Insights into seasonal dynamics of metalimnetic methane and hypolimnetic nitrous oxide in a deep Swiss alpine lake

Methane (CH₄) and nitrous oxide (N₂O) are potent greenhouse gases (GHGs) and are involved in ozone depletion. They are the second and third most significant GHGs contributing to climate change, with global warming potentials approximately 25 times and 300 times greater than CO₂. The contribution of freshwater lakes to CH₄ and N₂O emissions remains debatable. Oxygen-rich subsurface waters are recognized as hotspots of metalimnetic CH₄ flux, while oxygen-driven diffusion in deep water columns is expected to enhance hypolimnetic N₂O production in deep freshwater lakes. Therefore, understanding the seasonal dynamics of water-column CH₄ and N₂O in freshwater ecosystems is crucial for predicting their impact on future climate projections. Traditionally, both GHG gases have been thought to be primarily produced in sediments, with higher emissions occurring during the mixing events. However, the seasonal dynamics and characteristics of both water-column gases, including production in the water column which may contribute to atmospheric emissions, are often overlooked. Here, we hypothesize that both metalimnetic CH₄ and hypolimnetic N₂O might be present and emitted from deep lakes year-round. We will present our preliminary measurements of seasonal variations in CH₄ and N₂O concentrations in the deep Swiss alpine Lake Geneva. They relied on laser spectrometric probes called SubOcean, allowing in-situ and real-time observations of CH₄ and N₂O along the water column, with a remarkable depth resolution.