Investigating the effects of environmental change and biological processes on methane levels in lakes

This thesis explores the relationship between lake connectivity and surface methane (CH4) dynamics in deeper oligotrophic and mesotrophic lakes, with a particular focus on the influence of external factors such as humic/peatland dominated catchment. The thesis investigates the impact of increased colored dissolved organic matter (cDOM) input, through browning of aquatic environments, on CH4 dynamics in lakes. By examining lakes mainly affected by humic substances from peatland catchments, the study aims to elucidate the role of these environments in the global carbon cycle, especially considering predicted increases in lake browning (IPCC 2022). Through three key questions and corresponding studies conducted in diverse lake ecosystems, the thesis reveals several significant findings. Firstly, it demonstrates that lakes can transition from CH4 sinks to partial sources due to external enrichment from catchment-produced CH4. Secondly, it highlights the potential of browning to mitigate surface CH4 concentrations compared to clear water lakes. Thirdly, it indicates that browning favors mixotrophic phytoplankton dominance, suggesting a shift in community composition under altered environmental conditions. Finally, the thesis refutes the notion of mixotrophic phytoplankton as potential drivers of heightened surface CH4 concentrations, providing insights into the complex interplay between peatland dominated lake catchment, browning phenomena, and CH4 dynamics. These findings contribute to a deeper understanding of the surface CH4 dynamics in aquatic environments and underscore the importance of considering catchment influences in ecosystem management and climate change mitigation efforts.