Natural slicks on Lake Geneva: From small-scale effects to large-scale formation and distribution

Low wind speeds (< 4 m/s) are ubiquitous in many water bodies, yet the physical processes occurring at the air-water interface in this range are poorly understood. A notable example is smooth patches on the water surface, known as natural slicks, formed when biogenic surfactants dampen short wind waves. This thesis investigates natural slicks in Lake Geneva from two perspectives: (i) Their small-scale effects on surface and near-surface temperature dynamics and their impact on air-water exchanges, and (ii) the formation and distribution of slicks in the lake, dominated mainly by underlying large-scale currents. Furthermore, the thesis examines a range of interrelated physical processes in the low wind regime, including calm-to-wind-wave transitions, near-surface stratification, and the development of multiscale temperature fronts. For that, in situ observations were combined with airborne, shore-based and satellite remote-sensing data and 3D hydrodynamic modeling. Surface water sampling campaigns also confirmed the association between smooth patchiness and natural slicks. Natural slicks affect physical processes at the air-water interface by damping short waves. A notable example occurred when strong shortwave solar radiation-induced significant temperature gradients in a thin near-surface layer while wind speeds fluctuated around a 1.5 m/s surface wave generation threshold. While intense near-surface stratification was maintained within slicks, enhanced wave-induced mixing in non-slick areas was observed. As a result, sharp surface temperature gradients developed closely aligned with slick/non-slick boundaries. In contrast to previous studies carried out in oceans, warmer slicks were found under the studied environmental conditions. These results highlight the importance of wave-induced mixing under low wind conditions and underscore the potential role of slicks in air-water exchanges. However, determining the effect of slicks on surface fluxes using direct measurements is challenging due to their small spatial extent and the inherent wind variability at low wind speeds. To address this challenge, a series of field campaigns incorporating flux measurements were conducted, combined with wavelet analysis for evaluating short-term fluxes and dynamic flux footprint analysis. The results unveil how spatiotemporal flux variability under low wind conditions is affected by slicks, surface and near-surface temperature fluctuations, and short wind wave generation. This study is the first to use flux measurements on a moving platform with short-term flux estimations, offering a tool to address flux variability related to spatiotemporal heterogeneities.In addition to small-scale processes, strong winds can trigger gyres, eddies, and coastal upwelling in lakes. These processes can lead to the formation of fronts and filaments, which in turn create zones of intense convergence and downwelling where floating materials like surfactants concentrate when the wind subsides. This thesis investigates a striking ~10-km long "frontal slick" in Lake Geneva, developing on the warm side of a surface temperature front. Numerical modeling, supported by satellite data, showed that large eddies interact with density gradients associated with coastal upwelling, creating a strong convergent front. In lakes, these dynamics are common and considered a primary mechanism for large-scale slick formation, as well as highly efficient at transporting surface material.