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.