed temperate forest at Lanžhot in the Czech Republic (Central Europe). For this purpose, 18 bubble traps were installed at three stream sites and periodically sampled for gas volume and its CH₄ content from April to December. Diffusive CH₄ fluxes from water were measured at 14 days intervals with a floating chamber connected to a portable GHG analyser. Simultaneously, CH₄ exchange was determined on the forest ecosystem scale using the eddy covariance method (EC). We hypothesized initially that due to a relatively small area of water bodies in the EC footprint and a high probability of CH₄ consumption by soils, CH₄ emissions will be detectable by EC only in case that water bodies will create the potential CH₄ emission hotspots in the studied ecosystem. We found that the investigated stream was a significant source of CH₄ (mean 260 ± 107 mg CH₄ m^-2 day^-1) with ebullition as a dominant pathway (55 – 85%) of CH₄ release throughout the whole monitored time period. Furthermore, first EC results showed that the whole ecosystem is a small but constant CH₄ source as we observed an average emission flux of 16 ± 18 mg CH₄ m^-2 day^-1 over the period June to November 2021. In-depth investigations of the potential CH₄ sources and sinks within the studied ecosystem should answer the question of how the relative proportion of water surfaces and related CH₄ emission corresponds to whole ecosystem CH₄ fluxes.